Re: off topic air foils

cjuliano Wed Jan 11, 2012 6:12 pm

andrew wrote:
John Goddard wrote:I think you need to give the motor a fair bit of down thrust, it'll just keep looping otherwise.

John's made a very valid point.

If your motor is mounted as shown in the picture, the plane will be uncontrollable. Downthrust, or having the motor pointed downward relative to the angle of attack (the relative angle of the wing as the plane moves thru the air), is necessary to keep the aircraft from continually trying to climb, especially with high lift airfoils. Within a specific speed range, lift is related to the square of the velocity, so it goes up quickly as the speed increases. Before RC, freeflight planes had to be trimmed to fly on their own without pilot input. Downthrust was used as one of the trimming components. When under power and flying faster, the downthrust worked against the lift generated by the wing to keep the plane from ballooning or pitching up. It would be adjusted so the plane would fly in a slight climb. When the motor quit, the speed dropped off, as did the lift. But without the motor trying to pull the nose downward, the plane would be set up to fly in a fairly flat glide.

Downthrust is still used today on planes with lifting airfoils --- partially to compensate for lift to velocity squared relationship. When looking at your plane from the side, with the fuselage sitting as you think it will be angled in the air, the motor should be pointed slightly downward.

400g AUW is fairly light and the Surestart will haul it around quite well.

andrw

thanks I had no idea.
ill fix my engine mount then.

Kim Wed Jan 11, 2012 6:49 pm

Ditto to what the guys said...also, once you've gotten it going, don't be afraid to make fine adjustments to the engine's mounting angle using washers under the lower or upper mounting screws to continue fine-tuning it's behavior under power and in glide. You'll know you're close when it has a tendency to gradually climb under power, with a gentle transition to glide speed when the engine quits.

Ideally, the entire range of elevator trim you need should be just a few bleeps (or notches) up or down from center.

CG plays a role too, and I balance my planes for the glide, and adjust the angle of the engine for trim under power. This is just my deal, other guys may do it differently.

Kim

cjuliano Wed Jan 11, 2012 6:54 pm

thanks for the tip.

cjuliano Thu Jan 26, 2012 10:43 pm

can flat bottom air foils fly upside down

Mark Boesen Thu Jan 26, 2012 11:00 pm

Yes, with a enough elevator throw. Think about how a flat wing creates lift? Don't plan on doing a lot of outside loops though.

nitroairplane Fri Jan 27, 2012 1:45 am

cjuliano wrote:can flat bottom air foils fly upside down

Hes but for bat inverted performance or for any real aerobatics a symmetrical one it best.

John Goddard Fri Jan 27, 2012 5:46 am

With enough power and airspeed you don't nesesarily need any air foil, a tea tray can fly.

microflitedude Fri Jan 27, 2012 8:55 am

John Goddard wrote:With enough power and airspeed you don't nesesarily need any air foil, a tea tray can fly.

+1.

I call it "flying by the prop"

**RknRusty** Fri Jan 27, 2012 10:45 am

Brute force flying. That's what I've been accused of with my slat-winged planes like the Stuntman23. The slat wings do bare a lot of load during loops. And they keep it from descending too fast when the engine quits as long as you keep it nosed up a bit. I wonder how a control line fuselage with no wings except for a stabilator would fly. I've wondered but never tried it.

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