Counter rotating propellers
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From: Londonish
For whatever it’s worth, I took a wander down the flight line this weekend – only found 2 twins that counter rotated, both were rigged such that the down going blade was nearer to the fuselage, upgoing blade out. One was a piper ?senneca? might have been a Seminole – I know we have both! Not sure what the other was.
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From: flyover country USA
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...)
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...)
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From: very west
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?
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?
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From: Ontario, Canada
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
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
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From: flyover country USA
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.
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.
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From: Ontario, Canada
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.
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.
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From: Sale, Australia
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.
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.
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From: ITALY
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.
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.
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From: flyover country USA
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.
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.
Someone who's progressed beyond Aero 101 might let me know if I'm off-base here.
PS Thank you Brian for that excellent research!
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From: ITALY
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.
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.
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From: Ontario, Canada
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
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
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From: flyover country USA
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.
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.
