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Multi-Speed Propeller Air-Screws.......

xylstra

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The subject is MULTI-SPEED Propeller Airscrews........ "No,NO,NO!" - I DID NOT mean to say Variable-Pitch nor am I confused or mistaken. "YES!", multi-speed, as in changing gear in a car about to climb a hill.
It might very well be news to you but such propeller reduction drive gear systems were contemplated, designed and actually built and tested. In terms of chronological evolution the timeline can be broken down into two distinct phases: [1] early pioneering avaition beginnings when everything new was eagerly explored, not to mention that the virtues of the V-P Propeller were academically acknowledged and understood but the tangible engineering hardware had still yet to catch-up with the theoretical promise and the much later, more mature [2] late-1930's throught to late-1940's era when dreamers and 'celebrity engineers'' (!) dreamed monsters into existence, i.e. 3,000++Hp engines that had to confront, challenge and overcome the limiting barriers of physics, namely the Mach-limit of propeller-airscrew blade-tips exacerbated by the decreasing Mach speed with corresponding increase of altitude. In respect of the latter it matters not whether engine output is 3,000++Hp...., 30,000++Hp..... or even 30 million++Hp. All irrelevant if you can't then translate it into useable thrust and the only foreseeable solution to the dilemna in those days was resorting to ever larger blade-disc diameters (never mind the undercarriage length)!
I am on a quest, a mission if you will to peel back the layers of obscurity and blow away the swirling mists of time to reveal more of this long-lost and forgotten evolutionary technological dead-end. Someone has to care enough to bother, right? Never mind the fact that it's bloody interesting!
Not a solo effort - can't be done alone, need your help.......
So then, where did it all start? My earliest record (such as it is!) gives the 1st claim-to-fame to the pioneering experiments conducted at McCook Field in 1924 by the aviation research facility operated by the U.S. Army Corp of Signal Engineers ... confused? Unravelling of that mini-mystery: signalling was the domain of the Army's tethered observation balloon detachment. New-fangled airplanes were regarded by the senior Army 'brass' as simply untethered balloons with wings! Nutty logic but one way to shift that awkward nugget from the 'too hard' basket! This link provides a vivid description:
Apparently the 2-speed, V-12 Liberty engine conversion was first conceived in 1922. As an aside, all survivng descriptions of the McCook Field operation convey a wonderful pioneering, 'frontiersman' type of atmosphere, a veritable adults playpen for DIY inventors - "Oh, Halcyon days!!" - before it disbanded and ceased operations in the late 1920's in a hand-off to the newly formed NACA. Unfortunately, the rapid-fire, shoot-from-the-hip approach meant that records-keeping may not have been as good as it could have been or else the beaureacrats who closed the facility were historically short-sighted and cast valuable records and documents into the flames because very little of substance seems to have survived {go on then, prove me wrong; "dare 'ya" (plee-e-ase....)!}.
Don't know about you, but I'm having a lot of trouble believing that any aviation experimenter could look covetously upon the planetary gearbox from a FORD Model T and not see within it the makings of the perfect donor 2-speed propeller reduction gear for a few make-shift improvisations to try their hand at proving it'll work. Did they, or didn't they? Tell me please!
Anyway, I digress. back to the timeline:
Next up, almost simultaneously across the 'pond' the great and famous Arthur J. Rowledge (Oh come off it, surley you know your aero-engine history? Yess-ss, that's right, the guy who penned the design of the NAPIER 'Lion'.... oh God, you mean you've NEVER heard of that either? Have you only just arrived on Earth??) having just had a mutual falling-out with ex-employer NAPIER was hauled into ROLLS-ROYCEs' liferaft and then set about exercising his fixation with 'X' configuration aero-engines - his personal 'stamp' was all over R-R's "X"s. However rather early in the piece - around 1927 - he took a slight detour with a WW1-era V-12 'Eagle' donor to graft on and test a 2-speed propeller reduction gearbox which became the very last Mark XV in the 'Eagle' V-12 series and remained a one-off prototype (his name is on the patent that relates). Third-hand accounts vindicate it's predicted improved flight performance though independent corroboration is missing (and wanted!). Ironically, the 'Eagle's' generation-later H.24 sleeve-valve namesake was also proposed with a 2-speed propeller gearbox but insofar as anyone knows the proposal never progressed to the hardware stage and possibly not even to the drawings stage. Simply, no-one knows though it's solid concept is confirmed by its mention in a HAWKER fighter proposal brochure. These are BIG black holes in an important piece of R-R history that need to be filled - and "Yes", I've already contacted the R-R Heritage Trust - NOTHING found. "Damn!!"
So, can anyone fill in the blanks???
Back across the 'pond' LYCOMING were kicking around ideas for 2-speed reduction gearboxes for the '-5' and '-7' variants of the XH-2470 upper/lower deck H.O. 24-cylinder aero-engine and thence to the late/post-WW2 mighty XR-7755. No detailed information on the gear-boxes (e.g. drawings, pictures, etc) seems to be around. I've also been unable to do any patent searching not least of which because LYCOMING seems to have had a complicated ownership structure so I really have no idea what correct search term to use for the 'assignee' entry-field in ther U.S. Patent database. Can anyone supply this and all other 'missing' information to fill in the gaping holes?
......... and of course, how could anyone over look WRIGHT Aeronautical Corporation? They had an active 2-speed propeller reduction gearbox program relating to some equally innovative aero-engines, e.g. the WRIGHT XR-4090-3. Check-out this patent as but one example of the breed: https://patents.google.com/patent/US2225121?oq=inassignee:Wright+inassignee:aeronautical - ooh, and by the way the inventor, Wilton G Lundquist is another famous 'celebrity' engineer. I won't spoil the surprise of his many credits so I'll leave you to check him out.....
Now that your appetite for the subject has been whetted I want to include yet another interesting development in the field of multi-speed propellers the pinnacle (so far), in fact:
The STUDEBAKER XH-9350 Aero-Engine
...... "The what??" I hear you ask. Bet you've never heard of it and yes, it was intended to have a 2-speed propeller drive. If you haven't yet 'twigged' to what makes it so special, the type code should give you the clue - '9350' is cubic inches of displacement!!
Follow-up here for more information:
https://oldmachinepress.com/2018/11/12/studebakers-xh-9350-and-their-involvement-with-other-aircraft-engines/

So, where are we at? Astonishingly, so far (with the sole exception of one minor early 1930's German patent) the multi-speed propeller development 'landscape' seems to be an entirely British/American show.
"WHAT!!" "How could this be??!"
Where the hell are all the other major aviation nations - the French, the Italians and especially, the Russians?? The laws of physics are universal so the technical challenges of developing ultra-high output aero-engines are equally identical as are the likely technical solutions. Am I to believe that these nations can not, at the very least, have investigated multi-speed propellers? So where are the reports, the patents, drawings, pictures, technical papers, magazine articles, etc, etc??
I need your help! Send me EVERTHING!!
I have no expertise in the realm of formal Patent searching so please, if you are a patent attorney or examiner, but especially in non-U.S. countries where language is higher-tier barrier exacerbating the search difficulties then your assistance and offerings would be most appreciated. I would like to develop technical aviation contacts in these other countries so feel free to contact me - I don't mind broken English ('GOOGLE translate' will hammer the message back into shape!!).
 
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kitnut617

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I've been told the V-22 prop-rotor has 2-speeds, high speed for vertical flight mode, low speed for forward flight mode.
 

xylstra

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I've been told the V-22 prop-rotor has 2-speeds, high speed for vertical flight mode, low speed for forward flight mode.
Hi Kitnut617, I must admit to laughing: changing speed in level fight is one thing, but in VTOL mode? Ouch!! If true, I'd love to read the (posthumous) test-fight reports......
 

AeroFranz

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The V-22 cruise with 80% prop RPM. There is no gearbox, the engine is throttled back is my understanding.
Other tiltrotors, notably Karem's OSTR, have a gearbox with a gear ratio close to 2:1 IIRC.
 

kitnut617

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I'd admit I'm no expert on the subject, it was just something I was told. I did some calculations on prop tip speed on the V-22 and realized that the prop-rotor doesn't rotate very fast, somewhere around 250 rpm when in forward flight. That prop tip speed plus advertised forward speed puts it very close to the speed on sound at sea level. And as I know the V-22 isn't particularly noisy, having watched a flight demonstration at USMC Yuma a few years ago, it can't be going faster than the speed of sound.
 
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Basil

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The 2-speed propeller gearbox developments of the 1940s are interesting concepts to optimize propulsion efficiency for high performance piston engines although - akaik - no such gearbox was ever flown. All major US aero engine companies were active in this field (look google patents), especially Curtiss Wright. Hawker also intended to choose a 2-speed reduction gearbox into the final version of its P.1030 fighter.

Although I understand the intention to reduce propeller rpm at high forward speeds to avoid critical prop tip speeds it is not quite clear to me why you need it if you have a fully variable prop pitch control. Also the fastest turboprop aircrafts like the Tu-95 only have a single speed reduction gear.

However for helicopters a variable rotor speed is highly desirable, especially for future compound helicopters. Technical Universität Vienna (TU Wien) has recently patented a fully variable helicopter gearbox in cooperation with gearbox manufacturer Zoerkler.

 

xylstra

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Nonsense! What way do you have of knowing that 2-speed propeller reduction gearboxes weren't test-flown? Whilst no series-produced commercially sold aircraft, civil or military had them installed it is perfectly reasonable that a re-purposed experimental test mule incorporating a prototype gear-box may very well have flown. We simply just don't know.
.....what we do know (indisputably!) is that 2-speed propeller reduction gearboxes WERE experimentally flown in the 1920's - you clearly have not fully read my post.
Your lack of clarity on the need for multi-speed propellers corresponds to your lack of knowledge of the torque curve of the typical internal combustion engine. Simply throttling-back and coarsening up the pitch just 'ain't gonna get ya there with the blade disc diameters we're talking about.
 

Basil

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Well, we are all glad to have experts like you within this forum.
 

VTOLicious

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The 2-speed propeller gearbox developments of the 1940s are interesting concepts to optimize propulsion efficiency for high performance piston engines although - akaik - no such gearbox was ever flown. All major US aero engine companies were active in this field (look google patents), especially Curtiss Wright. Hawker also intended to choose a 2-speed reduction gearbox into the final version of its P.1030 fighter.

Although I understand the intention to reduce propeller rpm at high forward speeds to avoid critical prop tip speeds it is not quite clear to me why you need it if you have a fully variable prop pitch control. Also the fastest turboprop aircrafts like the Tu-95 only have a single speed reduction gear.

However for helicopters a variable rotor speed is highly desirable, especially for future compound helicopters. Technical Universität Vienna (TU Wien) has recently patented a fully variable helicopter gearbox in cooperation with gearbox manufacturer Zoerkler.

The abandoned Boeing A160 Hummingbird R-UAS had a two-speed-transmission as well
 

Viper2000

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The V-22 cruise with 80% prop RPM. There is no gearbox, the engine is throttled back is my understanding.
Other tiltrotors, notably Karem's OSTR, have a gearbox with a gear ratio close to 2:1 IIRC.
T406 has a free power turbine, so I assume that the rotor RPM is reduced by increasing blade angle whilst the gas generator tracks a rating.

These papers may be of interest:



https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060006384.pdf (background, but gives a really nice overview of where you can go with something rather like a T406).

Where the hell are all the other major aviation nations - the French, the Italians and especially, the Russians?? The laws of physics are universal so the technical challenges of developing ultra-high output aero-engines are equally identical as are the likely technical solutions. Am I to believe that these nations can not, at the very least, have investigated multi-speed propellers? So where are the reports, the patents, drawings, pictures, technical papers, magazine articles, etc, etc??
The reason that this field was dominated by American & British inventors was fuel quality. You need high octane or performance number fuel to run high boost at low RPM and unlock any benefit from this technology.

The argument in favour of a variable speed reduction gear is that you can slow the engine down to reduce friction losses whilst keeping the propeller spinning fast enough to maintain a sensible advance ratio. This is more efficient than throttling back because it removes pumping losses.

This argument was rendered obsolete by the pressure cabin, which allows the aeroplane to climb at maximum continuous power and fly faster, especially when combined with the supercharger & the variable pitch propeller (which is far less mechanically challenging).

It has nothing whatever to do with tip MN, which really doesn't make much difference.

Modern interest in variable speed rotors for tilt rotors is driven by a desire to reduce profile losses because the blade lift coefficient is two tenths of damn all in the cruise (the rotors are sized for hover; if hover efficiency is no object, their minimum diameter is set by (aircraft) pitch control requirements.

In the GT world, single shaft turboprops like to spin fast to maintain OPR, so I can see that a variable reduction gear would be attractive if cruise power is small, but again, if you have a pressure cabin it's much easier to climb to lapse the power away.
 

Basil

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If you look into the patent papers of the 1940s where the two-speed transmissions for high performance piston engine powered aircrafts were conceived you will notice that for high speed cruise the prop would turn in the slower gear for a given engine rpm. For take-off and climbing when flight speed was slow the prop would turn in the high prop speed gear. So - in contrary to an overdrive in a car - it was not intended for reducing engine rpm but to prevent reaching mach 1 prop tip speed by the combined vector of high forward speed flight speed and propeller rpm. Avoiding mach number tip speed was very much the driving argument for the two speed prop gear box development.
See for example Rudy Daub 's patents for Curtiss-Wright.
 

Basil

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Sorry, the second patent was already mentioned by Xylstra.
 

Maiwand1880

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The 1936 MAI Sokol VTO fighter project by F.N. Kurochkin had a two speed main gearbox so that the two propellers (driven by a single Hispano-Suiza 12Ybrs) would run at different, optimised rotational speeds, one for VTO operations and the other one for horizontal flight.
http://www.rp-one.net/sokol_vtol/sokol_vtol.html
and also on this forum.


MAI Sokol.jpg
 
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