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Some questions about the Heinkel He 219's Nacelle Thrust Line

TomcatViP

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Well that's what you get when you swap your engine with one having a lower power setting. The perfect balance is altered and the plane will cruise at a higher alpha just like when you reduce power.
 

edwest

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I do hope there will be a glossary in the book, and standard terms. Yes, the designers would have to make adjustments for a heavier or lighter engine, but original documents and photos would be needed to draw any conclusions. Ideally, some in-flight footage as well. I highly doubt that parts that were meant to be curved would be installed flat. If a machine press was not available then bending by hand could have been employed.
 

Zootycoon

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Well that's what you get when you swap your engine with one having a lower power setting. The perfect balance is altered and the plane will cruise at a higher alpha just like when you reduce power.
If your theory is correct, why didn’t they adjust the wing incidence to the fuselage as well? They’ve left the fuselage in the high drag orientation so they’ve only done half the job of getting the prefect balance.
 

TomcatViP

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Because it would need an extensive modification of the airframe, jigs etc...
Many successful planes did (and do) fly nose down for example when they got higher power engines. See the 109. See the B-52 at low alt.
The Drag curve (Cl/Cd) is sympathetic to minor alpha change (in positive and negative values). That helps you with gusts but also here.
 
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Zootycoon

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Except it doesn’t fly nose down
1592772969418.jpeg
I reckon there’s no engine tilt either.
 
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edwest

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Multiple photographic views are required. Along with illustrations showing structural changes, if any, and also close-ups of significant parts of the aircraft. Followed by a six view drawing.
 

TomcatViP

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Except it doesn’t fly nose down
View attachment 635749
I reckon there’s no engine tilt either.
I know it's getting complicated but the nose down attitude is for an engine power upgrade! It was a counter example.
Also in your picture. The plane is obviously not flying at high speed (close to max power) but cruising (or somewhat). You can see the wing attitude with positive pitch and the slight droop of the engine nacelle !
 

Zootycoon

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I know it's getting complicated but the nose down attitude is for an engine power upgrade! It was a counter example.
Also in your picture. The plane is obviously not flying at high speed (close to max power) but cruising (or somewhat).
It’s not complicated;- Here’s a typical Cl vs Airspeed with associated AOA, not the He219 but they’re all from the same laws of physics.
(credit Mechanics of Flight- Kermode)

1592856483052.jpeg You can see that the AOA change with increasing airspeed is rapidly tending towards a really small angle at much more than 300kts & certainly no where near the engIne tilt. Note also that at the highest speed the AOA goes negative so if your theory was correct the engine tilt is the wrong way.
So its simple;- there’s no need to tilt the engine to cater for the AOA change as a result of installing the less powerful engine.

Why is the aeroplane in the photo obviously not flying at high speed?
 
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coanda

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This has been a fun thread to read...

To the OP, you gotta go measure one, that's what you have to do if you want your drawings to be correct.

Please remember, its only correct for the aircraft you measure. It may not be correct to the manufacturers drawings or intent, it will also only really be correct to the side you measure (so do both!). The aircraft you measure will also not be correct to how it rolled off the production line. That's worth pointing out in your text - your drawings won't ever be nominal if you don't just reproduce the manufacturers drawings.

Your drawings represent 1g on the ground in the condition the aircraft is measured - not any other permutation which could well alter shapes, ground angles and cruise alpha and consequently dihedral and wing twist distribution.

I think a lot of the other discussion isn't really pertinent to the thread so won't address other points.
 

edwest

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I think the answer to the question is that fuel is conserved at cruising speed. More fuel is used getting up into the air. Flying faster is required to pursue and get within firing range of an enemy. Once there, careful speed control is used to get the aircraft in position to fire the killing shots, and then dropping and banking away to avoid debris and/or a possible explosion.
 

richdlc

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This has been a fun thread to read...

To the OP, you gotta go measure one, that's what you have to do if you want your drawings to be correct.

Please remember, its only correct for the aircraft you measure. It may not be correct to the manufacturers drawings or intent, it will also only really be correct to the side you measure (so do both!). The aircraft you measure will also not be correct to how it rolled off the production line. That's worth pointing out in your text - your drawings won't ever be nominal if you don't just reproduce the manufacturers drawings.

Your drawings represent 1g on the ground in the condition the aircraft is measured - not any other permutation which could well alter shapes, ground angles and cruise alpha and consequently dihedral and wing twist distribution.

I think a lot of the other discussion isn't really pertinent to the thread so won't address other points.

Hi - well that's the point, these ARE measurements from the last surviving airframe. See earlier answers. Drawings also appear to show downward tilt of engine gondolas. My point is this phenomenon is visible not only in the drawings, but in many contemporary photos
 

richdlc

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Except it doesn’t fly nose down
View attachment 635749
I reckon there’s no engine tilt either.
I know it's getting complicated but the nose down attitude is for an engine power upgrade! It was a counter example.
Also in your picture. The plane is obviously not flying at high speed (close to max power) but cruising (or somewhat). You can see the wing attitude with positive pitch and the slight droop of the engine nacelle !

the aircraft in the photo you've chosen is the one they used for ejection seat trials. I do have the speeds somewhere, but I would say you are correct, and that this shows a speed of around 400 km/h. The aircraft's top speed was just over 600 km/h
 

coanda

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This has been a fun thread to read...

To the OP, you gotta go measure one, that's what you have to do if you want your drawings to be correct.

Please remember, its only correct for the aircraft you measure. It may not be correct to the manufacturers drawings or intent, it will also only really be correct to the side you measure (so do both!). The aircraft you measure will also not be correct to how it rolled off the production line. That's worth pointing out in your text - your drawings won't ever be nominal if you don't just reproduce the manufacturers drawings.

Your drawings represent 1g on the ground in the condition the aircraft is measured - not any other permutation which could well alter shapes, ground angles and cruise alpha and consequently dihedral and wing twist distribution.

I think a lot of the other discussion isn't really pertinent to the thread so won't address other points.

Hi - well that's the point, these ARE measurements from the last surviving airframe. See earlier answers. Drawings also appear to show downward tilt of engine gondolas. My point is this phenomenon is visible not only in the drawings, but in many contemporary photos
Photos are only indicative for all the reasons outlined above. The drawings you have apply to that airframe only, strictly speaking. If you decide to normalise that set of drawings as 'this is what a He219 looks like' I think it appropriate for a note to be placed with the drawings saying as such.
 

TomcatViP

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@Zootycoon :
You don't read. I can't comment on something you think I said.
I made an illustrated suggestion trying to help. The topic is not me but a book on the Uhu and an author seeking suggestions.
Thanks.
 

Richard N

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I think the answer to the question is that fuel is conserved at cruising speed. More fuel is used getting up into the air. Flying faster is required to pursue and get within firing range of an enemy. Once there, careful speed control is used to get the aircraft in position to fire the killing shots, and then dropping and banking away to avoid debris and/or a possible explosion.
Concerning fuel consumption during a flight, the aircraft will be heaviest at the beginning of the flight and the wing will require its highest angle of attack and most power early in the flight to develop the required lift to fly at its necessary altitude and speed. As fuel is consumed and the aircraft gets lighter, wing AOA can be reduced leading to decreased drag and power required. Where the fuel winds up as it is consumed will also affect the trim of the aircraft. I think that the fuel management result that delivers the least trim drag with the least power needed is the desired goal.
 

newsdeskdan

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The thrust line of the He 219's nacelles does not appear to be angled downwards. This is from an AVA aerodynamics report of July 1944.

1.jpg

4.jpg

48.jpg
 

newsdeskdan

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However, it would appear that the rear portion of the nacelles was remodelled at some point. This is from a DVL aerodynamics report on the type circa May 1943.

Scan_0033.jpg
 

richdlc

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thank you, most interesting. I mentioned earlier that later models had extra fuel tanks in the rear of the gondolas which might account for a necessary redesign
 

riggerrob

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Post #59 shows a thrust line angled slightly downwards compared with the wing chord. Remember that wing-chord is measured from the centre of the leading edge - in a straight line - to the trailing edge.
I suspect the wing angle of incidence (usually measured relative to the fuselage) is optimized for climb. Mind you, the difference between the best angle of attack (wing-chord compared to relative wind/airflow) for climb is probably very close to the best angle for high-altitude loitering.
 

edwest

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In post #59, second image at the top under Gondel, the tip at the back end shows it is above the centerline.
 

Richard N

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Rich,

Since the Zoukei-Mura guys are the only people to have actually had hands on the airplane and measured it, have you spoken to them about how they chose the angles on their model? They had to resolve these issues before they cut metal on their molds.

Just because the aircraft was disassembled, that does not mean that there is no way to measure how the aircraft was rigged. There will be holes to match up in the nacelles, fuselage, and wings that tell where and how they all went together. Making guesses based on photos along with perspective, point of view, and optic distortion is guesswork at best and one photo can easily disagree with another depending on the differing circumstances under which the photos were taken. If that is all you have, you have to use it and do the best you can, but here you have an actual aircraft along with measurement done to create a model and the answers you are looking for are probably inside the measured data.

Richard
 

newsdeskdan

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Going back a little further, this is a Heinkel drawing from October 1942. It too deliberately shows the thrust line of the engine nacelles running completely in parallel to the fuselage - no downwards tilt.
October 1942 = no tilt.
May 1943 = no tilt.
July 1944 = no tilt.
I have another drawing from a June 1944 report showing no tilt too, but it doesn't really add anything different. Heinkel certainly seems to have commissioned a lot of aerodynamics work on the He 219. The only period He 219 drawing I can find with a suggestion of a tilt is the obviously inaccurate flugzeugtypenblatt from 1.1.42, which shows the nacelles tilted upwards at the front.
I can only conclude that the trust line of the engine nacelles of both prototype and production model He 219s ran parallel to the thrust line of the fuselage, regardless of the nacelles' external form.
Obviously, the He 219 was cancelled on November 9, 1944, with production officially scheduled to cease in February 1945, so it's possible that the last examples off the production line had something different but I've never seen any drawings that late and there don't seem to be any references to major engineering changes that late.

He 219.jpg

He 219 detail.jpg
 

richdlc

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many thanks guys.

regarding this image - could it really be so simple? That the answer is that the REAR of the gondola is above the centreline and the gondola isn't tipped forward at all??

Screen Shot 2020-06-30 at 16.03.41.png
 

richdlc

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Also, this document is most interesting. Our authors maintain that the aircraft was virtually unknown as the 'Uhu' by pilots and groundcrew until after the war's end. We do have a document (which I can't discuss here) which PROVES the official nickname, but I can't recall if I have seen this one:
Screen Shot 2020-06-30 at 16.05.30.png
 

richdlc

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Gents - I wonder if you could take a look at this copy I have prepared. It is based on all of your comments and suggestions, plus some extra info from other sources. This is going to be aimed at both the layman, and the more advanced reader, but I want to know if it makes sense or if it's dis-jointed gibberish. Please note that this information stops at post #59 in this thread. If you could make suggestions about improvements, whether I should include pictures / photos / diagrams (ALSO WHERE TO FIND THEM) then I would be very grateful!!

Thanks in advance...


He 219 Engine Gondola Slant: A Question of Thrust Lines, Design and Perspective.


There are certain aspects of the design of the He 219 which have sparked a great deal of conversation and speculation among historians, researchers and model makers. One of these is the apparent ‘downward’ thrust line of the engine gondolas when the aircraft is viewed from the side. Conversely, some would argue instead that the thrust-line of the engines appears roughly parallel to the ground, so, essentially, the angle of attack of the aircraft nose and wing would seem to be elevated for take off. Model kit manufacturers in particular have been trying to get this right, with varying levels of success, for some time. The shape of the engines / gondolas and their incidence to the wings also has a knock-on effect when producing scale drawings.

(insert an annotated image which shows the exhausts perpendicular to the engine thrust line etc.)

What could explain this ‘downward pointing’ engine phenomenon? Aircraft are very complex shapes and the parameters have precise definitions that do not necessarily relate to identifiable points on an airframe. Accurate developmental drawings and design testing records from the source, in this case, Heinkel, are required for a truly accurate and satisfactory conclusion. Unfortunately much documentation relating to the Ernst Heinkel Werke was captured by the Red Army in Vienna in 1945. This is rumoured to be stored at The Central Archives of the Ministry of Defence of the Russian Federation (TsAMO RF) in Podolsk, near Moscow. Russian researchers have found reliable information that the technical archives from Vienna were collected by a special Soviet team of qualified aviation engineers and directed to the Central Aviation and Hydrodynamic Institute (TsAGI) for evaluation and consequent distribution to the interested design bureaus. (1) (get source). Should this information come to light at some point in the future, it could reveal accurate dimensional and arrangement information, design studies, reports, calculations and correspondence. Without it, the researcher is merely speculating, and trying to accurately measure design parameters such as AOI (Angle of Incidence), airfoil chord, and thrust lines relative to longitudinal reference using photographs or even direct reference to actual airframes is all subject to the limitations imposed by a lack of access to original documents. Even hamstrung by this gap in our knowledge, it is possible to present a plausible hypothesis based on what we know about aerodynamics.

How do aircraft become airborne in the first place? In a simplified form: if we imagine a straight bar balanced on a fulcrum or pivot point, with a running motor and propeller mounted on top of the bar over the fulcrum, the bar stays balanced because the thrust or pull is balanced over that fulcrum. If we then move the motor to the side of the fulcrum and added a counter-balancing weight on the opposite side so the bar stays horizontal and run the motor again, the thrust would pull at an offset of, and towards the direction of the fulcrum. To bring things back into equilibrium, if we now point the motor away from the fulcrum the right amount, everything will go forward in the same direction as the vertical line of the fulcrum. Applying this to an aircraft, if the engine thrust line is very close to or parallel to a line through the centre of the leading edge and trailing edge of the wing, the plane will neither climb nor dive when power is applied. If the thrust line is significantly below and parallel to the centre of the leading edge and trailing edge line, the plane will climb under throttle requiring down trim to maintain level flight. When throttle is reduced in level flight, the down trim will have to be removed or the plane will dive downwards. To correct this, down-thrust or pointing the thrust line downwards and forward will allow the plane to fly level with little or no trim needed to fly level power on or off. Applying this logic to the He 219, when taking off the positive angle of the aircraft sitting on its landing gear means that as the plane nears take-off speed, the wing, being at a positive angle of attack, will lift the plane off the ground without elevator input if it is going fast enough. To shorten the takeoff roll, more power can be applied with up elevator to increase the wing's angle of attack and generate sufficient lift to take-off sooner.

When studying photos of the restoration process of the last surviving He 219 at the The Steven F. Udvar-Hazy centre (the Smithsonian National Air and Space Museum's annex at Washington Dulles International Airport USA), it would seem from studying the fuselage that the wing root is roughly aligned with the airframe's longitudinal axis. When the renowned model kit manufacturer Zoukei Mura of Japan were designing their superb 1/32 scale kit of the aircraft in 2012, they were given permission by the museum to use LIDAR (Light Radar) to create an accurate Three Dimensional (3D) model of the aircraft. LIDAR scanning uses laser light to accurately map the surface of an object in three dimensions, resulting in a high-definition 3D computer image of it. The image can then be fed into a CAD (Computer Aided Design) system, enabling designers to produce incredibly detailed models directly mapped from a real object, rather than recreating the shape from other sources such as blueprints or photographs. (2) This also means that highly accurate scale drawings can be created. (see p.xxx) At the time this book goes to press, LIDAR is still considered to be cutting-edge technology, and, in conversation with the publisher in 2017, the well-known

He 219 researcher Ron F. Ferguson concluded that the Zoukei Mura drawings were the best that he had seen. Close inspection of these drawings also shows the apparent downward tilt of the engine gondolas, in spite of the fact that at the time they were created, the Museum’s He 219 wings were still unattached to the fuselage. It is worth noting that an aircraft measured on the ground will not identically match one that has just rolled off the production line. The Zoukei Mura drawings represent 1g on the ground in the condition the aircraft was measured (engines and gondolas disassembled, engines and wings unattached to the fuselage), and not any other permutation which could well alter shapes, ground angles and cruise alpha (angle of attack) and consequently dihedral and wing twist distribution. For example, in the design stages the wing jig shape would also have been different to the 1g on the ground shape, which would be different to the in-flight shape. The key reference general arrangement drawing in aircraft design is called the ‘Ground Line Drawing’ which depicts the aircraft on its landing gear, at 1g and normally maximum take-off weight. The wing jig shape drawing is only used by manufacturing. The principal flight shape drawings are ones derived from wind tunnel tests, or more recently CFD (Computational Fluid Dynamics). Wind tunnel models start off as a very stiff representative of the flight, machined from ultra high tensile steel or teak. As the development progresses, models with representative stiffness or even parametric stiffnesses can be used to investigate the likely flight shape and dynamic stability, more commonly known as ‘flutter’, a further aeroelastic dark art. Bearing all of this in mind, we can see that drawings can never be nominal unless reproducing the original manufacturers drawings. The drawings made by Zoukei Mura apply to one particular airframe only. If we decide to normalise that set of drawings and say 'this is what a He 219 looks like' it is appropriate and important to remember this fact.

Sometimes the most important parameter in aircraft design is what is known as ‘cruise’. This is the flight phase that occurs when the aircraft levels off after a climb to a set altitude and before it begins to descend. The ‘thrust line’ (an imaginary line through which the resultant thrust acts, and which may refer to the thrust axis of one engine or of the whole aircraft) maximises the pull / push effect with the higher Cl/CD position for the wing. This is sympathetic to minor alpha / angle of attack change (in positive and negative values) and helps with wind gusts in flight and the subsequent stability of the aircraft. (CI/CD refers to ‘drag curve’. Because power must equal drag to maintain a steady airspeed, the curve can be either a drag curve or a power required curve. As airspeed increases, the propeller efficiency increases until it reaches its maximum. Any airspeed above this maximum point causes a reduction in propeller efficiency.) (3)

The He 219 was designed for the Jumo 222 engine in the 2,000 to 2,500 hp class, which would have resulted in an increased performance in flight. However, restrictions imposed on the Jumo 222’s development by the RLM (Reichsluftfahrtministerium, the Ministry of Aviation) meant that the type had to be equipped with less powerful engines. Heinkel turned instead to the DB 603. (4) The DB 603 weighed (dry weight) 920kg and the Jumo 222 1,088kg. If during the design phase engines are changed, with a radical effect on power, the thrust line has to be corrected as well as the angle between the chord line at cruise and the thrust line to reflect the variation in mass and / or centre of gravity (different wing pitch). A slightly ‘downward’ tilt of the engine gondolas would therefore produce something of a counteracting downwards force to compensate for the lighter and less powerful DB 603 engines. Less power equates to a lower cruise speed and an increase in pitch for cruise. Hence, since more angle of attack was needed, it is not beyond the realms of possibility to suggest that the engines had to be tilted downward to be perfectly in line during cruise. All the engine force would then act on traction. However, if this theory is correct, why then did Heinkel not simply adjust the wing incidence to the fuselage as well? We can see that the fuselage still has a high drag orientation. The answer could lie in the fact that these modifications would require an extensive modification of the airframe and wing jigs, as well as extensive further testing, and quite possibly Germany’s deteriorating war situation made this impractical.

When looking at the He 219 from various angles, it is obvious that the wing has several degrees of incidence, including twist for the ‘washout’ that can be observed outboard of the engine gondolas (washout is a characteristic of aircraft wing design which deliberately reduces the lift distribution across the span of an aircraft’s wing. The wing is designed so that the angle of incidence is greater at the wing roots and decreases across the span, becoming lowest at the wing tip). (5) Washout is most commonly used to tailor wing stall characteristics. It involves setting the wing-tip angle of incidence slightly shallower (nose down) to delay tip stall until after the wing-root is fully stalled. The goal is to maintain aileron (roll) control when the wing is partially stalled. Other ways to delay tip stall include changing the airfoil from root to tip, and adding stall fences (fixed aerodynamic devices attached to aircraft wings and more relevant to swept-wing aircraft, they are flat plates fixed to the upper surfaces parallel to the wing chord and in line with the free stream airflow, typically wrapping around the leading edge. By obstructing span-wise airflow along the wing, they prevent the entire wing from stalling at once, as opposed to wingtip devices, which increase aerodynamic efficiency by seeking to recover wing vortex energy.) (6) Although the He 219 does not have such design features as stall fences, the combined visual effects of twist and washout could certainly alter the viewers perception of the nacelles in relation to the rest of the airframe.

‘Aeroelastics’ (the amount of flight induced wing flexing, especially twist) is notoriously difficult to predict before a prototype’s first flight even for modern manufacturers, using the latest analysis technology. Aero-elasticity is a complex phenomenon that is inherent to the construction characteristics of the wing and the material used. So in a quiet, almost un-noticed process, it is routine to fine tune the wing twist based on real flight loaded observations as this makes a big difference to the cruise fuel burn. Aeroelastic wing structural twist, sometimes called ‘wing jig shape’, is different to washout but is yet another phenomenon which might alter the viewer’s perception of angles when looking at an aircraft. In aircraft design, the whole wing is deliberately built in a jig with a twist such that the flight loads will correct back to the design intended flight orientation. The washout is part of the design intent orientation, so its jig shape is likewise deflected back to the desired figures. Also the final optimal flight wing shape usually requires several iterations after the first prototype, normally with the first iteration being applied to the next wing design change. Wing jigs are generally made so that a twist adjustment is readily accommodated as the production run progresses. Quite a few examples of a complete airframe might therefore be built before the optimal design parameters are found (for example, when the Boeing 787 Dreamliner was developed in the late 2000s, the early production aircraft underwent a number of modifications as issues with weight and performance were uncovered. These ‘terrible teens’, so called because their production numbers were between the 10th and 20th Dreamliners built, were some of the first of the type to roll off of Boeing's production line at a time when the company had not quite completed the development process of the plane. Eventually many were sold off cheaply to various carriers. They required significant modifications, including heavy structural reinforcement that made them much heavier than the current versions of the 787. Some estimated that the alterations needed to make the ‘teens’ serviceable as commercial airliners cut as much as 1,150 miles off the Dreamliner's advertised range of 8,500 miles, which had a significantly detrimental effect on the aircraft’s greatest selling points of fuel economy and range.) (7)

Turning our attention to maximum speed of an aircraft in general, we must note that a greater impact on drag is drag related to lift. Many planes of the 1940s and 1950s-era flew with their nose pointing slightly down, for example the Bf 109 and the B-52 Stratofortress at low altitude (note however that jets often have offset thrust lines for a variety of reasons, with much greater effect than piston-engine thrust lines and we should err on the side of caution when comparing the flight characteristics of piston-engined and jet-engined aircraft.) What is certain is that other contemporary designs underwent wing alterations similar to that of the He 219. The P-38 Lightning (if we are being pedantic, the ‘Lightning I’) for example changed engine model and thrust lines from the XP-38. A direct comparison between models of the B-26 Marauder shows a change of 3 1/2 degrees in wing incidence upwards in the B-26G from previous models. This design change was to make take offs safer, and an example of compensations made for balance and stability issues.(8)

The He 219 A-7 variant and also some late A-2 variants had additional fuel cells in the rear of the engine gondolas. The Udvar Hazy He 219 is a very late A-2 variant and is fitted with such cells. Aircraft are designed to conserve fuel at cruising speed. More fuel is used getting up into the air. Flying faster is required to pursue and get within firing range of an enemy. Once there, careful speed control is used to get the aircraft in position to fire the killing shots, and then dropping and banking away to avoid debris and/or a possible explosion. When considering fuel consumption during a flight, the aircraft will be heaviest at the beginning of the flight and the wing will require its highest angle of attack and most power early on to develop the required lift to fly at its necessary altitude and speed. As fuel is consumed and the aircraft gets lighter, wing angle of attack can be reduced, leading to decreased drag and power required. Where the fuel winds up as it is consumed will also affect the trim of the aircraft. Generally, the fuel management result that delivers the least trim drag with the least power needed is the desired goal. Therefore, if we look at, for example, contemporary photographs of early and later variant He 219s we may well see a variance in the relative angles of the wings, engines and gondolas due to design changes implemented throughout its lifespan to increase fuel efficiency or whether that particular aircraft is shown at combat weight or empty.

It is also problematic for our perception of angles that the He 219 sits with such a pronounced nose-up attitude on its revolutionary tricycle landing gear. The angle of attack in high speed flight and the angle at which a plane sits on its undercarriage on the ground are very different things. The centre of gravity on a tricycle-gear aircraft will be slightly forward of a vertical line perpendicular to the ground-line and through the centreline of the main wheels. If the main wheels are too far forward, the aircraft can wind up sitting on its tail when there is no crew or fuel on board. The He 219’s landing gear is aft of the main wing spar and the heavy engines are forward of it. So the ‘moment couple’ will tend to nod the engines downwards while on the ground. (The moment of a couple is the product of the magnitude of one of the forces and the perpendicular distance between their lines of action M = F x d. In other words a couple refers to two parallel forces that are equal in magnitude, opposite in direction and do not share a line of action). (9)

With a propeller-driven aircraft there are also complexities when we view the aircraft with its engines running, and when the aircraft is taxiing because the engine and propeller produce ‘gyro loads’ or ‘moments’, not to mention the fact that the air flowing fore to aft has an upward vector on one side and downward vector on the other. Gyroscopic loads on an engine pylon occur when the aircraft is rotated at takeoff. This causes a reaction force from gyroscopic precession largely created by the fan disk (the propeller). This force is a turning force at 90 degrees to the direction of fan rotation and depends on the angle and rate of change of pitch of the aircraft. In simple terms it applies a twisting force on the pylon. Thus, when the propellers on the He 219 are spinning, our visual perception of the wings and engines can be momentarily altered.

Photographs, no matter how good, introduce their own distortions, as anyone who has photographed a tall building with a normal lens can attest. The focal length of the lens, the relative distances between parts of the image, the depth of field, how the photographer holds the camera, and even the degree to which the camera keeps the film flat and perpendicular to the axis of the lens can alter the ‘real’ shape of the object. The eye also introduces corrections/distortions of its own, which complicates the issue still more. It is therefore advisable to be cautious when drawing conclusions from photos alone. If we regard photographs of the He 219 taken from sideways-on, it is virtually impossible to avoid the nose and tail being off-axis and in a different plane relative to the engine nacelles and the wing tip. So it is at least plausible that the apparent, relative alignments in images of the aircraft are not conclusive.

The problem of accuracy extends beyond photography. Accuracy is always relative to the conditions under which observations are made, whether in photography or in historical research. There is no one, reliable source of truth. There are only reasonable judgments. For instance, while the colours in period colour photos do depend in part on colour specifications (FS, RLM, etc.), they also depend on variations in paint manufacture, storage, age/stability, and application methods, on ambient lighting, on weathering, on the time of day, atmospheric conditions, exposure, the chemistry and age of the photographic media, the mechanics of reproduction, shadows, scale effects, etc. So even the best photo cannot prove definitively what a given aircraft actually looked like at a given moment in time. Seen in this light, we can state again that factory drawings should be viewed with caution. Is a given drawing preliminary, corrected, final? Did manufacturing issues force major revisions that were subsequently lost or discarded, leaving the researcher with misleading, superseded sheets? Were the final drawings actually followed in all respects at every factory that built a given aircraft? Or were there locally made changes? Are the surviving drawings originals, copies, or copies of copies? On what media are they preserved? High-quality cloth, paper, or acetate sheets drawn with chemically stable, non-fading inks? Period blueprint sheets? Tracing paper? Microfilm? Photostat? Or are they later reproductions that try to reconcile conflicting, multiple originals? Are they forgeries? And so on. The bottom line is that the best possible accuracy is obtained by making sensible use of all available evidence and by understanding the provenance and history of that evidence as well as one can. There is never a ‘final word’, but that reality is what makes historical research so enduringly interesting.



(1)
(2) www.develop3D.com, 11 Nov. 2014
(3) www.flightliteracy.com
(4) 'Junkers Ju 388: Development, Testing And Production of the Last Junkers High-Altitude Aircraft' - Vernaleken (check page number)
(5) Wikipedia (perhaps get a better source)?
(6) Wikipedia (perhaps get a better source)?
(7) www.airportspotting.com, 6 May 2019
(8) (‘Warbird Tech Martin B-26 Marauder’, p.22 - Frederick A. Johnson)
(9) Wikipedia (perhaps get a better source)?
 

newsdeskdan

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If those are the best sources you can find for the tilted nacelle theory, and you want your book to be a serious and authoritative work on the He 219, I would simply delete that section of text. Period documents show that the thrust line of the nacelles, whatever their external shape, followed that of the fuselage exactly. It is as simple as that. Multiple contemporary documents support that contention - I'm sure I could find more if I tried - and there seem to be absolutely none to oppose it.
The Heinkel 'Uhu' document I posted is from Kew. And why can't you discuss frame 4797019 of NARA Microfilm Publication T321-50 'Betr.: Suggestivnamen fuer neue Waffen' here?

T321-50_1046.jpg
 

richdlc

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Hi - thank you very much for the suggestion. When I initially asked this question it was to gather information aimed at model makers because of the inaccuracy of some kits with regards to the shape of the nacelles and their apparent forwards 'slant' or lack thereof. Then I figured that perhaps this phenomenon might extend to the real aircraft too. My mini essay is therefore pure speculation. I initially wanted to include a section for model makers showing them how to correct erroneous kits. Then I figured, why not try and explain why they are doing it in the first place?

As for the documents you have posted, they are sensational! In fact this changes everything. I wasn't willing to discuss the document in post no.75 (from the NARA) because although we have a copy of the first page, I rather naively thought that no-one else would be privy to it. The problem is that many other authors (and veterans) didn't refer to the He 219 as 'Uhu' as far as we know. Once one of our authors presented us with the NARA document, we were all very excited because we thought that at last we had proof that it was, and under direct order of AH no less. Your copy is much better than ours - in fact, I will PM you about it now. I would also like to discuss the other document you posted. Thank you for posting them!!
 

Zootycoon

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“Applying this to an aircraft, if the engine thrust line is very close to or parallel to a line through the centre of the leading edge and trailing edge of the wing, the plane will neither climb nor dive when power is applied.“

This is incorrect;- Applying power in level flight whilst maintaining a constant AOA will cause the aircraft to climb as the lift on the wing is increasing. So as the power is increased the AOA must be reduced by trimming nose down. Generally the narrative is confusing aircraft trim and rate of climb/descent I.e climb and dive. If the aircraft is nose down and descending it’s in a dive, if nose up and ascending that’s a climb, but nose up or down can also be level flight. Nose up and descent can also be stalled, within wing born flight I’ve never seen nose down and ascending.

“When throttle is reduced in level flight, the down trim will have to be removed or the plane will dive downwards.“

This is not really correct ;-When power is reduced, level flight is maintained at zero descent by applying nose up trim albeit until the potential energy has been expended. At lower airspeed (when Cl max is reached) the aircraft can maintain a level attitude but with an increasing rate of descent.

“If the thrust line is significantly below and parallel to the centre of the leading edge and trailing edge line, the plane will climb under throttle requiring down trim to maintain level flight.“

This is incorrect;- all loads and moments need to be In balance at all times. Therefore thrust produces both loads and moments which need to be balanced by opposite acting loads and moments. These can originate from both drag and mass.

“To correct this, down-thrust or pointing the thrust line downwards and forward will allow the plane to fly level with little or no trim needed to fly level power on or off.“

This is only correct if reducing down-thrust produces a restorative moment about neutral pitch axis. I don’t know where it is on the 219 but all other aircraft where this know this would be incorrect. Conversely if increasing down thrust produces a nose up trim this would cause an excessive amount of trim drag at high speed which is really poor optimisation.

“Applying this logic to the He 219, when taking off the positive angle of the aircraft sitting on its landing gear means that as the plane nears take-off speed, the wing, being at a positive angle of attack, will lift the plane off the ground without elevator input if it is going fast enough. To shorten the takeoff roll, more power can be applied with up elevator to increase the wing's angle of attack and generate sufficient lift to take-off sooner.“

Again, if conventional this would increase the ground roll as the moment is acting against the elevator. Also the suggestion that take off could be achieved without a pitch change (Elevator input increasing the AOA) I recommend you avoid given the way all aero foils generate lift at low speed;- All aircraft shorten the take off by use of elevator.

“Aeroelastic wing structural twist, sometimes called ‘wing jig shape’, is different to washout but is yet another phenomenon which might alter the viewer’s perception of angles when looking at an aircraft.“

May I suggest;-

Wings, in particular those of high aspect ratio are inherently flexible. This means that when in normal flight and bearing the weight of the aeroplane they will be subject to distortions (both bending and twist) This is known as aeroelastic’s which is subject that covers both steady state and dynamic phenomena. A wing will typically have two types of washout;- geometric and aerodynamic(1). Geometric washout is sometimes called wing jig shape as it represents the shape of the unloaded wing as it’s built in the jig. Once removed from the jig it will bend and twist under its own weight. Likewise when it’s generating lift/drag and has masses such as engines installed it will deflect to a different shape. While wing bending is not normally a problem, unintentionally wing twist is highly detrimental. The performance, stability and control of the aircraft are dependent on the aero foil incident being correctly aligned to the on coming air flow. Hence the geometric washout must be such that it’s corrected by flight loaded deflection so as achieve the desired aerodynamic washout.

(1) General Aviation Aircraft Design-Applied Methods and Procedures by S Gudmundson;- Chapter 9 The geometric layout of the wing

I’ll add some more when I have some time.
 
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richdlc

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Wow, thanks mate! I really appreciate your efforts
 

Zootycoon

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many thanks guys.

regarding this image - could it really be so simple? That the answer is that the REAR of the gondola is above the centreline and the gondola isn't tipped forward at all??

View attachment 636330
The thrust line can only be perpendicular to the propeller disc. Hence a line joining the centerline of the prop shaft to the most aft point of the nacelle will be misleading.
 

Artie Bob

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I can only wonder what are the qualifications of some of the responders to this thread, are you engineers or have you any professional flying experience? The best advice was given by newsdeskdan, i.e. simply delete that section! If you have neither the necessary documentation nor the technical expertise to write coherently about the subject, leave it out.
Best Regards,
Artie Bob
 
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