Soaring Symposia Archive
posted by
Guy Byars'

Dick Schreder's Comments
Taken from
A Synopsis On Design - Present And Future
Proceedings of 1970 Symposium


(Snip ...)

One of the problems we are having now is getting people to use the simple flap. They have been approved by the CIVV for world competition in 1974, and by vote of the SSA Board of Directors for immediate use in the U.S. I have a list of the advantages of the simple flap and I would like to run through it. I know some of you people have flown HP-11's, HP-13's, or -14's, and you realize what some of these advantages are.

Most sailplanes, of course, that meet the standard class specification now have dive brakes, and they have some pretty serious drawbacks. First, when you pull out the dive brakes on the average ship that has the DFS type, the stalling speed increases. In other words, if you are coining in to land with open dive brakes, you are landing about 10 miles an hour faster. With a simple flap, you land about 10 miles an hour slower. This means that when landing in a very bad field, you will do less damage if you run into rocks or stumps, or a very rough terrain type of field. Also, you will land shorter. By actual test in the HP-14 with no wind, we found that we could land in 88 feet from the time we touched ground until we stopped. The flaps are more effective at low speeds.

Most of the DFS type dive brakes are very powerful at high speeds but as you slow up they get less effective. The DFS type of brake is very difficult to build because you have to have a slot in your wing anywhere from 5 to 8 feet long. From a structural standpoint, it is very difficult to build. As the wing flexes you get differential bending at that point. It also weakens your torque tube, your torque box, and it gives you problems when the wing flexes because the wing structure bends while the dive brake itself stays straight. Some of the European ships have solved this problem by having a small strip that is spring loaded so that it will lie on the surface as the wing bends.. Also, with all of this complicated structure right in the most critical part of the wing, cost of construction is seriously increased. The simple flap, on the other hand, is nothing but an aluminum triangular shaped box usually with no spar in it. They can be driven from one end so that no parts are required out in the wing to operate the flap.

Another problem with the DFS type dive brake is with extra drag when retracted. All of you who have owned ships with a DFS type brake have tried to fill them. When you are flying in a contest and get into the air, the wing bends, thus forcing the clay to pop out. You no longer have laminar flow over that section. The simple flap is lighter in weight because the mechanism and brake boxes are eliminated.

One of the reasons the standard class requires the speed limiting dive brake is that if a student pilot gets caught in a cloud he is supposed to be able to slow up the ship and get out of the cloud. I have found, in flying in Europe, especially in England, that every time you get up in clouds you usually get icing and usually come out of the bottom of the cloud with a load of ice all over the airplane and hope that it will melt off before you get to the ground. With the DFS type dive brake sticking out, that is the first thing that the ice builds up on and I am not too sure you will always -be able to get them back in the wing. When we flew in the Internationals over in England, we iced up in every cloud above 8000 or 9000 feet.

Another advantage of the trailing edge simple flap is, when making an approach to land, you can come in at a steeper angle than you can with any of the ships I have ever flown with the DFS type dive brakes. Also, when you put the flap down, it puts the nose of the ship down so you have very good visibility over the nose to see the field in which you are landing.

A further feature that you get with the trailing edge flap when you put the flap down is that the decalage angle between the wing and the horizontal tail is increased. This tends to increase the longitudinal stability. In most of the HP's, when you are making an approach with the flaps down, you can let go of the stick and be very stable. When the stick is pushed forward and then released the nose will pop right back up to the trim speed.

An additional flap advantage is realized on takeoff. You can get off the ground much quicker because down flaps have the effect of increasing the angle of attack of the wing and gives a higher lift coefficient.

Therefore, if you are taking off in a crosswind or under adverse conditions, you can get into the air much quicker than you can without a flap. I have mentioned that this type of flap does not require any special bracing in the wing as would be required with the DFS type.

When you are flying at relatively high speeds in gusty air with the trailing edge flap, you actually strengthen the wing when you put the flap down because it moves the center of pressure inboard towards the fuselage. If you are flying at high speed you can lock out at your wings and they will actually bend down because at the higher speeds the angle of attack of your wing outboard of the flaps is negative and you are getting negative lift on that portion of the wing. The reverse is usually true on the DFS type dive brakes because you kill off your lift in the area that has the brakes, and then the wing from the brake to the tip has to carry the weight of the ship. They had some severe problems with this phenomenon on the Dart. In some of the early tests, the wings bent up very sharply. Subsequent calculations revealed that the spars were below the necessary strength in this condition. The spars had to be reinforced before flight tests could be continued. There is also a possibility with a trailing edge flap of getting some improvement in performance at low speed and at high speeds by varying flap position settings.

There is a definite increase in performance when you are running at high speed and can put your flaps up slightly because it lines up the fuselage better with the airflow and instead of flying along with a nose down, the nose comes up and the ship trims in a more level attitude. At low speed there are flap advantages because you can fly a little slower, which allows a smaller turn radius and allows the sailplane to circle a little closer to the core of the thermal. On approach, you have much better visibility as you are coming into the field.


Question: The arguments that have frequently been heard of dive brakes over flaps is that in the landing approach, when you get slowed down with the flap configuration, what do you do when you get in sink and need to dump the flaps in order to make the field? The argument is that, well, if you've got dive brakes, you turn them off and that improves the flying characteristics so that you can get the extra penetration to make it in to the field.

Schreder: This is really the only valid question against flaps but it really doesn't have any bearing and I'll tell you why. In the first place, when you make an approach with any sailplane, it is my theory that you should have 15 or 20 mph above your stalling speed, according to how gusty it is. You use either your flaps or your dive brakes, not to control your air speed, but to control your glide path as you approach. In other words, you always use the same pattern around the field and then when it looks like you are too high you begin cranking in your dive brakes or your flaps; and you maintain that same speed all the way down to your flare out. Now, this, I'm sure, is the best method to make an approach. In the case of using dive brakes, as soon as you see you are a little low you immediately retract but you still maintain that same speed unless you have to slow down to reach the field. But, if you are coming in on a normal approach angle and you can see you are getting a little below it you retract your dive brakes, You do the same thing with flaps. If you are coming around on final and it looks like you are a little too low, you immediately retract your flaps and, of course, your glide flattens right out. You continue until you get back on your glide path where you crank them down again. Now. the one thing you don't ever do is leave your flaps down and just keep pulling the stick back trying to zoom in to the field, any more than you would leave your dive brakes out and keep pulling your nose up trying to hold altitude to get to the field. So, I really don't think it is a valid question. Now, it's true, if you left them out and kept slowing down until you got to your stall speed with the flaps down and saw you weren't going to reach the field, there is nothing you could do.

Question; The question is, what if you slowed down too much for a flaps up configuration and, in reality, you're saying that this should never happen.

Schreder: That's right. They don't know how to use the flaps. And, the only people who ask this question are the people who have never used them. Comment: I've been flying HP's a lot since '66 and the situation he describes will never happen to a fellow who has made two or three or four landings with them. One of the advantages that Dick mentioned was a steeper approach. That immediately takes the situation and wipes it out. On final approach I'm anywhere from 200 to 400 feet higher than I would come in with a conventional glider and I could either overshoot the field flaps up or put both flaps on and undershoot the field. With these type of flaps you've got such a wide angle that you will never find yourself doing it short.

Comment: The advantage, then, is the slower speed on actual touchdown?

Schreder: That's right. If you did get yourself into a position where you couldn't make the field, you would be much better off with flaps than you would with dive brakes because if you are going to run into a fence or you are going to go into the boondocks across the road from the airport, you are much better off with flaps because when you see you are going to go in there, you just crank them down all the way, and, if you have a 10 mph wind you touch down at about 25 or 30 mph, whereas if you had dive brakes, you'd be going maybe 10 mph faster, and if you put the dive brakes out, you'd be going even faster. So, this is the extra safety feature of the flaps.