I think I know the answer to this, and it has to do with cost, complexity and weight. However, a contrarotating propeller setup on a single-engine GA aircraft would it much easier to fly. Anyone who's driven dual-prop and single prop boats knows what I'm talking about. As a bonus, it would save CFI's from saying "more right rudder" 50 times during every flight lesson.

I'm befuddled. How did you put single-engine and contra-rotating in the same sentence like that. If you flipped it the other way, would you then not need more left rudder instead of right. Or am I just stupid.

AirKevin wrote:

I'm befuddled. How did you put single-engine and contra-rotating in the same sentence like that. If you flipped it the other way, would you then not need more left rudder instead of right. Or am I just stupid.

I think the OP is wondering why he can't turn his Cessna 172 into a Seafire Mk. 47:



One engine, two props rotating opposite directions.

AirKevin wrote:

I'm befuddled. How did you put single-engine and contra-rotating in the same sentence like that. If you flipped it the other way, would you then not need more left rudder instead of right. Or am I just stupid.

I think he is asking about coaxial contrarotating propellers:

https://en.wikipedia.org/wiki/Contra-ro ... propellers

In essence, he answers his own question. The complexity is unwarranted, especially in single engine trainers where simplicity is king. Maybe as you get into higher performance single engine aircraft, there could be a case for using one, but it would probably be easier and cheaper just to jump up to a dual engine aircraft, so there is no real development case for one, especially in the GA market.

adipasqu wrote:

AirKevin wrote:

I'm befuddled. How did you put single-engine and contra-rotating in the same sentence like that. If you flipped it the other way, would you then not need more left rudder instead of right. Or am I just stupid.

I think he is asking about coaxial contrarotating propellers:

https://en.wikipedia.org/wiki/Contra-ro ... propellers

In essence, he answers his own question. The complexity is unwarranted, especially in single engine trainers where simplicity is king. Maybe as you get into higher performance single engine aircraft, there could be a case for using one, but it would probably be easier and cheaper just to jump up to a dual engine aircraft, so there is no real development case for one, especially in the GA market.

Copy. Didn't know such a configuration even existed. Thanks.

Weight is a huge factor, as is complexity. Even now, you may trade cruise speed for noise, climb and smoothness when you add blades, let alone more moving parts. Adding a prop will remove at least 2 gallons of gas or other weight, and that is before you compensate for all the added weight for the mechanisms that keep them in place.

Oh, and a well rigged airplane will always require less rudder than something that had a hatchet job done.

Out of curiosity, how much does the proximity of the two propellers in the coaxial configuration affect performance? I imagine there would be a fair amount of turbulent air around both props. Or maybe just the aft prop. Or maybe not?

For a single engine aircraft, reliability of the engine is paramount. To add a counter rotating prop gives it new failure modes that didn't exist before, and could only be balanced by having multiple engines. That requirement might be suspended for military or stunt aircraft, but not for civil.

Welllll...if you had a conveyor belt looong enough.....

Zeke2517 wrote:

Out of curiosity, how much does the proximity of the two propellers in the coaxial configuration affect performance? I imagine there would be a fair amount of turbulent air around both props. Or maybe just the aft prop. Or maybe not?

There is of course an effect on the rear prop. But net-net performance is improved. As engine power increases, you need larger props. This eventually leads to tip speed issues, not to mention ground clearance issues. Two smaller props mitigate this issue. With two props you can convert more engine power into thrust, as it were.

The Antonov An-70 and Tupolev Tu-95 are examples of coaxial prop turboprops. At a guess, a LOT of work was done to optimise the scimitar blades on the An-70.





The Fairey Gannet actually had two turboprop engines integrated into the "Double Mamba", each driving one coaxial propeller. Because the British have to be different.

One engine could be stopped in flight to conserve unit, with the associated prop also stopping. IIRC during long missions the engines would "take turns", with each used for a few hours during the cruise.

Starlionblue wrote:

Zeke2517 wrote:

Out of curiosity, how much does the proximity of the two propellers in the coaxial configuration affect performance? I imagine there would be a fair amount of turbulent air around both props. Or maybe just the aft prop. Or maybe not?

There is of course an effect on the rear prop. But net-net performance is improved. As engine power increases, you need larger props. This eventually leads to tip speed issues, not to mention ground clearance issues. Two smaller props mitigate this issue. With two props you can convert more engine power into thrust, as it were.

The Antonov An-70 and Tupolev Tu-95 are examples of coaxial prop turboprops. At a guess, a LOT of work was done to optimise the scimitar blades on the An-70.





The Fairey Gannet actually had two turboprop engines integrated into the "Double Mamba", each driving one coaxial propeller. Because the British have to be different.

One engine could be stopped in flight to conserve unit, with the associated prop also stopping. IIRC during long missions the engines would "take turns", with each used for a few hours during the cruise.

Interesting info, and thanks! Have there been any examples of attempts to use the coaxial prop setup with two different propellers to maximize thrust? Obviously if it isn’t used commercially it’s not economically viable but I wonder about the attempts.

Zeke2517 wrote:

Starlionblue wrote:

Zeke2517 wrote:

Out of curiosity, how much does the proximity of the two propellers in the coaxial configuration affect performance? I imagine there would be a fair amount of turbulent air around both props. Or maybe just the aft prop. Or maybe not?

There is of course an effect on the rear prop. But net-net performance is improved. As engine power increases, you need larger props. This eventually leads to tip speed issues, not to mention ground clearance issues. Two smaller props mitigate this issue. With two props you can convert more engine power into thrust, as it were.

The Antonov An-70 and Tupolev Tu-95 are examples of coaxial prop turboprops. At a guess, a LOT of work was done to optimise the scimitar blades on the An-70.





The Fairey Gannet actually had two turboprop engines integrated into the "Double Mamba", each driving one coaxial propeller. Because the British have to be different.

One engine could be stopped in flight to conserve unit, with the associated prop also stopping. IIRC during long missions the engines would "take turns", with each used for a few hours during the cruise.

Interesting info, and thanks! Have there been any examples of attempts to use the coaxial prop setup with two different propellers to maximize thrust? Obviously if it isn’t used commercially it’s not economically viable but I wonder about the attempts.

What do you mean by "two different propellers"?

If I remember correctly from my military aero-d lectures nearly 30 years ago, during the 30's and 40's, there was a limit to the number of blades that could be used in a prop. This was due to aerodynamic interference. Once the engine power got past the point where 5 blades (the limit at the time) were required, the only solution was contra-rotating props. For any aircraft with less than about 2500hp, there is no real benefit for a complex contra-rotating prop.

btfarrwm wrote:

I think I know the answer to this, and it has to do with cost, complexity and weight. However, a contrarotating propeller setup on a single-engine GA aircraft would it much easier to fly. Anyone who's driven dual-prop and single prop boats knows what I'm talking about. As a bonus, it would save CFI's from saying "more right rudder" 50 times during every flight lesson.

All that added weight, complexity and cost just to avoid adding a bit of right rudder on takeoff. What on earth are you pricing your right leg muscles at?

aviatorcraig wrote:

btfarrwm wrote:

I think I know the answer to this, and it has to do with cost, complexity and weight. However, a contrarotating propeller setup on a single-engine GA aircraft would it much easier to fly. Anyone who's driven dual-prop and single prop boats knows what I'm talking about. As a bonus, it would save CFI's from saying "more right rudder" 50 times during every flight lesson.

All that added weight, complexity and cost just to avoid adding a bit of right rudder on takeoff. What on earth are you pricing your right leg muscles at?

Except that during stalls, the slipstream effect of the single-prop rotation induces spins and thus crashes. Yes, you can train to apply right rudder in slow flight and correct spins with rudder, but I wonder if a contra-rotating prop would negate the tendency to spin after a stall if the wings are level. I don't completely buy the complexity argument, either. Boat manufacturers have been making dual contra-rotating props on their motors for decades and they have proven reliable and the added cost isn't all that significant. The handling benefit in a boat is obvious...try backing a single-prop boat vs. a dual prop boat into a slip and it's night and day.

Didn't the big russian bombers with counter rotating props have exceptional performance for a turboprop?

btfarrwm wrote:

aviatorcraig wrote:

btfarrwm wrote:

I think I know the answer to this, and it has to do with cost, complexity and weight. However, a contrarotating propeller setup on a single-engine GA aircraft would it much easier to fly. Anyone who's driven dual-prop and single prop boats knows what I'm talking about. As a bonus, it would save CFI's from saying "more right rudder" 50 times during every flight lesson.

All that added weight, complexity and cost just to avoid adding a bit of right rudder on takeoff. What on earth are you pricing your right leg muscles at?

Except that during stalls, the slipstream effect of the single-prop rotation induces spins and thus crashes. Yes, you can train to apply right rudder in slow flight and correct spins with rudder, but I wonder if a contra-rotating prop would negate the tendency to spin after a stall if the wings are level. I don't completely buy the complexity argument, either. Boat manufacturers have been making dual contra-rotating props on their motors for decades and they have proven reliable and the added cost isn't all that significant. The handling benefit in a boat is obvious...try backing a single-prop boat vs. a dual prop boat into a slip and it's night and day.

But weight is way less of an issue for a boat than for a simple single engined plane. It's for the same reason the average Cessna 172 doesn't have air conditioning, heated windshield, electrically powered seats or a host of other things that are commonly found in modern day cars (or boats).

aviatorcraig wrote:

btfarrwm wrote:

aviatorcraig wrote:

All that added weight, complexity and cost just to avoid adding a bit of right rudder on takeoff. What on earth are you pricing your right leg muscles at?

Except that during stalls, the slipstream effect of the single-prop rotation induces spins and thus crashes. Yes, you can train to apply right rudder in slow flight and correct spins with rudder, but I wonder if a contra-rotating prop would negate the tendency to spin after a stall if the wings are level. I don't completely buy the complexity argument, either. Boat manufacturers have been making dual contra-rotating props on their motors for decades and they have proven reliable and the added cost isn't all that significant. The handling benefit in a boat is obvious...try backing a single-prop boat vs. a dual prop boat into a slip and it's night and day.

But weight is way less of an issue for a boat than for a simple single engined plane. It's for the same reason the average Cessna 172 doesn't have air conditioning, heated windshield, electrically powered seats or a host of other things that are commonly found in modern day cars (or boats).

I agree that weight is everything.

Here's a YouTube video showing a mechanically-simple gear assembly to produce coaxial contrarotating propellers in a boat. It doesn't look that all-that heavy.

https://www.youtube.com/watch?v=MkjD-U9yScs

The propeller for a C172 weighs 35 lbs. If a dual-prop assembly is used, the propeller size could be reduced and still produce the same thrust, so the weight penalty is not necessarily 2x1 for the props. What would the weight-penalty of the dual-prop system and gear assembly? I don't know, but my guess is ~60 lbs. It's also adding weight directly to the nose, which would change weight-and balance calculations for sure.

I fully understand the arguments against it, but I think it would be an interesting project to see how the plane handles during takeoffs, landings and stalls.

Zeke2517 wrote:

Out of curiosity, how much does the proximity of the two propellers in the coaxial configuration affect performance? I imagine there would be a fair amount of turbulent air around both props. Or maybe just the aft prop. Or maybe not?

It's actually a few-percent more effecient at turning power into thrust. Basically the second prop lives in the swirl of the first prop, which it can use for extra thrust. Done well, a single prop will put swirl into the air, which is less efficient. A coxial prop pair will produce no swirl, which is more effecient.

btfarrwm wrote:

aviatorcraig wrote:

btfarrwm wrote:

I think I know the answer to this, and it has to do with cost, complexity and weight. However, a contrarotating propeller setup on a single-engine GA aircraft would it much easier to fly. Anyone who's driven dual-prop and single prop boats knows what I'm talking about. As a bonus, it would save CFI's from saying "more right rudder" 50 times during every flight lesson.

All that added weight, complexity and cost just to avoid adding a bit of right rudder on takeoff. What on earth are you pricing your right leg muscles at?

Except that during stalls, the slipstream effect of the single-prop rotation induces spins and thus crashes. Yes, you can train to apply right rudder in slow flight and correct spins with rudder, but I wonder if a contra-rotating prop would negate the tendency to spin after a stall if the wings are level. I don't completely buy the complexity argument, either. Boat manufacturers have been making dual contra-rotating props on their motors for decades and they have proven reliable and the added cost isn't all that significant. The handling benefit in a boat is obvious...try backing a single-prop boat vs. a dual prop boat into a slip and it's night and day.

I dunno. I've done maybe hundreds of stalls, as has every pilot. Never had an accidental spin.

I don't think the problem you perceive exists. But that's cool; we are all here to learn........

kitplane01 wrote:

btfarrwm wrote:

aviatorcraig wrote:

All that added weight, complexity and cost just to avoid adding a bit of right rudder on takeoff. What on earth are you pricing your right leg muscles at?

Except that during stalls, the slipstream effect of the single-prop rotation induces spins and thus crashes. Yes, you can train to apply right rudder in slow flight and correct spins with rudder, but I wonder if a contra-rotating prop would negate the tendency to spin after a stall if the wings are level. I don't completely buy the complexity argument, either. Boat manufacturers have been making dual contra-rotating props on their motors for decades and they have proven reliable and the added cost isn't all that significant. The handling benefit in a boat is obvious...try backing a single-prop boat vs. a dual prop boat into a slip and it's night and day.

I dunno. I've done maybe hundreds of stalls, as has every pilot. Never had an accidental spin.

I don't think the problem you perceive exists. But that's cool; we are all here to learn........

You obviously never flew the Pa-38 Traumahawk.

And of course, the single bladed prop. It’s asymmetric, with half the blades of a normal prop. And it flies safely.

https://www.aopa.org/news-and-media/all ... -sensation

Charleytuna wrote:

Didn't the big russian bombers with counter rotating props have exceptional performance for a turboprop?

Yes, they have. Tu95 flies altitudes and speeds similar to turbojets.

However, I don't think it's contrarotating props that made it possible. It's more of a "let's put the most powerful turbine engine ever built" attitude, and contrarotating props are simple consequence of having to transfer torque from 15000 hp engines (Tu-95 MS version, the latest one; earlier used 12500 or 14800). Standard propeller would be too large and too heavy to fit.

Cheers,
Adam

The US struggled mightily for 2 decades to get a counterrotaing gearbox to work without tearing itself apart. They finally gave up at the end of the 50's. The Soviets stuck with it (using German tech) and made it work. Fast forward to the Pratt & Whitney 578DX and finally success, but then no one wanted it despite the superior performance. Gas was too cheap.

Fact of the matter is the counterrotating prop is inherently more efficient than a single rotation prop. Taking weight and complexity into account, that realistically begins with anything over roughly 2000 hp, if the RR Griffon is any indication. Although the Bugatti 100 had under 1000 hp and counterrotating props.

btfarrwm wrote:

I think I know the answer to this, and it has to do with cost, complexity and weight. However, a contrarotating propeller setup on a single-engine GA aircraft would it much easier to fly. Anyone who's driven dual-prop and single prop boats knows what I'm talking about. As a bonus, it would save CFI's from saying "more right rudder" 50 times during every flight lesson.

I have seen a few new GA designs with up this setup, the reason for the change is to do with change in method of propulsion that reduces complexity. For current engines you need to have variable pitch props due to the torque curve of the engines, newer designs that are coming out with electric engines have almost linear torque so they can have counter rotating fixed pitch props and just vary the speed thus a reduction in complexity with the addition of efficiency.

Just a crazy question... Would it be feasible to have 2 counter rotating turbine stages and shafts to drive 2 props?

kalvado wrote:

Just a crazy question... Would it be feasible to have 2 counter rotating turbine stages and shafts to drive 2 props?

I suppose it would be possible. But I think you'd run into issues keeping both turbines at the ideal speed for the respective prop..

The existence of the technology and lack of adoption for the use case implies that everyone who's investigated it so far concluded that the juice wasn't worth the squeeze. If you were to do an A-team B-team comparison on some light GA single, you'd probably get the following outcome:
A: single-stage prop
Baseline
B: dual stage prop (SWAG)
+5% propulsive efficiency
+50 lbs empty weight (IOW -8 gallons avgas, or about an hour cruise in a 172)
+Funky w/b
+mx costs for gearbox and additional prop inspections

If you want to make GA more efficient, engine thermodynamics are lower-hanging fruit. A good chunk of the fleet is still flying around with lycosaurs that could be replaced with FADEC diesels burning half the fuel for the same output. When you read the GA threads on the topic, it's something along the lines of "swapping lycosaur for $xx,xxx or $xxx,xxx diesel won't pay off in reduced fuel until I fly yyyy hours."

For GA, I could see BEV being a big disruptor because the existing GA fleet is so inefficient at a system level (old engines, 10:1 L/D, expensive 100LL). Once a few Bye Eflyers hit flightlines and can get 2-3 hour ranges, a lot of people will re-evaluate their options and go with those. Interestingly, the Eflyer is designed with a 20:1 glide ratio and several other aerodynamic refinements to reduce the energy need, but they still thought that single-stage prop was the way to go.

Starlionblue wrote:

kalvado wrote:

Just a crazy question... Would it be feasible to have 2 counter rotating turbine stages and shafts to drive 2 props?

I suppose it would be possible. But I think you'd run into issues keeping both turbines at the ideal speed for the respective prop..

thinking about it, just as a mental exercise...
turbine would likely end up a 3-spool (1+2 actually) optimized for specific prop configuration ($$$). RPM of 2 props may differ as there is no way to adjust 2 spools independently. However, that would be OK with independent pitch adjustment... And that is an interesting question of routing that adjustment to outer prop. That would have to go down the inner turbine shaft all the way to hot section.
Alternatively (hold my beer, please!) there could be a wireless control of pitch, powered by hub mounted generator, taking power from shafts counterrotation. Both props can be pitched that way to keep things uniform. Reliability would be... well, maybe.
Alternatively, a second prop may be just fixed. I fonder if traditional blade count (2-6 or so) would allow for significant efficiency gain.
OK, can I have my beer back? I need to think a bit to invent a turbofan....

On the other hand: While a twin-engine aircraft, the Cessna 336 Bugsmasher (I mean, Skymaster) essentially achieves counteracting torque with contrarotating props and centerline thrust. Rightfully or not, the FAA required a specialized multiengine rating to fly it:



I got a couple of hours in the plane (unfortunately) and MY GOD the noise and vibration, being so closely surrounded by two engines and props was unreal! Obviously this was long before noise-cancelling headsets.

GalaxyFlyer wrote:

If you didn’t have an AMEL, the FAA required an MEL that restricted the holder to centerline thrust only. An AMEL pilot didn’t need a special rating, tho. Curiously, the only other CLT plane I can think of is the T-38.

Before I got my AMEL, an instructor thought it would be a good idea to "transition" to the rating by getting a centerline-thrust MEL in the Bugsmasher first. Yeah, no thanks. I'd rather save my hearing. Then again Galaxy, I'm sure that's small potatoes compared to the noise in some of the aircraft you've flown. Out of curiosity, what was the noisiest cockpit you've piloted?

And RE: The T-38? So in training, you could transition to the aircraft before having an AMEL (or its military equivalent)? ...and as for the Talon being considered centerline thrust, is it because the exhaust pipes are so close together?

GalaxyFlyer wrote:

the only other CLT plane I can think of is the T-38.

GalaxyFlyer, I always understood the T-37 was also center line thrust. Is that correct?

It certainly didn't require much rudder in an engine-out situation.

e38

It's a cool idea, but as you noted in your original post, teaching the pilot to apply the appropriate amount of stompage to the right rudder pedal is a lot easier, cheaper and lighter.

btfarrwm wrote:

Boat manufacturers have been making dual contra-rotating props on their motors for decades and they have proven reliable and the added cost isn't all that significant. The handling benefit in a boat is obvious...try backing a single-prop boat vs. a dual prop boat into a slip and it's night and day.

Actually, prop walk can be very useful in a boat if you know how to use it. Sailboats utilize the effect in docking.