Counter rotating props
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I'm referring to the direction of prop rotation. I.e. why don't ALL twin piston aircraft have props rotating in opposite directions. Sorry I thought this was referred to as 'counter-rotating'. For example: Seneca does. Twinstar doesnt.
So you're saying it becomes a cost issue maintaining two different engine layouts?
Thanks for your reply.
So you're saying it becomes a cost issue maintaining two different engine layouts?
Thanks for your reply.
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The contra rotating propeller is better called a coaxial contra rotator, two props on the same shaft as UK Gannet.
Counter rotating is absolutely correct as in Seneca 11 etc, so reduces assymetric drag - I suppose. The handed propeller is same as counter rotating because 'hand' relates to direction of rotation, left hand or right hand, as viewed from behind.
I think it is correct saying that it is a spares, engineering and expense thing connected with engine swaps mostly.
Anyway - Superpilots are suposed to tell me that information! (Grin)
Counter rotating is absolutely correct as in Seneca 11 etc, so reduces assymetric drag - I suppose. The handed propeller is same as counter rotating because 'hand' relates to direction of rotation, left hand or right hand, as viewed from behind.
I think it is correct saying that it is a spares, engineering and expense thing connected with engine swaps mostly.
Anyway - Superpilots are suposed to tell me that information! (Grin)
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It's cheaper to make and maintain 1 engine and propeller type for an airplane instead of 2.
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Do you mean handed (i.e. props rotate in opposite directions on different engines e.g. A400M) or counter-rotating (i.e. two props on same shaft rotating in opposite directions e.g. Gannet/Bear)?
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jetstream 41 has counter rotating props. lower vmca and other control benefits. no critical engine etc.. you certainly dont need 2 different engines, just a slight change to the gear box meaning a slight weight increase. its a turbo prop though.
The Lockheed P-38 Lightning had counter-rotating props as well. As in the Seneca/Seminole/Chieftain the props were laid out so that the left one rotated clockwise and the right one rotated anti-clockwise (when viewed from behind). All the benefits of a lower Vmca, no critical engine, etc.
Trouble was, the props produced an unacceptable down-wash on the rather large tailplane at high power settings so they swapped the rotation. Now they had an up-wash at high power settings (which apparently was OK) but also a higher Vmca. As far as I know this was the only time that this arrangement was tried on a production aircraft. Anyone else have any more info?
Trouble was, the props produced an unacceptable down-wash on the rather large tailplane at high power settings so they swapped the rotation. Now they had an up-wash at high power settings (which apparently was OK) but also a higher Vmca. As far as I know this was the only time that this arrangement was tried on a production aircraft. Anyone else have any more info?
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Trouble was, the props produced an unacceptable down-wash on the rather large tailplane at high power settings so they swapped the rotation.
The graph shows the improvement in the pitching moment compared to the prototype and the British same direction rotation. Hence Kelly Johnsons comment "It made for a better gun platform".
Thanks Brian, very interesting.
If I am reading that graph correctly, Lockheed chose that configuration because it gave the smallest change between power-on and power-off pitching moment coefficient. Is that correct? If so, are you aware of any other aircraft that had similar problems and solutions?
Thanks and regards,
Eckhard
If I am reading that graph correctly, Lockheed chose that configuration because it gave the smallest change between power-on and power-off pitching moment coefficient. Is that correct? If so, are you aware of any other aircraft that had similar problems and solutions?
Thanks and regards,
Eckhard
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Correct. Am not aware of any others, am only aware of the P-38 because the question as to why the prop rotation was changed is a perennial question among aficionados.
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Change in pitching moment can be very important in the missed approach.
While not speaking specifically to the P38, pitching moment changes present a stability problem on a number of aircraft which have props up near the nose and big HP engines.
With higher aircraft pitch angles, the prop normal force at high power (consider a side view of the aircraft and compare the flow through the prop disc to flow over a wing) gives a large nose up force vector resulting in a significantly destabilising nose up pitching moment - due to the change in flow direction at the prop disc... the low IAS doesn't help the tail's ability to control the pitch problem.
The addition of (pitch) stability augmentation systems (by whatever name they may be known) on GA piston-to-turboprop developments is a typical response to this problem ... the turboprop mod pushes the engine way out forward to fix CG problems and the prop normal force results in a big nose up pitching moment delta when power is run up during the miss ... crash, burn, die if the aircraft becomes longitudinally unstable .. as can happen if the prop effect outweighs the basic aircraft's desire to pitch down (trim speed stability).
Fix is SAS or power restrictions during the miss.
While not speaking specifically to the P38, pitching moment changes present a stability problem on a number of aircraft which have props up near the nose and big HP engines.
With higher aircraft pitch angles, the prop normal force at high power (consider a side view of the aircraft and compare the flow through the prop disc to flow over a wing) gives a large nose up force vector resulting in a significantly destabilising nose up pitching moment - due to the change in flow direction at the prop disc... the low IAS doesn't help the tail's ability to control the pitch problem.
The addition of (pitch) stability augmentation systems (by whatever name they may be known) on GA piston-to-turboprop developments is a typical response to this problem ... the turboprop mod pushes the engine way out forward to fix CG problems and the prop normal force results in a big nose up pitching moment delta when power is run up during the miss ... crash, burn, die if the aircraft becomes longitudinally unstable .. as can happen if the prop effect outweighs the basic aircraft's desire to pitch down (trim speed stability).
Fix is SAS or power restrictions during the miss.
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Fix is SAS or power restrictions during the miss
the prop normal force results in a big nose up pitching moment delta when power is run up during the miss ... crash, burn, die if the aircraft becomes longitudinally unstable
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The certification process will pick up on the problem as there are boxes to be ticked for missed approach handling and stick loads.
However, the potential is significant. I did a course many moons ago with some folk who used a particular lightie turboprop conversion as one of their test platforms... ie they had heaps of test data. The stick forces in the missed approach (without SAS) reversed .. ie requiring a very definite push force to maintain a below trim speed situation.
I didn't cite Types as it is a long time since I have reviewed a manual or flown in any of the usual culprits. However, I certainly can recall seeing a power restriction on either a Conquest or Cheyenne on which I did some flying quite a few years ago. Merlin might be another contender ? The normal prop force problem generally is a concern on conversions where a piston derivative gets revamped to a turboprop.
Can't comment on the TBM .. never had any involvement with that bird.
However, the potential is significant. I did a course many moons ago with some folk who used a particular lightie turboprop conversion as one of their test platforms... ie they had heaps of test data. The stick forces in the missed approach (without SAS) reversed .. ie requiring a very definite push force to maintain a below trim speed situation.
I didn't cite Types as it is a long time since I have reviewed a manual or flown in any of the usual culprits. However, I certainly can recall seeing a power restriction on either a Conquest or Cheyenne on which I did some flying quite a few years ago. Merlin might be another contender ? The normal prop force problem generally is a concern on conversions where a piston derivative gets revamped to a turboprop.
Can't comment on the TBM .. never had any involvement with that bird.