Desperately seeking prop alignment help!

[notchu]

I'm fairly new to this and have an ARF trainer from Thunder Tiger. The trouble is that if you hold the plane so that the bottem of the wing is level, the firewall (and prop) is not vertical. The prop seems to want to pull the plane downhill by about 10 to 15 degrees. I've never attempted a flight with it as I need to know if this is normal. Being an ARF the firewall is factory installed. Should I shim the motor mount to make the prop perpendicular to the lower edge of the wing?

Any help anyone can offer is greatly appreciated.
Thx, Notchu

[Dave Robelen]

Hi Notchu,
It is perfectly normal (and desirable) to have the prop angled down a small amounbt in a trainer. The key is "a small amount" I would consider 3-4 deg. a resonable angle, but 10 deg. sounds excessive, and would likely make trouble trying to get the plane to trim correctly.
Cheers, Dave

[notchu]

Thanks Dave,

I was about to level it. With your advice I'll stop a bit shy of level and call it good. A more accurate measurement yields a rise of 1.375" per 10". If trig serves that's about 7.8 degrees. It looks pretty silly now because with the lower edge of the wing level the top of the 11" prop is almost 1.5" in front of the bottem of the prop.

Thanks again, Notchu

[Dave Robelen]

Hi Notchu,
In this case using the bottom of the wing is perfectly acceptable. The more customary method for setting thrust angles is to measure relative to the centerline of the fuselage. This, because the wing may have some LE up incidence which can confuse matters. Since the fuselage is obviously not marked, it is quite reasonable to fasten a string next to the engine center, and pull it taught along the fuselage side with the other end in the center of the tail cone . This will usually end up with a line that is parallel to the bottom of the stabilizer, and a good reference to work from. When I mentioned 2-3 deg. down, this is really the reference I was thinking of. As long as it is tilted down slightly, the exact amount is no big deal. 7.8 deg. is truly excessive, regardless of where you measure from.
Cheers, Dave

[EdM]

When power is applied such as at takeoff and the nose pitches upward noticeably, there is a need for down thrust in the motor mounting. The reason for this is that the longitudinal pitching moment of the aircraft responds to the thrustline below the CG and the wing drag above the CG which combine to produce the upward pitching moment. This is undesireable because it puts the aircraft near to stall when close to the ground. The down thrust is rarely more than 5 deg. (measure .017 inches/inch/degree).
Right thrust is required because of the swirl in the prop wash striking the fin and rudder at an angle of attack that produces a left yaw with right hand props (clock wise rotation looking forward). needs to be compensated. The only other moments produced by a prop are P-factor effect which is discribed as a yawing moment produced by nose pitch up or down which produces a differential in prop blade lift in the horizontal plane and of course gyroscopic moments. Many times a slight amount of fin offset is used to reduce the severety of P-factor. EdM from NH