kram

My Feedback: (7)

If the wing and the stab are at 0 degrees respective to the fuselage, what effect will 2 degrees of down incidence on the engines have???:


A) Will it pull the nose down and thus require Up elevator trim, or:

B) Will the plane travel horizontally through the air along its thrust line, thus causing the wing to fly at a relative 2 degree positive angle, causing the nose to go up and require Down trim??

I understand that by asking this question I demonstrate my lack of aerodynamic fundamentals, and I throw myself at the mercy of the court.

Thank You,

mt

av8r4aa

My Feedback: (82)

All good questions Kram.
Here is my take on it.
Any downthrust on any plane is for pitch stability.
Look at the North American T-28 .
That prop is pointing 3 or more degrees down.
I used to work on full size T-28 airplanes and I had the same question.
According to the operating handbook on that Navy plane it said.
The reason for the downthrust is to minimise pitch changes with power changes.
That means pull the power back (idle) the plane does not pitch up.
Conversly when appling lots of power the plane does not pitch down.

Now as to your question of wing incedence the wing could not care where the propulsion is coming from.
IE pusher engines or jets. The wing will fly at a given aoa angle of attatck for the respective speed,.
I would guess that that is why the engines are downthrust,
Look at the most basic trainer they have gobs of downthrust (model)
Thats my dollar on that.

As you have seen in your models slow the plane down as in landing the nose comes up
Same thing as at work
Super 80 has about 8 degrees up trim on landing.
full flaps and gear down

jrf

My Feedback: (551)

kram:

It depends on your airfoil.

I assume your incidence measurements are taken from the chordline of the wing, not it's bottom surface, and I further assume that you are not using a fully symmetrical airfoil. Semi-symmetrical and flat-bottom airfoils make lift at 0 degrees incidence. The faster you go, the more lift, even at 0 degrees. The down thrust is there to counter the higher lift at higher throttle settings.

The forces affecting an airplane in flight are always a compromise. If the wing incidence, engine thrust offset, CG and center of drag are properly balanced the airplane will fly level with little or no trim change as speed changes.

Given my assumptions above, I would expect your B 17 to fly with the fuselage level and no elevator trim required.

Of course that also assumes that the designer got the forces properly balanced and you got the CG right.

Jim

kram

My Feedback: (7)

Makes sense.

Thanks!

mt

jimcork1

My Feedback: (4)

JFR is correct. Once in my way back past I was a flight instructor for a small flying club called the USAF and taught applied aeronautics to students. The airfoil in subsonic flight is a key contributor to down thrust. If you have a flat bottom high camber wing the increase speed will generate more lift for a given angle of attach unless a down vector is applied to reduce the angle of attack for a given trim setting and cancel out the lift to maintain level flight as the power is increased. Same but in reverse for pulling power off.

Happy flying.

Now supersonic flight is another discussion, dealing with pressure waves and points for lift generation.

Uhmmmm wonder how a rocket flies with out wings..