Counter rotating propellers
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Counter rotating propellers
I am in need of tribal wisdom....
I will be involved in testing a twin engined aircraft, upon which counter rotating engines are being installed. I am not aware that it would have flown this way before. I would like to understand better the reason for the choice which will be made (before I can influence it) as to which engine on which side.
I'm expecting to see the left hand engine on the left wing, as this makes the two propellers rotate toward each other at the top. This seems to be the common arrangement, but is it the best?
Here's the logic argument going on in my mind:
We are choosing the added expense and nuisence of the odd engine so as to improve single engined handling on the critical engine. The critical engine is commonly thought of as being the left. This, because at high power, it is creating torque, which must be overcome with flight control input. The torque of a right hand engine on the right wing will cause a rolling force to the left, which is the same direction toward which flight control inputs will be required to maintain straight and level flight on only that engine, so it is good. That same right engine on the left wing would create a torque force which will roll the plane further away from straight and level flight, and thus require maintaining even more control input, so Vmca goes up because of incresased drag and/or need for speed to make more effective those flight controls at increased deflections.
However, With counter-rotation engines arranged so as to have the propellers moving toward each other at the top, the resultant increased angle of attack (and local lift increase) from the up flowing propwash is on the outboard (of the nacelle) area of the wing, not the inboard. Thus the additional lift on the outboard area of the wing will require more control to counteract that lift 'way out there, rather than the lesser effect of the increased upward flow propwash acting on the inboard area of the wing, which would seem to balance everything out.
So, is it a case of the two opposing propeller forces cancelling each outer out somewhat, but the torque still having the greater effect of the two forces? Are the counter rotation engines so arranged only on the basis of the "feel" of Vmca during flight test of a type, or is it possible that both engine arrangements should be tested to see what the actual difference is? Is there tribal wisdon of the engine swapping thing having been done during other twin flight testing, to actually validate this, or is the comparision of the Vmca handling with a right engine on both sides enough to not bother going any farther?
Thanks for your thoughts...
Pilot DAR
I will be involved in testing a twin engined aircraft, upon which counter rotating engines are being installed. I am not aware that it would have flown this way before. I would like to understand better the reason for the choice which will be made (before I can influence it) as to which engine on which side.
I'm expecting to see the left hand engine on the left wing, as this makes the two propellers rotate toward each other at the top. This seems to be the common arrangement, but is it the best?
Here's the logic argument going on in my mind:
We are choosing the added expense and nuisence of the odd engine so as to improve single engined handling on the critical engine. The critical engine is commonly thought of as being the left. This, because at high power, it is creating torque, which must be overcome with flight control input. The torque of a right hand engine on the right wing will cause a rolling force to the left, which is the same direction toward which flight control inputs will be required to maintain straight and level flight on only that engine, so it is good. That same right engine on the left wing would create a torque force which will roll the plane further away from straight and level flight, and thus require maintaining even more control input, so Vmca goes up because of incresased drag and/or need for speed to make more effective those flight controls at increased deflections.
However, With counter-rotation engines arranged so as to have the propellers moving toward each other at the top, the resultant increased angle of attack (and local lift increase) from the up flowing propwash is on the outboard (of the nacelle) area of the wing, not the inboard. Thus the additional lift on the outboard area of the wing will require more control to counteract that lift 'way out there, rather than the lesser effect of the increased upward flow propwash acting on the inboard area of the wing, which would seem to balance everything out.
So, is it a case of the two opposing propeller forces cancelling each outer out somewhat, but the torque still having the greater effect of the two forces? Are the counter rotation engines so arranged only on the basis of the "feel" of Vmca during flight test of a type, or is it possible that both engine arrangements should be tested to see what the actual difference is? Is there tribal wisdon of the engine swapping thing having been done during other twin flight testing, to actually validate this, or is the comparision of the Vmca handling with a right engine on both sides enough to not bother going any farther?
Thanks for your thoughts...
Pilot DAR
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Actually, I thought they kept the down going blades as close as possible to the CofG because the downgoing part of the prop disc produced more thrust due to the blades higher AofA when the aircraft is slow and has a high nose attitude (or close to Vmca).
Keeping the most thrust producing half of the prop disc close to the fuselage reduces rudder required for any given airspeed, or conversely you can fly slower before you run out of rudder.
Thats my 2c anyway
Cheers
Keeping the most thrust producing half of the prop disc close to the fuselage reduces rudder required for any given airspeed, or conversely you can fly slower before you run out of rudder.
Thats my 2c anyway
Cheers
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Going back to my Engineering Flight Testing many years ago :
1)If the propeller are counter-rotating (whichever way): no critical engine.
2)If the propeller are counter rotating inner-tip-down : no critical engine , the Vmc is less than....
3) Propeller counter rotating outer-tip-down : no critical engine, however the Vmc is more than 2), as the outer blade (going down) has more thrust (due to its higher relative AoA) than the inner one.
4) propeller with same direction of rotation : the critical engine is that opposite to the sense of rotation (clockwise: critical engine is the left, because if it fails, the asymmetry is higher du to higher arm from the down-going right-engine propeller blade)
For what refers to engine 'torque' : torque transmits itself all over the aircraft , it is not important if the item generating torque is on the left wing, on the nose or on the right wing ; its effects are the same.
I hope to have given you some useful information
1)If the propeller are counter-rotating (whichever way): no critical engine.
2)If the propeller are counter rotating inner-tip-down : no critical engine , the Vmc is less than....
3) Propeller counter rotating outer-tip-down : no critical engine, however the Vmc is more than 2), as the outer blade (going down) has more thrust (due to its higher relative AoA) than the inner one.
4) propeller with same direction of rotation : the critical engine is that opposite to the sense of rotation (clockwise: critical engine is the left, because if it fails, the asymmetry is higher du to higher arm from the down-going right-engine propeller blade)
For what refers to engine 'torque' : torque transmits itself all over the aircraft , it is not important if the item generating torque is on the left wing, on the nose or on the right wing ; its effects are the same.
I hope to have given you some useful information
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The downward - vs - upward blade effects are commonly known as "P-factor", and is probably the dominant effect regarding Vmc.
I note the Lockheed P-38 used counterrotating props, but with the tips travelling OUTBOARD on top. This resulted in a very high Vmca, but allegedly improved combat manuverability.
Also note that if the vertical fin(s) is in the rotating propwash, a yawing moment will probably be observed. This is very common in SE prop aircraft and is what we usually call "torque". The fin will probably be offset to reduce this effect.
I note the Lockheed P-38 used counterrotating props, but with the tips travelling OUTBOARD on top. This resulted in a very high Vmca, but allegedly improved combat manuverability.
Also note that if the vertical fin(s) is in the rotating propwash, a yawing moment will probably be observed. This is very common in SE prop aircraft and is what we usually call "torque". The fin will probably be offset to reduce this effect.
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Torque is torque, Slipstream effect on the vertical stabiliser is airflow, they are not the same.
In a SE a/c torque will be felt as a rolling tendency in the opposite direction from the direction of rotation of the propeller. Airflow is never "torque". Rudder bias or offset fin are to counter airflow effects as well as the rolling tendency on ME a/c in the event of critical engine failure.
In a SE a/c torque will be felt as a rolling tendency in the opposite direction from the direction of rotation of the propeller. Airflow is never "torque". Rudder bias or offset fin are to counter airflow effects as well as the rolling tendency on ME a/c in the event of critical engine failure.
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Torque is a physical moment (measured in lb-ft, N-M, etc.) about some axis. Prop shaft torque is about the longitudinal axis, is reacted through the engine mounts into the airframe, and will be countered in the ailerons.
But the "swing" or "torque" pilots experience on rapid power application is about the vertical axis, and is countered by rudder input. (Right-hand prop rotation, right rudder...). This is the airflow effect (prop vortex on the vertical fin).
In both cases, as we slow to near Vs, we may run out of rudder or aileron authority, and the airplane does unwanted things.
I think we are talking about the same thing, blueplume, but I just wanted to get the terminology straight.
But the "swing" or "torque" pilots experience on rapid power application is about the vertical axis, and is countered by rudder input. (Right-hand prop rotation, right rudder...). This is the airflow effect (prop vortex on the vertical fin).
In both cases, as we slow to near Vs, we may run out of rudder or aileron authority, and the airplane does unwanted things.
I think we are talking about the same thing, blueplume, but I just wanted to get the terminology straight.
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You could of course, set up the two scenarios in something like X-Plane Fight Sim, which includes P-factor in its aerodynamic model... to see if you can measure/sense the difference between two clockwise or anti-clockwise props.. Austin Meyer, the author might take that on for a lot less than swapping two engines over!
NB. Having the downgoing blade nearest the fuselage seems to make sense from a Vmca perspective...
NB. Having the downgoing blade nearest the fuselage seems to make sense from a Vmca perspective...
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BTW:
The prop shaft torque is easily calculated by the mechanical formula:
SHP = Prop RPM x Q (torque in lb-ft) / (33000 / 6.28...)
OR
Q (torque in lb-ft) = 5252 x SHP / Prop RPM
I suppose one could try to calculate the yawing torque, but it varies with airspeed etc. and thus is trimmed out empirically with either fin offset or rudder tab.
The prop shaft torque is easily calculated by the mechanical formula:
SHP = Prop RPM x Q (torque in lb-ft) / (33000 / 6.28...)
OR
Q (torque in lb-ft) = 5252 x SHP / Prop RPM
I suppose one could try to calculate the yawing torque, but it varies with airspeed etc. and thus is trimmed out empirically with either fin offset or rudder tab.
Last edited by barit1; 26th Sep 2008 at 03:26.
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Can remember P factor.
Seneca 11 had counter rotating props.
In the theory, there was no critical engine for VMCa.
In the practice, the loss of the left engine, No1, was slightly more critical than the loss if the right.
Refresher...
Conditions for determination of Vmca.
Aircraft in take off configured.
Critical engine not fethered.
Other engine full power.
Cof G at max aft position.
Sea level.
MAUW.
Indicated Vmca decreases with altitude.
So, Vmca demo in PA30 can be quite bad when stall speed reached at same time as Vmca.
These are the more thoughts for your appreciation.
The Gannet had counter rotating props also - both on the nose spinner.
Seneca 11 had counter rotating props.
In the theory, there was no critical engine for VMCa.
In the practice, the loss of the left engine, No1, was slightly more critical than the loss if the right.
Refresher...
Conditions for determination of Vmca.
Aircraft in take off configured.
Critical engine not fethered.
Other engine full power.
Cof G at max aft position.
Sea level.
MAUW.
Indicated Vmca decreases with altitude.
So, Vmca demo in PA30 can be quite bad when stall speed reached at same time as Vmca.
These are the more thoughts for your appreciation.
The Gannet had counter rotating props also - both on the nose spinner.
Last edited by Der absolute Hammer; 26th Sep 2008 at 16:14. Reason: Error to be corrected.
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Actually - wasn't the Gannet a twin-engine machine, with each engine driving a separate prop?
The Macchi M.C.72 - see video - also was a twin (note the starting sequence at 0:53). Interesting that the specs claim it was a single engine, and while there may have been a single crankcase, there were clearly two crankshafts!
The Macchi M.C.72 - see video - also was a twin (note the starting sequence at 0:53). Interesting that the specs claim it was a single engine, and while there may have been a single crankcase, there were clearly two crankshafts!
Actually, the P-38 prototype had counter-rotating props,but the a/c suffered from vibn/resonance/something nasty,so pre-prod/prod went non handed.
The Hornet (Aahh,DeHavilland) was handed,props turning inboard from the top and looking forward. The Gannet has/d counter-rotating,2 engines,separate drives; the Shackleton and Bear both had/have contra-rotating props,as did late Spits/Seafires/fangs/RedBaron ie engine drives both props thru` the gearbox.
And if you read the A400M blurb, it has counter-rotating props on each wing,ie inboards go one way ,outboards t`other way....all to do with downwash/slipstream.....Syc
The Hornet (Aahh,DeHavilland) was handed,props turning inboard from the top and looking forward. The Gannet has/d counter-rotating,2 engines,separate drives; the Shackleton and Bear both had/have contra-rotating props,as did late Spits/Seafires/fangs/RedBaron ie engine drives both props thru` the gearbox.
And if you read the A400M blurb, it has counter-rotating props on each wing,ie inboards go one way ,outboards t`other way....all to do with downwash/slipstream.....Syc
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sycamore, I was about to call your bluff, but it appears you are correct regarding the early Lightnings of the RAF. They not only had same-rotation engines, but no turbos either. I found this at:
About the P-38
However, all US P-38's did counter-rotate. In fact there's a limerick set to a ditty tune "Fighter Pilot's Lament", one chorus of which goes:
"Don't give me a P-38
With propellers that counter-rotate;
She loop and she'll spin
and she'll augur you in,
Don't give me a P-38!"
About the P-38
However, all US P-38's did counter-rotate. In fact there's a limerick set to a ditty tune "Fighter Pilot's Lament", one chorus of which goes:
"Don't give me a P-38
With propellers that counter-rotate;
She loop and she'll spin
and she'll augur you in,
Don't give me a P-38!"
Last edited by barit1; 4th Oct 2008 at 01:34. Reason: missing syllable in poem
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The British only took delivery of three P-38's. After testing they realised what a dog it was and cancelled the rest which mostly went to US training units in a variety of mod states. Depending on who you believe the lack of turbos is given as due to the shortage of supply, or at that stage of the war the US ban on the export of turbochargers. The Brits wanted the engines to be interchangeable with those of the Curtiss H.81A Tomahawk, which had been ordered in great numbers, hence the engines being same handed.
The prototype had outward rotating props. Have seen no authorative account of why the change in rotation to inward. The engines of the prototype were fitted with failure prone epicyclic reduction gearboxes and subsequent engines had spur reduction, difficult to see that that may have had an influence.
The prototype had outward rotating props. Have seen no authorative account of why the change in rotation to inward. The engines of the prototype were fitted with failure prone epicyclic reduction gearboxes and subsequent engines had spur reduction, difficult to see that that may have had an influence.
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Fairey Gannet AS.
Contra rotating.
Two Mamba engines, each driving a propeller.
Operational fuel. Kerosene.
Extended range on one engine.
No asymetry problems.
They were a bit pretty really.
Contra rotating.
Two Mamba engines, each driving a propeller.
Operational fuel. Kerosene.
Extended range on one engine.
No asymetry problems.
They were a bit pretty really.
Barit1,and all, apologies ,you are correct that the P38 had counter -rotating
props,but outwards,,as different from the prototype...must check book before scribing,but it`s been a long week !! Interestingly the prototype Whirlwind had counter-rotation props as well,but went to same rotation in production; pity we didn`t put a couple of Merlins in it,or even Allisons..
I guess we should say the Gannet had co-axial counter-rotating props,in a similar manner to Kamov helicopter rotors are co-axial,but coupled by the gearbox at constant rpm.....confused ?
Dar, back to the original Q,I would suggest if not already done that some form of `tunnel` testing has been done to give an indication of whether inward,or outward rotation is more beneficial with respect to increased /decreased slipstream effects ,resonance near fuselage/prop tips,buffeting of tailplane etc.As you can see ,history reflects things that sometimes `seem like a good idea` don`t always work as expected... However, hope it does work...
props,but outwards,,as different from the prototype...must check book before scribing,but it`s been a long week !! Interestingly the prototype Whirlwind had counter-rotation props as well,but went to same rotation in production; pity we didn`t put a couple of Merlins in it,or even Allisons..
I guess we should say the Gannet had co-axial counter-rotating props,in a similar manner to Kamov helicopter rotors are co-axial,but coupled by the gearbox at constant rpm.....confused ?
Dar, back to the original Q,I would suggest if not already done that some form of `tunnel` testing has been done to give an indication of whether inward,or outward rotation is more beneficial with respect to increased /decreased slipstream effects ,resonance near fuselage/prop tips,buffeting of tailplane etc.As you can see ,history reflects things that sometimes `seem like a good idea` don`t always work as expected... However, hope it does work...
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Thanks Sycamore.
The arrangement on the subject aircraft has just been changed to meet the designer's intent, and I will fly it soon. I'm trying to be as attuned as possible to the possible affects, and if those effects are as favourable as intended. If I can, I'll be flying an original configuration as well, to see the difference, but that may not be so clear, as there is a power change involved as well. It will be a change in Vmca I'll be closely watching for...
Pilot DAR
The arrangement on the subject aircraft has just been changed to meet the designer's intent, and I will fly it soon. I'm trying to be as attuned as possible to the possible affects, and if those effects are as favourable as intended. If I can, I'll be flying an original configuration as well, to see the difference, but that may not be so clear, as there is a power change involved as well. It will be a change in Vmca I'll be closely watching for...
Pilot DAR
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Der Absolute Hammer
Aircraft in take off configured.
Critical engine not fethered.
Other engine full power.
Cof G at max aft position.
Sea level.
MAUW.
Though may seems strange, remember that the 'Take off configuration' for what refers to demonstration of Vmca, is with all the flaps settings allowed for take off on the AFM .. but ALWAYS wih the Gear UP !! (FAR 23)
Aircraft in take off configured.
Critical engine not fethered.
Other engine full power.
Cof G at max aft position.
Sea level.
MAUW.
Though may seems strange, remember that the 'Take off configuration' for what refers to demonstration of Vmca, is with all the flaps settings allowed for take off on the AFM .. but ALWAYS wih the Gear UP !! (FAR 23)