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

Foo Fighter

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A little harsh imho, perhaps respect other people trying and TALKING about these topics. It's what keeps us coming back.
 

dan_inbox

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A little harsh imho, perhaps respect other people trying and TALKING about these topics. It's what keeps us coming back.
Trying is OK, _if_ it is described as such.
When it is not, there is precious little difference with someone talking out of his @ss or simply generating false information. Which is not OK.
Unfortunately, it is not always described as such.

Not directed at you in particular, just supporting Artie Bob's call.
 

Foo Fighter

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Thanks Dan. Fair enuffski, no worries. Not trying to pick holes in anyone either, Artie, sorry if I came across like that.
 

edwest

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It is alright to include speculation and tentative conclusions. As a researcher with 40 years of experience, conclusions, tentative or not, are good only at the time of publication. More documents may come to light, as is the case here. I will also add that I've read 'historical' articles online that are distorted, incomplete and biased. And some without the necessary statements regarding any speculation. Trophy brigades (Трофейные бригады) was the Russian name for their exploitation teams.

I have a background in photography and perspective drawing. The following example is meant to be simplified. Say you have an aircraft that is exactly 30 feet long. Let's give it a fuselage height of 8 feet. Now imagine the fuselage, minus landing gear, suspended above the ground at the same height as your eyes, with your eyes at the middle of the 8 feet. You are standing at the exact middle lengthwise at 15 feet. You cannot stand too close since part of the aircraft will be outside your field of view. So you back up until you can see all of it.

Now, take a picture. If you could enclose the floating fuselage in a rectangular box that fits it exactly while being open on your side, you will see the upper left and right sides tilt downward toward the back. The lower left and right sides tilt upward. On a fuselage with a circular cross section, this will be difficult to see. It would be more noticeable in the tail assembly. That describes the distortion in photos taken from the side and assumes at least one meets the requirements I've given.

This is called one point perspective. It can be likened to standing in the middle of a straight railroad track that goes to the horizon. You know the track is straight but both of the rails converge in the distance.
 

iverson

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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??
I'd say so. The shape of the nacelle--almost a tear drop but up-swept at the rear--invites the impression of tilt, particularly if perspective distorts the spinner, the propeller blades, and ends of the nacelle.
 

iverson

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It is alright to include speculation and tentative conclusions. As a researcher with 40 years of experience, conclusions, tentative or not, are good only at the time of publication. More documents may come to light, as is the case here. I will also add that I've read 'historical' articles online that are distorted, incomplete and biased. And some without the necessary statements regarding any speculation. Trophy brigades (Трофейные бригады) was the Russian name for their exploitation teams.
...
This is called one point perspective. It can be likened to standing in the middle of a straight railroad track that goes to the horizon. You know the track is straight but both of the rails converge in the distance.
This brings us back to the nature of research.

We hear a lot of different views on authoritative sources, most of which are correct up to a point--as long as they don't get doctrinaire. For example, historians tend to view contemporary documents as authoritative, while scientists are likely to give more credence to physical artifacts--photographs, components, or even eye-witness observations. Nowadays, neither give much credence to secondary sources. Yet neither approach guarantees authenticity. Original documents may be preliminary, incorrect, misleadingly reproduced, incomplete, or outright false. And as we have seen, photographs can misleading due to perspective, color sensitivity of media, etc. On the other hand, secondary sources may be based on primary sources that are not presently available (almost all scriptural and classical literary/historical research is based on such sources). So, in some cases, the much derided secondary source is MORE reliable than all the available primaries put together. The bottom line is that no single source or set of sources is authoritative except relative to other available sources, critically examined.

Critical analysis is thus as important as sourcing. At a minimum, no source can be true unless what it appears to show is compatible with logic. This and several other of the more interesting above posts depend on logical assessment of the photographic evidence in light of known physical and mechanical principles. News Desk Dan's posts are convincing because they relate documents at different points in the airplane's design history and show that an angled thrust line is logically unlikely. None of them in and of itself proves anything about the "real," as-built airplane.

Finally, there was a question about qualification above. In my view, this is the least compelling support for an argument. Taken to extremes, reliance on particular qualifications can be highly misleading. Just look up the latest retractions of scientific papers published by highly qualified persons at prestigious institutions. In reality, the main value of qualification in anything is that it provides methodical training in logic and research methods (I've made my living for many years in software and computer development on the basis of degrees in Medieval European literature). So, for best results, on the evidence presented, not on the presenter.

Given the above, I still say that the mere fact that a statement is wrong, whether poorly researched or not, is no justification for disdain. I'm not pointing at any individual by any means. But I have noticed a growing tendency to intolerance in several topics of late. Tolerance for ignorance is the mark of a real expert. Intolerance suggests insecurity not authority. So my advice is to educate patiently. Stress what you don't know whenever you assert something that you do. Don't condemn. Because once questions bring condemnation, no will want to ask them.
 

edwest

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Well said. My own research work relies on primary and secondary sources, photographs and other contemporary documents. Then yes, analysis occurs. Regarding German aircraft, some captured film footage survives of the particular aircraft in flight as is. I won't go into any design changes that may have occurred. No doubt all of the necessary adjustments were made before delivery.

People tend to follow those with qualifications. I've read scientific papers and watched some retracted for improper procedure and other mistakes. Do some correct their errors and try again? Yes. Others are simply discredited and are not heard from again.

Some training in logic should be there but much fine research occurred in the past by people who presented their case well, but who appeared to have worked it out without a degree or specialized training. No one can know what training an author has, and a list of degrees may not mean much when confronted with a particular problem. Without any training in aircraft design, and the reasons why aircraft look as they do, the researcher should contact others who do have a working knowledge to assist him. Or educate himself.

Condemning others is common on the internet. Add some profanity and perhaps the offender will stay quiet or withdraw. It's quick. It's easy. I mean, who wants to waste their time? It takes time to tell the other person why he is wrong. Showing disdain for some idiot? Sure. Why not? Don't patiently correct him or show him why you are right or think you're right.
 

Justo Miranda

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It is alright to include speculation and tentative conclusions. As a researcher with 40 years of experience, conclusions, tentative or not, are good only at the time of publication. More documents may come to light, as is the case here. I will also add that I've read 'historical' articles online that are distorted, incomplete and biased. And some without the necessary statements regarding any speculation. Trophy brigades (Трофейные бригады) was the Russian name for their exploitation teams.

I have a background in photography and perspective drawing. The following example is meant to be simplified. Say you have an aircraft that is exactly 30 feet long. Let's give it a fuselage height of 8 feet. Now imagine the fuselage, minus landing gear, suspended above the ground at the same height as your eyes, with your eyes at the middle of the 8 feet. You are standing at the exact middle lengthwise at 15 feet. You cannot stand too close since part of the aircraft will be outside your field of view. So you back up until you can see all of it.

Now, take a picture. If you could enclose the floating fuselage in a rectangular box that fits it exactly while being open on your side, you will see the upper left and right sides tilt downward toward the back. The lower left and right sides tilt upward. On a fuselage with a circular cross section, this will be difficult to see. It would be more noticeable in the tail assembly. That describes the distortion in photos taken from the side and assumes at least one meets the requirements I've given.

This is called one point perspective. It can be likened to standing in the middle of a straight railroad track that goes to the horizon. You know the track is straight but both of the rails converge in the distance.
Optical distortions?
 

sienar

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Every camera lens suffers from geometric distortion to some degree. Focal length causes perspective distortion if its not a "normal" focal length. Still its possibly to make a very basic estimate from photos...
219 nac 2.png 219 nac.png
 

edwest

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Yes, optical distortions. A camera lens mimics the human eye which takes in an image and 'knows' it looks right. We just accept things, especially large objects, as looking right from childhood. Try the following:

Take an overhead drawing (plan view) of any aircraft. The drawing shows the aircraft as if the viewer were high above it. Every line drawn through the aircraft will be parallel and true. Now take that drawing and tilt it 45%. Draw straight parallel lines from left to right across the aircraft. Every line on the aircraft is now at an angle to the ground.

The same occurs when a 3D object rotates in the air. Take a large model you can hold. Make sure you hold it at eye level and turn it around. It's apparent shape changes as it turns in space.

For those who wish to look at the mechanics of perspective drawing, which must be used by artists to render their beautiful and accurate drawings, go here:

The camera lens avoids all of the drawing. In fact, shortly after the camera was introduced, painters were told to give up painting. The camera did a better and quicker job. And was cheaper.
 

iverson

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Yes, optical distortions. A camera lens mimics the human eye which takes in an image and 'knows' it looks right. We just accept things, especially large objects, as looking right from childhood...
On the other hand, such distortions, when understood, became the basis for interpreting aerial photographs and converting images into intelligence. A knowledge of perspective combined with geometry and optics lets you determine the dimensions of objects, etc. Seen in this light, the common split-vertical arrangement of reconnaissance cameras deliberately creates distorted--stereoscopic--images and thereby extracts more information than could be had with a single, normal image.
 

edwest

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Yes, reconnaissance cameras do this. The Germans were aware of this. The accurate measuring of shadows was important. I have seen a photo series where a truck filled with fake aircraft parts goes to a fake airfield. A fighter is assembled and even wheels could be dispensed with. Struts were used to make sure this aircraft, which had the correct dimensions, was the correct height off the ground.
 

AeroFranz

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Rich, i apologize if i missed something (i shamelessly only skimmed your writeup), but in your discussion of flight trim i did not see much about the elevator, which is after all the main effector for trim.
Because of changing weight, speed, and power setting, it's nigh impossible to trim an aircraft longitudinally for all flight phases. Designers rely on the elevator to make the necessary adjustments. The rigging of the flying surfaces, from a designer's point of view, can be considered optimum when you minimize trim drag over the course of the flight conditions you care about the most.
Say you're a reconnaissance aircraft; then you'd probably want your trim drag to be low somewhere around the mid-point of the mission while flying at your loiter speed, because that's where you spend lots of time. You can the minimize your fuel burn and design the smallest possible vehicle that still fulfills the mission. A fighter or a dive bomber would face different requirements and different riggings.
But even this is position, which makes sense to me, might fail for a number of reasons, some hard to guess...you mentioned some of these, things like the weight increases, installed power changes over the initial planned ones..or maybe it's something that has nothing to do with performance or aerodynamics but relates to ease of manufacturing. These things, if they don't cause too much performance penalty, are better handled with using the elevator for trim and not go through with extensive redesign that might disrupt production.
All this to say that unless you have primary sources telling you why the rigging was done a certain way, and this does not conform to a typical set up, you can only speculate as to the why. Oh, i forgot, engineers screw up a lot too! even translating wind tunnel tests to full scale does not guarantee success!
 

edwest

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There is a published photo of a full-size Bf 109 suspended in front of a wind tunnel. Engineers cannot make too many mistakes or the aircraft will fall to enemy action.
 

edwest

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PB 39170. Scholkemeier and Röber. Kraftmessungen und rudermomentenmessungen an einen original-leitwerk der He 219 mit rumpfstummel (Force and control surface moment measurements on an original He 219 control surface unit with fuselage stump). (Deutsche Luftfahrtforschung UM 2141) Nov 1944. 30 p[ages].

This is a report of the "Luftfahrtforschungsanstalt Hermann Göring - Institut für Aerodynamik, Braunschweig." The influence of diagonal flow on lift and control surface moment of the elevator unit is of minor nature. The same applies to influences of angles of side slip and the lateral rudder deflection on the elevator float angle. The auxiliary lift flap with a greater trailing edge angle proved to be more efficient than a flap with out trailing edge. Very little difference was noted in the effect achieved by the four lateral control surfaces investigated except that the rudder float angle was found to be greatly dependent on the pitch angle of the elevator unit in the case of the control surface with overhauling balance. Data tables, graphs, diagrams, and photographs are included. In German.

--

The above was published in Bibliography of Scientific and Industrial Reports, Volume 4, Issue 11. Published by U.S. Office of Commerce, Office of the Publication Board, 1947.

At the time, a microfilm copy or photostat of the report could be purchased.
 

AeroFranz

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" The influence of diagonal flow on lift and control surface moment of the elevator unit is of minor nature. "

what they're describing here is spanwise flow, which is the correct English term. Must be a translation thing, but i think this is what they're talking about.
Spanwise flow is either caused by sideslip, wing sweep (not our case), or the presence of something that alters the pressure distribution away from what an "infinite wing" would see. The latter case includes immersing the wing in the propeller slipstream, or near the wingtip where the wingtip vortex induces some side components. I suppose the presence of bodies like a nacelle can also change pressure distribution locally and thus flow direction.

Anyway, if you put the airplane in a crab, the air no longer runs parallel to the longitudinal axis of the fuselage. They're saying the effect on elevator effectiveness is small. This is not surprising; i am no pilot, but how much sideslip can you put on an aircraft like that? 15 degrees? You find yourself in a situation where the elevator hinge axis is not perpendicular to the airstream. The cosine of 15 degrees is 0.96...so you would expect that most forces remain the same.
My interpretation is that they are not discussing the absolute effectiveness of the elevator in general; rather, they are looking at the effects of normal amounts of yaw on the elevator authority, and finding that these effects are small.
 

AeroFranz

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There is a published photo of a full-size Bf 109 suspended in front of a wind tunnel. Engineers cannot make too many mistakes or the aircraft will fall to enemy action.
You are giving engineers too much credit! (source: i am an aeronautical engineer) :D
I wish this wasn't the case, but even with the best of intentions and lots of work, they make plenty of mistakes. Engineering is hard! Otherwise the history of aviation wouldn't be littered with cases of failure that was preventable in hindsight.
 

TomcatViP

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" The influence of diagonal flow on lift and control surface moment of the elevator unit is of minor nature. "

what they're describing here is spanwise flow, which is the correct English term. Must be a translation thing, but i think this is what they're talking about.
Spanwise flow is either caused by sideslip, wing sweep (not our case), or the presence of something that alters the pressure distribution away from what an "infinite wing" would see. The latter case includes immersing the wing in the propeller slipstream, or near the wingtip where the wingtip vortex induces some side components. I suppose the presence of bodies like a nacelle can also change pressure distribution locally and thus flow direction.

Anyway, if you put the airplane in a crab, the air no longer runs parallel to the longitudinal axis of the fuselage. They're saying the effect on elevator effectiveness is small. This is not surprising; i am no pilot, but how much sideslip can you put on an aircraft like that? 15 degrees? You find yourself in a situation where the elevator hinge axis is not perpendicular to the airstream. The cosine of 15 degrees is 0.96...so you would expect that most forces remain the same.
My interpretation is that they are not discussing the absolute effectiveness of the elevator in general; rather, they are looking at the effects of normal amounts of yaw on the elevator authority, and finding that these effects are small.
Verticals do stall. Just like a wing. Side slip is restricted by stall like any out of straight flight escape (although other aero effect my interact before).

At high speed for example (supersonic), aerodynamic loads might overstress structural integrity (read opening post in latest B-58 thread) at an hard to predict low beta (sideslip angle).
 

AeroFranz

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Verticals do stall. Just like a wing. Side slip is restricted by stall like any out of straight flight escape (although other aero effect my interact before).

At high speed for example (supersonic), aerodynamic loads might overstress structural integrity (read opening post in latest B-58 thread) at an hard to predict low beta (sideslip angle).
i feel like there's a confusion about what we're describing here. Flying surfaces stall if they exceed a certain angle of attack, which is measured between the incoming airflow and the chordline of the surface. The angle the German report was talking about (at least in my interpretation), was the in-plane angle formed between any of the chord lines of the surface and the incoming airflow. You are not changing the angle of attack of the surface. I'm attaching a picture, imagine that's a horizontal tail with an elevator at zero alfa, or a wing with ailerons or flaps. Left is normal flight, right you introduce a bit of in plane angle.
What the report is saying is that the force you can generate with the movable surface is not much diminished if you're in yawed flow. Nor should you expect there to be a whole lot; lots of planes have swept hinge lines on their control surfaces and they work fine within reasonable angles. yawed flow.JPG
 

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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)
There are quite a few basic errors in this paragraph, that if I’d bought the book would lead me to question everything!

Cl/Cd is the Lift to Drag ratio. If you were plot its curve, you would have the drag polar, which would link to the angle of attack. This is a key aerodynamic efficiency factor for maximising either range or endurance.

Power is definitely not equal to drag to maintain airspeed. Drag is a force (SI unit of Newtons) and is equal to Thrust.
 
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