lml@lancaironline.net Mailing List Archive

Colyn wrote:.

I suspect they are effective down to a lower speed than precise-flights though?

Could be useful for spot landings on short final to a hay field?

My speed brake effect is greatly diminished at lower speeds, but rises again as the angle of attack begins to rise, in the form of higher sink rate. So during my recent BFR when the instructor wanted to see a series of "forced landing" I made the customary approach over the airport and then a descending 270 degree circle approach to the runway adjusting radius to get the desired outcome.

We found that the speed brakes could be used with good effectiveness to provide a second adjustment (besides adjusting radius), increase of sink rate and thus descent angle, during the final stages of approach. You can also leave them out and flare as long as you are operating with 120 knots which is my forced landing best glide speed anyway. So a secondary benefit with the speed brakes that others might try is to aim for 35-50% of the way down the runway initially during forced landing practice, and then shorten the approach at the end with application of speed brakes.

Y'll be careful out there, y'hear!

Fred

On Sep 4, 2012, at 11:16 PM, Frederick Moreno wrote:

Colyn wrote: Tell us more about those hydraulic speed brakes...

A flap hydraulic cylinder is mounted cross wise on the back of the main spar and pushes and pulls a bell crank that pushes and pulls two heavy duty marine grade push pull cables that then snake out through the fuselage and out to the speed brakes. The whole assembly sits under the pilot and copilot seats, and is covered by a molded carbon cover to keep back seat passenger feet from getting involved. The cylinder is operated by the same type of valve used for flap actuation. The valve is located in the center vertical console panel between but below the gear and flap handles and has its own unique knob.

The push pull cables terminate in a four bar linkage in each wing that serves two purposes. First the horizontal motion of the cable is converted to vertical motion which pushes the speed brake panels up and down. The panels are in the conventional position, hinged at the front with a carbon piano hinge covered with a glider gap seal, and are about 18 inches wide and 3.5 inches in the flow direction. They move up 45 degrees,and position can be controlled just like flaps. The four bar linkage is also configured so that in the down position the panels are locked down by an over center movement so they can not float up, a common problem with some Precise-Flight speed brakes in turbulence.

Overall they work about the same as the Precise-Flight speed brakes, but are infinitely adjustable which turns out not to be much of an advantage. I either put them out or don't. And having a very tight fit on the top of the wing, there is no leakage of air from inside the wing out the top of the wing which reduces drag a smidgen.

It was a hell of a lot of design and fabrication work to save the price of Precise-flight brakes (which I never much liked) in favor of a hydraulic valve, cylinder, and some cables. Not recommended.

Fred