STOL cuff boosts Cessna stats

Originally Published “Air Alaska” circa 1989

By Everett Long

"She called me and said she wanted to get rid of some aluminum. I saw the stuff in a box and asked what it was. She told me that her husband was experimenting with this – but nobody seemed interested, so she just wanted to get rid of it."

Marshall Quackenbush, a retired flight engineer for a major airline, describes how a revolutionary STOL (Short Take Off and Landing) modification for single engine Cessna’s almost went the way of the scrap heap.

"Marvin Davis, a Lockheed (Aircraft Corp.) engineer built the STOL kit for his brother’s 170B. "Quackenbush said. "He wanted to get his 170 into tight places."

From NASA research on spin/stall recovery and the wind tunnel at Lockheed, Davis developed the Sportsman STOL cuff. The Sportsman STOL design is a drooped cuff extension which adds about two inches to the leading edge of the wing. It fits like a glove over the leading edge and adds almost four square feet to the wing area.

"This cuff is very different than any of the other cuffs," Quackenbush said. "None of the other cuffs increase the wing area like this does. It almost doubles the mean camber which almost doubles the air across the top of the wing."

More air flows over the wing increasing flap efficiency and aileron control. Stall performance and spin control are dramatically improved. A common hazard to student pilots and hunters is the low speed high bank turn with cross controls. The lower wing stalls, instantly flipping the plane inverted at low altitude. "I have had people try to get the airplane to go into this inverted condition," Quackenbush said, "and nobody has been able to do it yet."

When Davis developed the STOL kit for the Cessna 170B he completed the Federal Aviation Administration STC certification. He continued development work for other wings and was planning to market the Sportsman STOL cuff after he retired. Davis died before he could realize his dream.

Davis’ wife tried to interest others in the STOL kit unsuccessfully and that is when she decided to clean out the garage. "I asked her to let me take it and see what I could do with it," Quackenbush said. "I put it on my old 172 and I was amazed. The first time I took off I knew I really had something."

NASA was doing some research by adding a leading edge cuff to the outer section of the wing for better spin control. "It was the same kind of cuff." Quackenbush said. "I called NASA and explained what I was doing. They came out made a full size tracing of the airfoil and said that it was almost the same thing that they were doing."

"If you try to go into a full power stall – it will not stop flying – and never go into a stall. I went along with the FAA when we certified the 182. They couldn’t get it to stall. They had to get a stall (speed) figure so they had to go into a dive, pull it almost straight up to a near stall – just to get a figure. They established 36mph at full gross for the Sportsman STOL, Cessna 182."

"My biggest customers are floatplane pilots. You can’t fool a float plane pilot on aircraft performance when every take off is a maximum effort. The airplane’s more stable and you have aileron control, even in a stall. The biggest advantage is safety – all these other things are benefits," Quackenbush added.

"Another customer uses a 206 for parachute jumping. Before, he always had to use maximum takeoff performance and almost all of the runway. Now he makes an intersection takeoff, and reduced his climb to 10,000 feet by three minutes."

Aileron gap seals are included in all kits except the Cessna 206. "I don’t include wing (stall) fences," he added. "I don’t think they are worth carrying around. NASA’s research agrees. I also think flap gap seals are a detriment. When the flaps are up you gain some speed, but with the flaps down you loose efficiency," said Quackenbush.

Another added safety feature is the change in the glide ratio. It has almost doubled the gliding distance for the same speed. When asked about claims of increase speed Quackenbush said. "I don’t claim a speed increase, which Davis did in his notes. I have customers who say it has, usually with float planes."

Jim Magoffin Jr., Fairbanks, compares over ten years of flying a Cessna 185 on floats to a Sportsman STOL modification. "Really makes a difference on performance. I’ve been flying this plane since 1977. I noticed that on takeoff my usual technique was to get up to about 45 or 50mph indicated, lift the right wing, accelerate, then lift up the other float. With the STOL kit I get up to about 43 to 45mph indicated and it just lifts right off the water – without raising a wing.

"The times I have used a maximum liftoff it really wants to climb fast, before you get anywhere close to the published maximum climb angle speed. It is a remarkable difference and you get off the water quite a bit quicker."

"Another remarkable change is when I’m coming in to land," Magoffin said. "I used to come in at 80mph indicated, go into a nice flare and touch down. Now I come down at 70 to 60mph indicated. When I flare it doesn’t feel like it is trying to drop out or descend real quick. It feels like it still has a lot of lift at very slow airspeed. I think that actual touch down is now about 50mph or a little less."

"It feels more stable at low speeds. Before at 70mph things would get a little mushy. Now it has very good aileron control down at 60mph. On wheels it was even better than that, like, at about 55 indicated it feels real good. You don’t feel like your getting on the edge of control," Magoffin added.

"When it was still on wheels I tried some stalls with very little power, about 12inches of manifold pressure. By the time it started to buffet and stall it was real close to 30mph indicated. It would not drop past the horizon. It would lower – pick up speed to about 40 and would be flying again. I would hold it and it would slow to 30 again, dip the nose, and come right back to 40. It was remarkable. I’m real happy with it. It feels like you have a lot more safety built into the plane. It feels better when taking off and landing."

"You need to talk to Gary Pease at Fish and Game," Magoffin said. "They have this kit on their 185. Gary is the kind of pilot who is very meticulous about his flying. I’ve seen him come in and land with that STOL kit on, and it almost looks like he is flying a Super Cub."

"We had looked at several options, including the Robertson STOL kit," said Gary Pearse, Fairbanks, a biologist and pilot for the Alaska State Fish and Game sport fisheries division. "I have about 200 hours in a Robertson converted 185. Then we heard about this cuff (Sportsman STOL) sold through Turbo Tech in Vancouver, Washington. They referred me to LindAir in Victoria, B.C.

"All cuff manufacturers claimed the same thing, improved stall performance, better lift over drag ratio, better glide. I asked pilots who had other kits on their 170s and 180s. None could come up with any real advantage. So I called LindAir about the Sportsman kit – they were ecstatic. They said performance was as good, if not better, than figures claimed by the STC holder." The Sportsman STOL kit was installed on Fish and Game’s 1979 Cessna 185.

"Our main use is on floats in the summer," Pearse said. "We do a lot of low level tracking of radio tagged animals and fish. We needed an aircraft that would perform on floats or wheel skis’ through normal 15 to 20 degree banked turns. Without giving a stall indication at observation speeds."

"The 185 would not do this with standard wings. At 85knots we were getting a stall buzzer and real loose, soft ailerons. When tracking tagged Pike we get some real tight turns. With standard wings. I had gone up to high altitude and did a lot of stalls. When it lets go out of a turn – coming out of it at less than 500 was difficult. And we needed to get within 500 feet of the ground to track these fish," Pearse continued.

"After installation of the cuff we could operate at 65knots. In a turn – without high power application – without stall indication. We took 15 to 20knots in a turn off the safe maneuvering speed.

"On floats I cut my takeoff distance by two thirds at gross on a hot 85-90 degree Fairbanks summer day. There were times when we would have to abort and unload part of the airplane. I have not done that since installing the new cuff," Pearse said.

"Here is another problem we had with the standard Cessna cuffed wing. We were getting into the air in ground effect and the tail would drop slightly – and hang there. At 55 knots you couldn’t make a positive climb out until you gained more airspeed. With the cuff we come out of the water at 40-42knots."

Pearse recorded stalls on floats at 42 statute miles per hour full flaps. "We are getting a clean stall at 48, and that is with full aileron control down and through the stall."

"On wheels or skiis they (Sportsman STOL) recommend 30 degrees flap prior to takeoff. I have tried 30 and 30 degrees with the standard wing. The 185 tends to wallow and skip a lot in deep snow. On wheels the gear would kind of come in and go out again two or three times and bounce down the runway. Now with 30 degrees of flaps and tail low, at a takeoff speed in the low 40s, it is immediately airborne and established in a climb attitude," Pearse added.

"One thing I could never do nicely, after 1700hours in the 185 on floats, was to make a nice full flap landing. I would come in, flare for landing at minimum airspeed. The airplane would either enter a high rate of descent close to the water – or would stall and make a hard arrival. I could not make a nice landing."

"Now I can come in at about 55 statute mph, full flaps – flare the aircraft – reduce the power – remain in ground effect above the water – and settle on."

"In a glassy water situation an abrupt arrival is a disadvantage. Before I would carry about 20 or 30 degrees of flap with a 200 or 300 feet per minute rate of decent. Now I can come in with a normal 500 fpm rate of descent. Flare above the water and let it settle with full flaps at minimum airspeed, land in calm water and in a short restricted area. I could not do that before. The only difference is that cuff."

Northland Aviation Services, located on the East Ramp of the Fairbanks International Airport is the statewide distributor for Alaska. Jon McIntyre, Northland’s owner said, "We’ve installed other kits before, such as Mid-America and Horton. The results from our customers were never very definite. But with the Sportsman they all come back grinning from ear to ear. It is an entirely different airplane."

"Low speed handling is solid as a rock," McIntyre added. "In a full stalled condition you can still pick a wing up with half aileron deflection. Before you would have to go lock to lock to have any effect. You have to re-learn to land the airplane. It puts the same airplane into the same airstrip with a greater margin of safety," said McIntyre.

"We have removed a Mid America cuff for guy and installed the Sportsman cuff. There was a dramatic improvement. The biggest problem is the disbelief people have about claims concerning this modification. Myself included. That is – until they get a chance to fly one. It is a 180 degree turn around in belief. It is hard for people to accept just how good this cuff is, and does what it does.

"You will have to get used to flying at least 5knots slower on landing or you will overshoot your touchdown. I recommend that a pilot works this 5 knots at a time. On the first try you may overshoot the runway." The Sportsman STOL cuff greatly adds to the low speed performance of the Cessna wing. But more important is the added safety." McIntyre concluded.