Another thought on engine down thrust

This weekend one of the other club members and I were talking about down thrust. Something to ponder.. If the engine is pointed slightly down and the airframe is still flying level doesn't this effectively increase the pitch of the inboard prop blade and reduce the outboard.

Yes but the plane isn't flying "Level" It has some positive angle of attack--perhaps a few degrees. It has to in order to develop lift with a symmetrical wing. The engine downthrust would align the engine more directly with the airstream. Not sure this is the real reason some like it or not.

P-factor is caused by the difference in apparent pitch of the right side v. left side of the prop when the airflow is not stright down the axis.

FWIW

Curt

> . . .
> . . . The engine
>downthrust would align the engine more directly with the
>airstream. Not sure this is the real reason some like it or
>not.
> . . .

>Curt

So, during inverted flight, you get . . .

Different precession, p-effect, and thrust angles, which averages or cancels what out with what ???

Larry Fulwider

Guys, I gave up trying to think of all the nuances of downthrust/ right thrust, positive stab.

What I did do was try it out on a plane I was building.

About 2 degrees of downthrust, a little out thrust, maybe about 1 degree, and slightly positive, about 1 degree or slightly less, at the stab.

The result.

My plane trimmed out with out a droopy elevator, it grooves both upright and inverted. Manuevers are honest.

I'm happy, even if I don't completely understand all the reasons why it works.

I am guessing that if you really built a plane where it was completely symmetric about the longitudinal axis, then the motor would be at 0-0 to geve the same upright and inverted. You really don't see too many planes that are really symmetric. Otherwise you wouldn't see positive stab offset either.
This is assuming you really want the plane exactly the same upright and inverted. Maybe the biggest issue with symmetry isn't the plane but is your hand! No one ever said it is the same for inside and outside maneuvers!

>Guys, I gave up trying to think of all the nuances of
>downthrust/ right thrust, positive stab.
>
>What I did do was try it out on a plane I was building.
>
>About 2 degrees of downthrust, a little out thrust, maybe
>about 1 degree, and slightly positive, about 1 degree or
>slightly less, at the stab.
>
>The result.
>
>My plane trimmed out with out a droopy elevator, it grooves
>both upright and inverted. Manuevers are honest.
>
>I'm happy, even if I don't completely understand all the
>reasons why it works. John Miller

John,
Any interest in describing your measuring technique for these small angles, (I'm assuming you haven't already and I just missed it).

Thanks,
Kim Mortimore

>I am guessing that if you really built a plane where it was
>completely symmetric about the longitudinal axis, then the
>motor would be at 0-0 to give the same upright and inverted.
>You really don't see too many planes that are really
>symmetric. Otherwise you wouldn't see positive stab offset
>either.

I don't think so. A 0-0, in-line airplane is almost certainly more prone to inside/outside asymmetry than a conventional layout. You still have pitch effects of precession, and that appears to be the cause of the tendency to need downthrust, positive incidence, elevator droop, etc.

To me it's very clear that a lot of the beneficial "asymmetries" are the result of needing to compensate for the pitch precession effects. Maybe a perfectly "symmetrical" airplane with contra-rotating props would really be the same but we don't usually do that.

Brett

Brett,
Just to make sure I understand what you mean, the precession you are referring to is due to the nose of the plane "turning left" in normal upright flight? It is true that this should give rise to a pitching up moment (if I have my right hand rule correct), and that some downthrust or down elevator or + angle of stab attack should compensate.

If this is the correct view, I am guessing that flying clockwise---like in the old days--- would then be built with everything just the opposite---which is what a CCW plane flying inverted effectively is seeing.

Or using a opposite rotating engine would require putting everything "backward" too.

Interesting, it would be a fun experiment, but a lot of work just to see the opposite trim work.

You don't need to understand John, It's something that "ALWAYS" works - If you need a detailed explanation, just ask RJ next time you see him!

Jim Pollock

>You don't need to understand John, It's something that
>"ALWAYS" works - If you need a detailed explanation,
>just ask RJ next time you see him!
>
>Jim Pollock

I've had a detailed conversation with RJ about this subject, and he explained it pretty well, but, as he, you, and now, I have said, it "ALWAYS works".

As for measuring these small angles, they really aren't that small.

A degree is about 1/8" in a foot I believe.

I use my Cad system to layout my incidences, and build them into the structure. I use a pair of incidence meters to verify before final attachment.

If you set your incidence meter up on the shaft of your engine, you can easily see, and set your wing 1.5 to 2 degrees positive to the thrust line. Move the meter from the shaft,without moving anything, to the stab, and set it about 1 degree negative to the wing. You're done.