Downthrust

[Ken Culbertson]

In most of my prior designs I have built in 1-2 degrees, usually 1, of downthrust.  Since moving to electric I have not thought to ask if that is still a good idea?  FYI I will be using a BadAss 3515 and either an 11-6 three blade or 12-6 two blade prop with normal rotation.  Thrust line is 1/2" above wing center, stab is 1 1/2" above.  Wing 2 1/4" thick 11" chord w/o flaps.  19" tail moment.   I have tried the opposite rotation thing and I don't see a clear advantage.

Ken

[john e. holliday]

If the design has a full semitrical airfoil in wing and stab/elevator with every thing set in line  why would you need down force?

[Brett Buck]

If the design has a full semitrical airfoil in wing and stab/elevator with every thing set in line  why would you need down force?

    You are continually yawing to the left, which way does precession make the nose go?

      Brett

[Ken Culbertson]

If the design has a full semitrical airfoil in wing and stab/elevator with every thing set in line  why would you need down force?

It has nearly the same effect as negative incidence in the stab.  Even though the components are symmetrical, the layout is not.  The Stab is above the thrust line and the Wing is below.  I never knew the technical reason for it in my IC planes.  I learned it from someone who learned it from someone else.  Supposed to help in preventing hunting so I am told. 

Ken

[Dennis Toth]

Ken,
If you have decided to use conventional counter-clockwise rotation (tractor), then all the normal trim rules apply as with IC. If you commit to clockwise rotation (pusher) the trim rules are the opposite. As Brett said because of the yaw induced by our circular flight path CC rotation creates an upward pitching moment (reaction to a spinning gyro is 90 deg to the applied force), for C rotation motors it creates a downward pitching moment hence trim goes just the opposite of the tractor rotating motors.

If you are on the fence then set up everything at zero - zero and make the stab removable so it can be trimmed up or down depending on the prop chose.

Happy New Year,              DennisT

[Ken Culbertson]

Ken,
If you have decided to use conventional counter-clockwise rotation (tractor), then all the normal trim rules apply as with IC. If you commit to clockwise rotation (pusher) the trim rules are the opposite. As Brett said because of the yaw induced by our circular flight path CC rotation creates an upward pitching moment (reaction to a spinning gyro is 90 deg to the applied force), for C rotation motors it creates a downward pitching moment hence trim goes just the opposite of the tractor rotating motors.

If you are on the fence then set up everything at zero - zero and make the stab removable so it can be trimmed up or down depending on the prop chose.

Happy New Year,              DennisT

Thanks.  When you have been in this sport forever there are some things that you learned as a kid and just *do", with no clue what the science is behinds it.   The epoxy is set now so the 1 degree is built in.  Life is good - Thanks

Ken

[Bruce Shipp]

With electric and four mounting bolts for the motor, I haves used thin AN washers to shim the top or bottom of the motor. This was with the mounting bulkhead installed with no up/down thrust.  I found it easy to go from down thrust to up thrust when switching from standard to reverse pitch and back again.