Counter rotating props
 

Why don't all twin piston aircraft have counter-rotating propellors?

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.

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)

This one is still 'in the making'! :p

It's cheaper to make and maintain 1 engine and propeller type for an airplane instead of 2.

exactly.. to make a prop spin the other way, you need a whole new engine type to the one on the other wing. that means 2 engines that need to be tested and certified for the aircraft which is an expencive business rather than just the one.

I believe the Gannet was both. i.e. two engines, with two props contrarotating on coaxial shafts, albeit turbine engines.

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?

Not quite the whole story. The prototype was written off after only 11:50 hours of flying, 7:45 hours of this being its record breaking trans continental flight, so little testing had been carried out. The prop rotation decision was the result of wind tunnel work following the crash, when investigations were made into the aircrafts power on/off pitch stability. In the "no power" condition the main wing airfoil generates a pitching moment (aerodynamic centre of lift versus aircraft centre of gravity) that must be counteracted by the horizontal stabiliser over the range of acceptable CG for the aircraft. The elevator must also be able to control the effects of engine power on this parameter. This requirement is further complicated by the dynamics of slipstream airflow, which alters the flow of air over portions of the wing (changing the centre of lift) and empennage exposed to the propeller induced flow. The objective selected by Lockheed was to minimise the change in these control forces over the speed and power range of the airplane.

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".
http://i101.photobucket.com/albums/m...m227/M0001.jpg

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

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.

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.

Lighty Cessna drivers are more than aware of the big elevator push required with a flapped overshoot, but could you give a type that requires a power limitation John. 208, PC12?
The TBM 700 seems to have more than its share of accidents in the approach/overshoot phase. Might this be a pointer?

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.