Motor Incidence

[James Frolik]

I am concerned about the motor incidence (http://www.rcgroups.com/forums/attachment.php?attachmentid=1783244) on my Graupner Me-323 Gigant. (The link takes you to a photo in the Scale Electric Models colume.)

This model should have 0-degrees incidence, but I have a feeling there is about 0.5- to 1.0-degree positive, or upward, incidence—on all 6 motors! This wouldn't be good for the model: huge glider wing, short tail moment, documented tendency to stall in more-than-shallow turns (but I've built in washout that's actually not called for in the plans).

Does anyone here see what I'm seeing. There is a shadow effect in the photo because the spinners and props are all flat black. But anything noticeable on the No. 1 engine (most visible in the photo) is the same on the other five. The model is flat on all wheels, battery inside up front, and it's essentially at its AUW less the receiver and ESC.

Any comments?

[BMatthews]

Consider that the airfoil's center line is NOT the flat bottom of the airfoil but instead the line from the extreme leading edge to the extreme trailing edge. If you recheck the props with that line I think you'll see the same 1 to 1.5 degrees of DOWNthrust that I do.

Nice looking model. All the best of luck with the first flights.

[James Frolik]

Oh, good point!

In relation to the wing's leading edge (and flat bottom), that's true: the incidence is 3-degrees of DOWNward thrust.

Although the construction plans note the alignment should have zero incidence to the flat level plane of the fuselage (on the ground); or you could also say zero incidence to the horizontal stab.

My mind is possibly playing tricks on me now. The positive sloped incidence of the wing makes things hard to judge. I also lack any mechanical or instrumental means to accurately measure my notion.

[MarkusN]

Another important thing to remember: Downthrust is not so much about angle but about lever arm and thus moment about the lateral axis of the plane. Both the drag of the plane and the thrust of the motors exert such moments. The net effect of them should compensate for the changing moment of the stab with changing speed.

A plane with a large, draggy fuselage below the thrust line like the Gigant probably needs less downthrust; might even require upthrust.

[James Frolik]

Downthrust is not so much about angle but about lever arm and thus moment about the lateral axis of the plane. Both the drag of the plane and the thrust of the motors exert such moments. The net effect of them should compensate for the changing moment of the stab with changing speed.

A plane with a large, draggy fuselage below the thrust line like the Gigant probably needs less downthrust; might even require upthrust.Like vintage1 (who posted an answer in the Scale Electric Models colume), you too have opened my eyes! Much appreciated. I still have lots to learn about aerodynamics.

[eflightray]

A much better way to check for motor thrust angle is to replace the prop with a piece of wood or cardboard, the bigger the better. That way you truly do have a flat surface to measure or eye-ball against.

As MarkusN pointed out about highish thrust lines and thrust angles, quite a few of the flying boats have up thrust.

[BMatthews]

So after all this it pretty much comes down to "leave it alone and fly the darn thing" :D

Everything said so far is all good and I can't see anything that doesn't pertain to your case. And in any event you've got throttle control so you can always ease back to reduce the power effects if any.

[James Frolik]

Well I guess the input here is right. Unless Tinkerbell appears and warns me of imminent danger, I won't decide any further about the incidence until after the first flight. But I won't "leave it alone..." until I add the receiver and ESC.

At the moment the Rx is being double-checked by Schulze since it had sat for about 18 months after a serious plane crash. Although it worked okay after that crash I decided not to use it again in a plane until now, but just for safety I gave it to Matthias Schulze at the Sinsheim fair and asked him to inspect it.

Thanks again.

[Brandano]

if the plane doesn't need trim adjustments at different throttle levels, then the thrust angle is right. Changing the engine's angle won't change the way the plane enters a stall, there's enough "rest of the plane" to take care of flight behavior as it is. The really critical thrust angle on a plane like this (multi engined, single finned) would be on the plan view. I'd expect each engine to point further outward the further it is from the centerline, to handle engine out situations better. But engine failures are so rare with electrics, and the fin is so large that probably it isn't worth the bother to make any adjustment.