Dennis Johnson
Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.
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 Posted: Mon Jun 28, 2010 6:39 am Post subject: Speedbrakes |
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My Lancair Legacy has Precise Flight speedbrakes. But are they spoilers or speedbrakes?
The primary purpose of a spoiler is to kill the lift on top of the wing. Typically, they are relatively long and narrow. Long to cover a significant portion of the wing and narrow because for the purpose of killing lift, they don't need to be very wide.
The primary purpose of a speedbrake is to slow the airplane by acting as a flat plate drag device.
The Precise Flight speedbrakes on a Legacy (and some other designs) are installed on the top of the wing. That makes them neither fish nor fowl. Since they are on top of the wing, they kill lift along the span of the wing equal to at least their width, and therefore, act as spoilers.
However, their shape makes them inefficient as spoilers. Their flat plate area is nearly square (they are as tall as they are wide), which means for a spoiler, they are much taller than needed and cover too short of the span of the wing. So in that regard, they are speedbrakes.
As a practical matter, Precise Flight speedbrakes are both spoilers and speedbrakes. We know that parasite drag increases as the square of airspeed. This means that at high airspeed, the effect of the speedbrakes on parasite drag is hugely more than at low speed. Deploying them at high speed generates a large amount of parasite drag. At low speed, the drag effect is relatively small while the spoiler effect is large.
They act more like speedbrakes at high speed and more like spoilers at low speed. Since I typically only use mine at high speed to slow down, for me, they are speedbrakes. I don't normally use them at low speed because those stubby little wings need all the lift they can get when flying slowly.
By the way, I rarely use my speedbrakes. I feel like deploying them is an admission that I used poor planning and am just wasting gas. However, there are times when it makes life so much easier. There are a few mountain airports I visit where for safety it's nice to come in high over a ridge and drop down quickly to pattern altitude before entering downwind. And there are times on IFR flights when approach control keeps me high until the last minute.
Dennis
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