Propeller Design

[MIflyer]

From Aviation Magazine, April 1942.

 

[PFVA63]

Thanks for sharing

 

[MiTasol]

Excellent material for the next time prop design becomes a forum topic.

Thank you.

 

[xylstra]

From Aviation Magazine, April 1942.
View attachment 604986View attachment 604987View attachment 604985

.... but nothing on 2-speed propellers!!

 

[MiTasol]

At that stage the in the USA two pitch was pretty much ancient history so not applicable to the article.

 

[xylstra]

At that stage the in the USA two pitch was pretty much ancient history so not applicable to the article.

No, no, NO MiTasol! You didn't read what I said: 2 - SPEED, NOT Pitch, i.e. changing gears as with a car.

 

[Michael Hope]

No, no, NO MiTasol! You didn't read what I said: 2 - SPEED, NOT Pitch, i.e. changing gears as with a car.

The blade pitch change is how the propeller changed the engine load to change the engine RPM, like changing gears. The Two position propeller had a a low pitch setting and a high pitch setting. In the Hamilton Standard counterweight type propeller, you used oil pressure for low pitch, high RPM, and then for high pitch, lower RPM, no oil pressure, and counterweights.

 

[MiTasol]

No, no, NO MiTasol! You didn't read what I said: 2 - SPEED, NOT Pitch, i.e. changing gears as with a car.

I have been working in the industry since 1962 and the only two SPEED propellers I have ever worked on (and heard of) are those fitted to the Allison 501/T-56 series engines. Those are only two speed because the engine has a ground idle of 10,300 rpm and runs at 13,820 rpm (1,020 prop rpm) for absolutely everything else from taxi to take-off. As Michael Hope says - everything else is obtained by varying the blade angle.

The only aviation engines I know of with propeller gearboxes that relate to car gearboxes are some of the airship engines that had a gearbox that provided forward and reverse. They did not have high and low gear and the reverse thrust was only a percentage of the forward thrust as the blade aerodynamics were "backward". Some airship engines could run in either direction and had to be stopped and restarted "in reverse" to provide reverse thrust. One British airship (the R-101) carried five engines - four providing forward thrust and one that was along for the ride and was only started if reverse thrust was required.

 

[swampyankee]

If I remember (my reading!), there were a couple of aircraft from the 1930s which were trialed with two-speed gearboxes; the gearbox provided no benefit, especially once variable-pitch (and constant-speed) propellers were developed. Despite this, the Lycoming XR-7755 was fitted with a two-speed gearbox (and each camshaft had two sets of lobes, which could be switched in flight, with one set for economy and the other for power).

 

[MiTasol]

Thanks, SwampYankee.

I was not aware of those engines but then again, apart from the Napier Nomad, I tend to ignore experimental engines.

I suspect that Lyc would have been a nightmare to maintain and I doubt the benefits compensated for the weight gain and maintenance issues it added.

 

[xylstra]

I have been working in the industry since 1962 and the only two SPEED propellers I have ever worked on (and heard of) are those fitted to the Allison 501/T-56 series engines. Those are only two speed because the engine has a ground idle of 10,300 rpm and runs at 13,820 rpm (1,020 prop rpm) for absolutely everything else from taxi to take-off. As Michael Hope says - everything else is obtained by varying the blade angle.

The only aviation engines I know of with propeller gearboxes that relate to car gearboxes are some of the airship engines that had a gearbox that provided forward and reverse. They did not have high and low gear and the reverse thrust was only a percentage of the forward thrust as the blade aerodynamics were "backward". Some airship engines could run in either direction and had to be stopped and restarted "in reverse" to provide reverse thrust. One British airship (the R-101) carried five engines - four providing forward thrust and one that was along for the ride and was only started if reverse thrust was required.

Then you both don't know as much as you think you do: See my separate post on the subject. Multi-speed (NOT pitch!) Propellers are a specialist technical research topic of mine. Such 2-speed (yes, that's right speed!) Reduction gear-boxes were developed for such aero-engines as LYCOMING, culminating in the XR7755, the STUDEBAKER XH-9350, also developed by WRIGHT and that's only the U.S. contingent. Over in England there was ROLLS ROYCE in the 1920's and Armstrong Siddeley also had a hand in it.
Back to school for you two!

 

[MiTasol]

But none of those ever made production did they.
That shows they were not viable options.

 

[xylstra]

But none of those ever made production did they.
That shows they were not viable options.

It also shows that you haven't studied the topic. Like I said, "back to school...!" 2 + 2 does not equal 5!
Your assumption that no production = failure is erroneous. The development of multi-speed propellers was centred on military applications. Program cancellations leading to cessation of work had nothing whatsoever to do with any failings of the change-speed propeller. None!

 

[Michael Hope]

It also shows that you haven't studied the topic. Like I said, "back to school...!" 2 + 2 does not equal 5!
Your assumption that no production = failure is erroneous. The development of multi-speed propellers was centred on military applications. Program cancellations leading to cessation of work had nothing whatsoever to do with any failings of the change-speed propeller. None!

What is the purpose of having a propeller, I assume that what you are describing would that have the blade pitch set at only one angle, that had a gear to drive at it at two different RPMs without the blade pitch changing?

A single pitch, i.e. fixed pitch/ground adjustable, is only efficient at one RPM, any other RPM is just a comprise of efficiency.

Can you explain in more detail if the propeller has a gear that makes the blades change pitch, if so then that is a variable pitch propeller.

Can you please state if the blade pitch does or does not change in your example.

 

[swampyankee]

Even with variable pitch, engines have a curve of rpm vs power where efficiency is best. Overall, though the complication of a multi-speed gearbox isn't worth it. One does wonder about the combination of a fixed-pitch propeller and a CVT....

 

[nuuumannn]

but nothing on 2-speed propellers!!

What's the point when you have a constant speed unit where the pitch is regulated based on the engine RPM? You set the condition lever/s to Max and put the power lever/s to where you need them to be and the props do as they should; why faff around with gears? Just added and unnecessary complication.

 

[Shortround6]

There were several reasons for two speed propellers, even with variable pitch. Yes, the US military was interested for some of the experimental engines of the late 30s and end of the war.

Problems include clutches or speed changing mechanisms. The weight of even a single gear set that can handle 1000lb (let alone the monster engines of mid to late war) and the resultant weight of a two speed drive may have been part of what doomed the concept. The Lycoming H-2470-5 with two speed prop drive managed to be about 300lbs heavier than the single speed prop drive H-2470-1. The H-2470-7 managed to add counter rotating propeller shafts and keep the two speeds for only another 125lbs.

Somebody may have thought there was an advantage in cruise flight???
Perhaps an advantage when operating at different altitudes?

However unless the advantages actually outweigh the problems (will the better cruise outweigh the 300lbs or better of extra weight per engine or would 50 gals of extra fuel per engine give about the same range? Higher initial cost, higher overhaul cost.
Were the prop makers able to introduce new blade designs that offered similar performance with less complication.

H-2470 engine with two speed drive AND counter rotating propellers.

 

[MiTasol]

The Lycoming H-2470-5 with two speed prop drive managed to be about 300lbs heavier than the single speed prop drive H-2470-1. The H-2470-7 managed to add counter rotating propeller shafts and keep the two speeds for only another 125lbs.

What was the percentage weight increase of the -5 over the -1? That is probably a better indication of the weight penalty as 300 lb on a 3,000 lb engine is ten percent which is serious enough but on a 2,000 lb engine it is 15% which would be almost impossible to justify unless there is a massive fuel saving. Were there even any projected fuel savings? If no that is a minimum of 1,200 lb of payload lost on a four engine aircraft if there is no additional weight penalty from additional oil cooling and the additional structure to carry the heavier engine. On a single seater there is a lot more mass that must be forced to change direction when maneuvering.

I note wiki says the engine weighed 2,430 lb and produced 2,300 hp but not which version that is (and those figures may well be for two different versions). If the numbers are for the -1 that was already an overweight engine compared to the other engines coming on line. Most Napier Sabre versions were under 1 lb/hp and according to wiki the Lycoming is already carrying a 130 lb penalty.

Add to that the structure to carry that extra 300lb of engine sounds minimal but if the aircraft is stressed to just 7.5g that means supporting an additional 2,250lb before getting into the other structural factors.

 

[xylstra]

What is the purpose of having a propeller, I assume that what you are describing would that have the blade pitch set at only one angle, that had a gear to drive at it at two different RPMs without the blade pitch changing?

A single pitch, i.e. fixed pitch/ground adjustable, is only efficient at one RPM, any other RPM is just a comprise of efficiency.

Can you explain in more detail if the propeller has a gear that makes the blades change pitch, if so then that is a variable pitch propeller.

Can you please state if the blade pitch does or does not change in your example.

You are EXACTLY right! The propeller is only at its maximum efficiency at one one speed for a given pitch. There are three modes for which a change-speed gear-box is desired but the one case that generated the most intensive development activity concerned the USAAF's determination at the close of the 1930's that a bespoke, ultra-high output (5,000-8,000Hp) 'bomber engine' was required for super-heavy, long (very long!) range bombers and transports. These engines would typically have low specific outputs due to their huge displacements so they required maximum thrust for take-off and climb-out but once settled in at cruise altitude the power gear-ratio was simply too high to maintain propeller rpm's once the engine was throttled back to lowest sfc point on the rpm range so a second optimal gear ratio was necessary (torque to spare!). Blade pitch change was still available on both ratios. gear-box fabrication was undertaken by Merz Engineering, Ohio. For further details, consult GOOGLE patents, Wright's designs at least, are there.

 

[xylstra]

There were several reasons for two speed propellers, even with variable pitch. Yes, the US military was interested for some of the experimental engines of the late 30s and end of the war.

Problems include clutches or speed changing mechanisms. The weight of even a single gear set that can handle 1000lb (let alone the monster engines of mid to late war) and the resultant weight of a two speed drive may have been part of what doomed the concept. The Lycoming H-2470-5 with two speed prop drive managed to be about 300lbs heavier than the single speed prop drive H-2470-1. The H-2470-7 managed to add counter rotating propeller shafts and keep the two speeds for only another 125lbs.

Somebody may have thought there was an advantage in cruise flight???
Perhaps an advantage when operating at different altitudes?

However unless the advantages actually outweigh the problems (will the better cruise outweigh the 300lbs or better of extra weight per engine or would 50 gals of extra fuel per engine give about the same range? Higher initial cost, higher overhaul cost.
Were the prop makers able to introduce new blade designs that offered similar performance with less complication.
View attachment 605794
H-2470 engine with two speed drive AND counter rotating propellers.

ROLLS ROYCE also investigated a two-speed propeler drive for the 'EAGLE' H-24 sleeve-valve but not sure how far the design work progressed, definitely no hardware though.