Driftdown

Driftdown

Well Known Member
For a properly aligned Rotax 912ULS engine on an RV-12, where should the engine be pointing, as the airplane sits on the ramp?

I suspect, in azimuth, pointing straight ahead. And in elevation, slghtly drooped, maybe a couple of degrees, below the airplane longitudinal axis.

Please correct me if this is wrong.

For those engineers out there, in elevation (vertical alignment), why is it preferable to have a slight droop, with reference to the longitudinal axis of the airframe?
 
Downthrust is good

for a high wing airplane, Ideally the thrust line should go through the center of gravity so there;s no change in pitch with power application/reduction. You can rarely do that but we (us aero engineers) like to try. The wing pitching moment, which changes with camber (i.e.flaps), speed, angle of attack, etc also complicates the the trim problem a lot as any RV-12 pilot knows.

Wayne 120241/.143WM still waiting for prop
 
yankee-flyer said:
for a high wing airplane, Ideally the thrust line should go through the center of gravity so there;s no change in pitch with power application/reduction. You can rarely do that but we (us aero engineers) like to try. The wing pitching moment, which changes with camber (i.e.flaps), speed, angle of attack, etc also complicates the the trim problem a lot as any RV-12 pilot knows.

Wayne 120241/.143WM still waiting for prop
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Thank you Wayne. That makes sense to me.
So for, a high wing airplane, it would not be uncommon for the design to have a slightly spinner low, line of thrust. Correct?

What about directional offset, to the right, to compensate for p-factor (assuming clockwise prop rotation)?
 
Yep

Somewhere there is a similar thread where I noted that it is not uncommon for prop-driven (I guess to be correct, TRACTOR prop-driven) airplanes to have a few degrees of down thrust and either a few degrees of right thrust, the vertical tail not parallel to the centerline, an asymmetric vertical tail, or some combination of the above. The RV-12 has none of the above so we stand on right rudder for takeoff and use quite a bit of trim. On the other hand, it's a lot easier to build that way.

Wayne
 
yankee-flyer said:
Somewhere there is a similar thread where I noted that it is not uncommon for prop-driven (I guess to be correct, TRACTOR prop-driven) airplanes to have a few degrees of down thrust and either a few degrees of right thrust, the vertical tail not parallel to the centerline, an asymmetric vertical tail, or some combination of the above.
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Okay . . . thanks Wayne.:)
 
yankee-flyer said:
so we stand on right rudder for takeoff and use quite a bit of trim.
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When you build an airplane that has a pretty high power to weight ratio like all the RV's (even the RV-12), have you will always have to use a bunch of rudder in high torque/p-factor conditions. This is true for all the RV's. Even the ones that do have vertical stab and engine thrust offset.

The offsets are primarily for cruise flight. They would have some influence during take-off and climb, but not much.

As for pitch trim, that is largely a function of the min to max. speed ratio (but also a few others like pitching moment induced by flaps, flap induced airflow change over the horizontal stab, etc.).

If you flew an airplane that climbed at 70 kts, cruised at 70 kts, and descended at 70 kts, you probably almost wouldn't need adjustable pitch trim (some low perf. airplanes don't have it).

I don't think the RV-12 requires any more use of pitch trim or rudder than most other airplanes, and probably less than a lot of them.
 
Thank you.
Points well taken. Especially, "offsets are primarily for cruise flight."

From what I gather here, normal offsets, for a standard, clockwise prop rotation, would be slightly nose low and slightly to the right.
 
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