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

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

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

Hmmm, Useful thoughts.... Thank you....

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.

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.

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.

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...

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.

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.

I'm still appreciating your thoughts....

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!

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

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!"

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.

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.

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...

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

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)

Daniel_1100

Of course!
Sorry to have forgoten that important fact.
All up weight.
After clean up.
Full power.
Full weight.
CG back for Moment Arm.

A, perhaps, bone but nonetheless serious question; does the downgoing blade inboard arrangement feed additional energy to the wing tip vortices?

Hmmm - never thought of that one.

The propwash vortex certainly reduces AOA downwind of the descending blade, and increases AOA behind the ascending blade.

Let's assume right-hand rotation of both engines (i.e. a typical twin). The spanwise CP is thus moved inboard a bit on the right wing, and outboard a bit on the left wing. (I think that's right...) With both turning, there's a bit of roll to the right, which may or may not equal the left-hand roll reaction due to prop-shaft torque.

Now if the left prop stops turning, the CP on that wing moves inboard a bit, reducing the right-hand roll moment. But P-factor tells us the left engine is critical, because the thrust vector is outboard of the RH prop centerline.

So I guess the P-factor issue is mitigated partially by the accompanying wing spanwise CP shift. (Honest, I've only had half-a-beer, Yank brew at that...) :=

Pilot DAR,

You could do worse than inspect and fly a couple of Piper Twin Comanche's. Firstly, Try an unmodified PA 30, then try a PA 39. Reason I suggest this is, Piper had a go at producing a light twin with non-handed engines (PA30). Then later fitted counter-rotating engines (PA39). Still a number of these aircraft around, and the manufacturer put a fair bit of effort into making them both work properly. Probably the easiest and quickest way for you to determine what your project aircraft should 'feel' like, and what kind of basic handling differences exist. These aircraft types also have fairly comprehensive POH's.

The reason I suggest an unmodified PA30 is, a number have been retrofitted with counter-rotating engines by STC.

Hope this helps.

camlobe

P.S. How about letting us know how you get on?

Yes, Camlobe,

Flying the PA30 and 39 are in my plans. My original question was brought about by the factor which barit1 has identified, that is the local AOA increase resulting from the flow from the upgoing propeller blade. A very well informed aerodynamisist friend suggested that it was desireable to have that higher local AOA more inboard that outboard, so that it's differential effect would be less if an engine quit. Thus the upgoing blade would have to be on the inboard side. All of the other information I have seen does support tips inward at the top though, as opposed to what my friend has suggested, I'm just keeping my mind open, and trying to observe everything I can.

The engine choice has been made for the modified aircraft, and it's nearly flight ready. My task is to evaluate it's handling for design compliance. I hope to fly the original version, with two right hand engines, to form a basis for comparison, though power increase will affect the observations. The original aircraft are getting hard to come by in flying condition, due to engine problems.

Cheers, Pilot DAR

So we have two opposing schools of thought:

1) The very well informed aerodynamicist friend likes to have the CP moved inboard, meaning outboard rotation, to reduce rolling moment presumably. I won't argue his logic, except to point out the following -

2) Every conventional twin I'm aware of has its critical engine defined (or at least described) by P-factor theory; and when Piper chose to create the PA-39, and thus reduce Vmca, it rotated the props inboard to minimize P-factor.

In the absence of empirical tests disproving #2, I'm inclined to go that direction.

Yes barit1,

I agree, after due consideration.

The value in the excercise for me is that now I know why I think this is the best arrangement, instead of just going along without having thought it through!

During my flight testing, I will be paying great attention to all of the factors, to make my best determination. If I can fly the two right engine version, I'll be carefully looking for the differences!

Your thoughts are appreciated...

Pilot DAR

Now that I have learned how to post photos, I can offer a very pleasant Ontario autumn photo taken last week, during the return from flight testing one of my other projects - completely unrelated to the counter rotation question.

http://i381.photobucket.com/albums/o...AR/9169001.jpg

Finally an answer to the P-38 prop rotation, courtesy of the good folk at the Aircraft Engine Historical Society.

When the XP-38 was written off in the crash at Mitchell Field it had a total of 11:50 hours on the airframe, 7:02 of that being the record breaking flight from Los Angeles to New York. Naturally, in the 4:48 hours devoted to testing, little had been achieved in envelope expansion, particularly the high speed regime where tail buffeting was first observed on the follow on YP-38. The XP had a short life of two weeks, first flight being the 27th January, 1939, and crashing 11th February. The YP was a substantially redesigned and differed greatly in detail to the XP, the first YP rolling off the production line in September 1940. Detailed wind tunnel testing during the design of the YP resulted in the production of the attached graph detailing power on/off stability with various prop rotation directions. Hence “Kelly” Johnsons comment the chosen direction of prop rotation made it a better gun platform. No consideration was given to any “critical” engine aspect. Interesting too that the direction chosen for the prototype was the worst possible choice, and that rotation in the same direction enhanced stability to some degree, albeit introducing torque effects and handling issues at the stall.
http://i101.photobucket.com/albums/m...m227/M0001.jpg

Pilot DAR

if I can add something to this interesting topic ; I do not think that 'recent' GA aircraft suffers from lack of aileron authority ; this means that the 'upflow ' help effect on the 'good' wing is very secondary to the negative difference in asimmetry around the vertical axis (yaw) from the downgoing blade thrust (P factor) ; this need to be counteracted by rudder which authority is indeed more limiting in the engine-out conditions.
In other words, I am pretty sure that very seldom you will find problems in keeping the wings level also in an engine-out situation(whatever propeller roatation), as ailerons are generally sufficiently powerful to raise watever wing, and corresponding forces are not vey high.
Instead, you for sure will found 'difficulties' (both from authority and forces) in keeping the heading fixed in an engine-out condition, using rudder/pedal force.

Please keep the Flight Test Community updated on your new project.

After a little consideration, I have another thought - and it may be quite insignificant, but here goes:

By moving the spanwise CP inboard (i.e. outward rotation) the outer wing panels work less hard, and thus the tip vortices are not as strong. Isn't this increased (equivalent) aspect ratio?

If so, and the climb performance is marginal, then outboard prop rotation might give some climb benefit.

Someone who's progressed beyond Aero 101 might let me know if I'm off-base here. :uhoh:

PS Thank you Brian for that excellent research!

By moving the spanwise CP inboard (i.e. outward rotation) the outer wing panels work less hard, and thus the tip vortices are not as strong. Isn't this increased (equivalent) aspect ratio?
If so, and the climb performance is marginal, then outboard prop rotation might give some climb benefit.

Maybe yes, but I suppose that this need to be balanced with an increased rudder deflection (more drag, less Rate of Climb) due again to the greater asymmetry of the outboard-downgoing blade.

I absolutely agree. And I suspect the final answer will be dependent on the exact case details.

Thank you for your very infomative thoughts, they are very appreciated!

The majority of wisdon certainly holds that inward rotating is better than outward. Obviously, common light twin design supports this. I will take every opportunity to familiarize myself with the various effects.

Though completely unrelated to my original enquiry, in this week's test flying of the Basler turbine DC-3 pictured, I could not detect a critical engine. The left is said to be, but the plane was well stalled before Vmca, so I was reacting to a stall instead of a limit of rudder authority. I attribute this to all of the external equipment hanging from the plane, as the stall speed seems to have increased somewhat. I will take future opportunities to evaluate critical engine in this aircraft, as well as my other project aircraft, in a few weeks.

Cheers, Pilot DAR

Interesting observations, Pilot DAR.

I note that when PanAm upgraded DC-3s to R-2000 power they limited them to the original 1200 hp. to avoid Vmc issues - but were able to draw this at higher altitude for South American ops.