LARGE-SPAN FLAPS FOR SAILPLANE APPROACHES AND LANDINGS

By Stephen du Pont

Soaring July 1963 page 10-12

(Article provided by Mike Thompson)

I have been asked many times recently about the use of large-span flaps in place of spoilers on sailplanes. One of the most important recent advancements in sailplane technology is the large-span flap. There is an unfounded fear in the minds of many sailplane pilots that a sailplane without spoilers would be difficult to fly.

I am reminded of the old Department of Commerce attitude when Hawley Bowlus and the late Richard duPont first equipped the Albatross II with flaps in the late 1930's. The Department held that it was necessary to actuate the flaps with a crank of many turns to prevent someone from letting them off too suddenly. This is a little like requiring a multi-turn crank to control spoilers to prevent putting them on too suddenly. I once tried pulling spoilers on a popular sailplane just prior to touching down. I still have imprints in my nether side of rivet heads in the seat as they pushed up through five inches of foam rubber, and it is a tribute to the Schweizer brothers' excellent product that I didn't bend it. Of course, the answer is simply that sailplanes are flown in landing approach, right onto the ground at a save speed margin above stalling speed.

Let us consider some flight characteristics of a sailplane with large-span flaps and no spoilers. (See table for suggested fight use of flaps.) Dick Schreder designed HP-10 without spoilers because he didn't want to spoil the smooth surface of the laminar-flow wing for soaring by cutting slots in it which could never be smooth in flight.

HP-8 had both spoilers and flaps. The flaps on HP-10 reach from one aileron to the other, having a span of 34 feet out of a 48-foot wing. They come down about 50 degrees on the HP-10 Schreder prototype, 75 degrees on the Helisoar HP-10's and 90 degrees on Dick's HP-11. With 50 degree flaps the HP-10 at 55 mph flies at an angle of about 25 degrees nose down and it's coming down fast. There is nothing to prevent you from flying it all over the sky this way, and the thing to remember is that at 55 mph the flaps can be released immediately. At such a nose-down attitude if flaps are released suddenly, you accelerate downhill fast in a sure enough dive, but the ship doesn't stop flying. Instead, you can keep the speed from increasing by leveling out as you let flaps off. All that happens is the ship is suddenly gliding 55 mph at 35-to-1 glide. Yank on full flaps again, keep the airspeed up by dropping the nose, still at 55, and you are coming down as steep as you could possibly desire, with strong drag to make it easy to hold that speed. If you apply flaps in an extreme fashion and at a higher speed as just described you go through an instant of high lift which would be expected to, and does, "balloon" the sailplane. This couldn't matter less, as it is only momentary. But, consider the terrific advantage before ore even after touch-down of being able to let air brakes off part way and increase the lift over the dive-brake as well as the no-brake condition. You can jump last-minute fences, ditches, rock, etc., by dint of the fact that around landing speed, putting flaps from high-drag position into the high-lift position, say 25 degrees, literally picks you up over the obstruction. The best approach technique is to set flaps at 35 degrees down, crank on more for steeper glide, and crank in less for a more shallow glide. Best flap for landing probably in the high-drag area.

HP-10, with nearly 6-to-1 wing loading for cross-country penetration, is a fast ship with an excellent glide angle up to around 36-t0-1. With 12 degree flaps it becomes as easy to land as a lower wing-loading ship with spoilers alone. The large-span flaps make it absolutely an easy ship to fly; first be sure you know what to do with them.

It isn't wise to get into a single-seater sailplane with spoilers for the first time unless you are briefed on how spoilers act and what they are for. The same goes with large-span flaps. Anyone who can soar relaxed in any sailplane, and is also familiar at first hand with the effective flaps of, say, a Cessna 150 or 182 can fly the HP-10. They need merely to know what the flaps are for and to have been briefed on the flight speed characteristics of this sailplane, especially throughout the flap range. Getting into small fields in any strange sailplane comes with practice, and this goes without saying. But it is a delight to fly and it's easy as pie with large-span flaps.

A further comment is the addition of some aileron droop to supplement the 12 degree flap for low sinking speed. Schreder does this automatically in the HP-11 in the vicinity of the 12 degree range but not far above or below it.

One of the penalties of large-span flaps is that when the wing has a wide chord, the addition of large flap loads produces a totally new load path through the wing structure causing increased structural considerations over a ship with spoilers alone. Pitching moments are large with large-chord wings and large flaps, also. But, by keeping the wing chord narrow (on HP-10 it is only 28 inches) and by designing a wing beam that is an efficient torsion member, the flap load becomes easy to handle from an engineering standpoint, and the pitching moments are held down.

FAA isn't saying much in CAR-5 about dive brakes except that the structure must be thoroughly substantiated. It remains to be seen what it costs to present the case of flaps as terminal velocity dive brakes so FAA will accept it. But it is a fact that the improvement from the pilot standpoint and safety to flight characteristics of contest grade sailplanes is so marked by the use of large-span flaps, that there can be no question of the high merit of the concept. In sailplanes of a more utility nature also, I am sure that large-span flaps can improve safety and soaring performance to a marked degree provided of course that the pitching moments are kept within reason.

With large-span flaps, spoilers become an unnecessary extra. Spoilers just can't improve the soaring ability. Large-span flaps can be put up 5 degrees or so above zero setting for lower drag at high speed, down 12 degrees to improve sinking speed while soaring or down 60 to 90 degrees as dive brakes or as quick-stop landing brakes. You will hear a lot of nice things about large-span flaps on sailplanes now that they are coming into use.

The characteristics of flaps are considerably different than those of spoilers and different flight techniques are required. Below are points to remember:

A. Flaps increase lift, spoilers decrease lift. Hence, to maintain a constant altitude above the runway, the nose must be lowered with flaps and raised with spoilers.

B. Flaps decrease landing speed, spoilers increase landing speed.

C. Always start takeoff run with flaps full up position if ailerons are interconnected. This practice will greatly increase aileron effectiveness at low speed.

D. Small flap deflections will increase the "Floating" tendency and may extend the distance covered before touchdown.

E. Large flap deflections increase drag tremendously and make steep angles of approach necessary to maintain flying speed.

F. Flap deflections increase the effective angle of attack of the wing and result in a stall at lower fuselage angle of attack.

G. Complete stall with full flap is approximately the same attitude as the "at rest" position of the ground.

H. When cranking in more flap close to the ground during landing, it will be necessary to push the stick forward to avoid "ballooning" due to the additional lift.

I. Always approach for landing at 60 mph. Maintain this speed while cranking flaps up or down to adjust you approach angle.

J. You can safely retract flaps at 60 mph to extend you flight if you get too low.

K. Never come in low and slow with large flap settings since you can do nothing to extend you glide if you get too low.

L. Full flap deflections provide extremely high drag and steep approaches and, unlike most spoilers, they are very effective at low speeds.

M. If the ship is floating down the runway and doesn't want to slow down, you just don't have enough flap cranked down.

After landing, always hold stick back during landing roll to keep tail wheel on ground for positive directional control. Pushing the stick forward can result in a ground loop especially in high grass.