Spoilers and their "aspect ratio"... height vs. length

[CactusJackSlade]

Hello all,

Anyone have input as to shape/size of spoilers? I'm assuming there is a point of no return, but what is the best "aspect ratio" for a flip up type spoiler?

What would be better at the same surface area of 15 sq. in.?

A 1"x 15" or 1.5" x 10"?

The chord of the wing in the spoiler area is 7.5"... I would think the chord would have something to do with a point of no return... or is it somewhat a matter of structural integrity?

Oh, I also plan to try to get close to 90 deg. of travel out of them for full stopping power...

Thanks all!

CJS

[Ollie]

In a vertical dive, there is no lift so spoilers only add drag and spoiler area determines effectiveness.

In a glide, spoilers increase drag in two ways and decrease lift. The drag increase in a glide is due to the frontal area of the spoilers and to the induced drag caused to interrupting the lift distribution. Higher aspect ratio spoilers will be somewhat more effective than low aspect ratio spoilers particularly at slow speeds where induced drag is a larger proportion of the total drag.

The greater the span of the spoilers the more lift will be affected and the more the maximum glide angle will be steepened.

Spoilers not only reduce top speed but they increase stalling speed so that they narrow the available speed range at both ends of the flight envelope.

Spoilers alter the pitch trim to a more nose-down trim condition. Spoilers are too big when they exceed the control power of the elevator to provide pitch trim. The higher the aspect ratio of the spoilers for a given area, the greater the effect on pitch trim.

The location and fit of the spoilers affect the disruption of the flow over the wing when they are closed. Because the boundary layer on the wing thickens as the flow progresses from leading to trailing edge, the more aft the spoiler location on the chord, the easier it is to hide the flow disruption effects in the bottom of the boundary layer.

[Frank B]

Originally posted by Ollie
Spoilers not only reduce top speed but they increase stalling speed ...

Does this mean that the spoilers should not be up when landing?

Normally I use the spoilers to control speed when approaching landing, but I keep them up until the plane is on the ground. Should I "down" the spoilers during the last part of the landing to avoid stalling? I have tip stalled a couple of times, maybe it could have been avoided?

[Ollie]

The main benefit of spoilers in the landing pattern is to steepen the glide angle. That makes it a lot easier to avoid overshooting the landing zone. Spoilers can be up during the landing pattern if adequate airspeed is maintained. It is always a good idea to have plenty of stall margin in the landing pattern, especially on planes that have a tendency to tip stall. Now that you know about the affect of spoilers on stalling speed, you know not to try to slow down too much until the plane is almost touching the ground. If the plane is close enough, almost touching the ground and doesn't have a tendency to tip stall, a gradual landing flair that results in a stall, at or just before touch down, is safe enough. In this maneuver the airspeed must be reduced slowly enough that the plane continues almost touching the ground without regaining any altitude. In this maneuver the drag of the spoilers offsets the drag reduction of flying in ground effect.

[CactusJackSlade]

Hey Thanks Ollie, I somehow you'd be the man to answer this one - thanks. When I get done with my "experiment" this winter I will post photos.

CJS