“If gyros are supposed to be the safest aircraft in the world, why are there so many gyro crashes?” my non-flying buddy asked. It’s usually some sort of gyrocopter PIO, PPO or sinking so I answered; “Some of the older gyro designs aren’t dynamically stable in pitch. Low-time pilots get these gyros to oscillating, lose control, and crash,” I said.
“You’d better explain what dynamically stable in pitch means,” my friend said. “I’m sure it’s got nothing to do with singing.”
“Okay, Pitch is the axis of movement fore and aft of the center of gravity,” I said, “If a gyro is dynamically stable in pitch, it will pitch nose down in an updraft and nose up in a down draft or remain pitch stable and just rise with the updraft or sink with the downdraft.”
“So what happens if a gyro is not stable in pitch?” my friend asked.
“Let me introduce you to Larry Neal, a gyro pilot for almost 30 years, who has designed, built, and tested his own gyros since the early 70s,” I said. “He’ll tell you about Pilot Induced Oscillation or PIO and Power Push Overs or PPO.”
Larry Neal Explains: Pilot Induced Oscillation (PIO) and Power Push Over (PPO)
“Gyrocopter PIO and PPO are usually caused by a gyroplane that is dynamically unstable with a high thrust line and no horizontal stabilizer,” Larry said. “A high thrust line occurs when the thrust vector of the propeller is above the vertical center of gravity of the aircraft.”
“What is a high thrust line, again?” my buddy asked.
“When the thrust line is above the center of gravity, that’s called a high thrust line,” Larry said. “This thrust offset puts a torqueing moment around the center of gravity of the aircraft and causes it to want to pitch forward upside down. The only thing keeping it from flipping upside down is the drag from the rotor blades at the top of the mast.”
“So, with a high thrust line, the thrust from the engine is trying to tip the gyro forward,” my friend said. “And the drag from the rotor blades prevents that from happening. But what if the drag from the rotor blades disappears?”
A power push over can happen in less than 2 seconds…
“If the drag from the rotor is absent for less than a couple of seconds, then the gyro can flip forward upside down, and the gyro pilot will fall inverted to his death, a victim of the Power Push Over,” Larry said. “Because the high thrust line and drag of the rotor are continually fighting each other, the gyro is pitch unstable.”
“Oh, great,” said my buddy. “Is that the only thing that can make a gyro unstable?”
Another design flaw discussed
“Another cause of instability occurs when the lift vector of the rotor Is in front of the horizontal center of gravity of the gyro,” Larry said.
“What’s a lift vector?” my friend asked.
“When rotor blades are developing lift, that lift should be measured from an imaginary line drawn straight up perpendicular through the center of the plane of rotation of the rotor blades. That’s a lift vector,” I said.
“Sines the rotor blades are on top of the gyro, you can extend the lift vector line downward to see its relationship with the center of gravity. With gyros, the lift vector of the rotor blades is slanted backwards from the vertical about nine degrees.”
“When a gyro pilot flying a gyro with this design flaw gets into an updraft, the nose of the gyro pitches up. In a down draft the unstable gyro pitches down. The low-time, self-taught, or inexperienced pilot usually over-controls by pushing the control stick a lot more than is needed.”
“I can see why the pilot would over-react,” said my buddy. “If I flew into an updraft and the nose of my gyro pointed way up, I’d want to bring it back down pronto.”
“When the gyro reacts by pitching more than the pilot intended, he puts another over-controlled input in the opposite direction and a Pilot Induced Oscillation is started,” Larry said.
“This oscillation usually increases in amplitude as the gyro’s airspeed slows down. Usually at the end of the third oscillation, the gyro flips upside down from the thrust onset as the rotor is unloaded.”
“What do you mean when you say ‘the rotor is unloaded’?” my friend asked.
“Remember, the lift and drag of our rotor blades is created by air flowing up through them in autorotation,” Larry said. “When air goes in over the top they slow down very fast.”
“And unless the pilot is trained to recognize porpoising or PIO and reacts immediately, the PIO can become PPO,” I said. “But a horizontal stabilizer would help, wouldn’t it, Larry?”
“A gyro with an adequate horizontal stabilizer would help counteract this high thrust line torqueing moment, as long as enough airspeed is maintained for it to function,” Larry said. “It would also be more stable in dampening out the pitching moment from updrafts and downdrafts.”
“My friend visualized a gyro pitching up, then down, then up more, then down more, then up even more, then flipping upside-down and tumbling to the ground. All the color disappeared from my friend’s face as he asked, “What could be done to prevent such a horrible accident?”
“The REAL SOLUTION is to not fly a gyroplane with a high thrust line and no horizontal stabilizer,” Larry said, “why take a chance when a simple design change can save your life?”
Preventing PIO and PPO Accidents
“As a gyro instructor, I try to persuade my students to only purchase a proven design that is center-line thrust and is equipped with an adequate horizontal stabilizer,” Larry answered. “What’s center-line thrust?” My friend asked.
“You can see it in a single-engine Cessna. Draw an imaginary line through the center of the propeller and straight back,” I said. “The line goes through the cockpit and through the vertical center of gravity of the airplane. When the thrust line goes through the center of gravity, it’s called center-line thrust.”
Speaking of his gyro students, Larry continued, “If they already have a gyro that isn’t centerline thrust, I try to persuade them to change it into centerline thrust before they fly it. I tell them to either fix it or sell it, but don’t fly it. Your life is worth more than money.”
“Will center-line thrust prevent PIO and PPO?” my friend asked.
“The center-line thrust gyros that I have flown are practically PIO and PPO resistant,” Larry said. “There isn’t any torqueing moment to flip them upside down. I recommend a thrust line that is exactly center-line or up to and not more than two inches under the vertical center of gravity.”
“What can a pilot do to avoid getting into PIO?” my friend asked.
“Most gyro flying can be done by moving the stick no more than one inch in each direction,” Larry said. “I also stress the importance of not moving the stick around a lot.”
“But what if the pilot gets into an oscillation anyway?” my friend persisted.
“If the gyro gets into some oscillation that is not wanted, he should hold the stick steady and reduce power. A gyro will settle out of most situations into a simple vertical descent,” Larry said. “Over-control has caused a lot of accidents.”
“Most gyro accidents have lack of gyro flight training as part of the cause,” I said. “Gyro flight training is so hard to get that some people think they can teach themselves to fly. But too many of them end up seriously injured or dead.”
“The situations you’ve been talking about involve wind,” my friend noted. “Can PRA chapters stop their low-time members from flying when it’s too windy?”
“We can’t stop anyone from flying, but we stress common sense and give personal advice as to whether our pilots are ready for a certain amount of wind,” Larry said. “We recommend that they accumulate several hours of solo flight time in no wind situations. They can work into flying with more wind as their flight skills and confidence levels increase.”
“New and low-time pilots can get into oscillations when there isn’t any wind, too,” I said. “But as Larry says, they need to start their solo flying in no wind to learn about how easily you can get in to a gyrocopter PIO situation.”
“If they only fly in no-wind situations, how can they ever learn to fly in wind?” my buddy asked.
“Our club owns a two-place center line thrust Air Command Tandem Elite trainer. I have trained students in up to 40 MPH winds,” Larry said. “Once they have learned to not move the stick around a lot and to use more rudder and throttle while flying, they realize that a properly designed gyro can handle wind very well.”
“Then I don’t understand why all gyros aren’t centerline thrust with horizontal stabilizers,” my friend said.
“The first popular small gyro was the Bensen GyroCopter, designed by Dr. Igor Bensen,” Larry said. “It had an aluminum rock guard below the propeller, but this wasn’t really a horizontal stabilizer. It was too small and located too close to the horizontal center of gravity to be effective in dampening out a PIO.”
“What about the thrust line on the Bensen GyroCopter?” my friend asked.
“The thrust line is about three inches above the vertical center of gravity on the stock Bensen gyros, and they will PPO in a heartbeat after a PIO,” Larry said.
“It seems like a lot of gyros don’t have horizontal stabilizers,” my friend said.
“All of the certificated gyroplanes like the Pitcairn, the Air and Space 18A, and the McCullough J2 were designed by real engineers and have huge horizontal stabilizers,” Larry said. “Only the homegrown variety don’t have them.”
ABOVE IMAGE: This pre-1992 Air Command single-place gyro has no horizontal stabilizer and has a high thrust line. The thrust line from the center of the propeller (shown in black) is high above the gyro’s center of gravity leaving gyrocopter PIO a real possibility. The new Air Command International company says this old gyro is not airworthy because it has no horizontal stabilizer. They recommend upgrading old gyros to centerline thrust. (Photo by Kerry Cartier)
“Back in the 80s, a Bensen gyro pilot designed a Rotax-powered gyro and named his company Air Command,” Larry continued. “His gyro had a seven-inch high thrust line, a short rear keel, and no horizontal stabilizer.”
“And was it a safe gyro?” my friend asked.
“After several Air Command accidents in Great Britain, the British government grounded all Air Command gyros in the United Kingdom,” Larry said.
“The Australians were smarter than the Americans, because in 1990 the Australian Sport Rotorcraft Association required horizontal stabilizers on all gyros,” I said.
“Didn’t anyone in the U.S. object when gyro kits were sold without horizontal stabilizers?” my friend asked.
“One man did,” I said. “I’ll tell you his story.”
Preventing PIO and PPO Accidents
“The original Air Command company was sold to two brothers in Florida. Far as I know, they had difficulty delivering kits,” I said. “Then in 1992, Red Smith bought the company’s assets and moved them to Texas. He didn’t buy the backlog of undelivered kits, but I’ve been told he delivered every Air Command kit that had been ordered, and paid for it out of his own pocket.”
“Red Smith is a professional aeronautical engineer,” I said. “He examined the original Air Command design, then determined it was not airworthy and should not be flown due to possibility of gyrocopter PIO. Then he did something unprecedented. He issued the first kit plane company equivalent of an Airworthiness Directive.”
“What’s an Airworthiness Directive?” my friend asked.
“The Federal Aviation Administration issues an Airworthiness Directive when there’s an unsafe part on a certificated aircraft,” I said. “Certificated aircraft can be bought in ready-to-fly condition, like Cessnas and Pipers. What was different was that Red Smith put out a safety bulletin describing unsafe parts on un-certificated aircraft for kits of earlier versions of the Air Command gyros.”
“And he said…” my friend said.
“Red Smith of Air Command International said that the old Air Command gyros were not airworthy unless they had horizontal stabilizers, dual-redundant masts, and a new control system to replace the pump-stick.”
“Try that in English,” my friend requested.
“Dual-redundant mast means the mast has two vertical sections, each with a cross-section of one inch by two inches, and either piece could support the weight of the gyro if the other one broke. They replace a single mast that had a two-inch by two-inch cross-section,” I said. “The new control system gives the same amount of rotorhead movement from the same amount of control stick input, making the control stick less sensitive.”
ABOVE IMAGE: This Air Command International single-place gyro has a horizontal stabilizer and has been upgraded to center line thrust. The front keel was raised 14 inches so the thrust line from the center of the propeller (shown in black) intersects the center of gravity, located approximately at the pilot’s navel. The use of a horizontal stabilizer and center line thrust enhance pitch stability against gyrocopter PIO. (Photo by Kerry Cartier)
“In 1992, Red Smith’s Air Command International offered an upgrade kit so all the older Air Command gyros could become safer flying machines,” I added. “A decade later, the upgrade kits are still available.”
“Did the U.S. Government require everybody to upgrade their old Air Command gyros?” my friend asked.
“Of course not! They aren’t certificated aircraft,” I said. “There are still maybe 3,000 old Air Command gyros out there somewhere, and most of them were never upgraded.”
Air Command International Makes Center-Line Thrust Upgrade
“In the year 2000, Red decided all his new models would have center-line thrust in addition to horizontal stabilizers to reduce gyrocopter PIO,” I said. “He wanted all center-line thrust changes on his newer designs to be backwards-compatible, so older Air Command gyros could be retrofitted with them, too.”
“Red remembered the 3,000 or more older Air Command gyros that needed upgrading, so he issued another Airworthiness Directive-type statement encouraging owners of the older gyros to upgrade them to center-line thrust,” I said, “Then he priced the upgrade kit at about a thousand bucks.”
“What do the upgraded gyros look like?” my friend asked.
“I have the first upgraded Air Command International gyro,” I said, “I had a 1994 version with dual – redundant mast, horizontal stabilizer, and the new control system. Larry Neal installed the center-line thrust upgrade, which puts the pilot seat 14 inches higher so the engine thrust line goes through the center-of-gravity.”
I added that my gyro looks kind of odd, with the high seat and a nosewheel that looks like it came off a chopper motorcycle (or maybe a chopper minibike). It can fly, stable, hands-off and feet-off, so I could care less how it looks.
Upgrades Are Available For Other Gyros
“Do any other companies offer upgrade kits?” my friend asked.
“A pilot named Ken Janulewicz pioneered a horizontal stabilizer upgrade kit for the RAF 2000, a Canadian gyro. He no longer sells them, though he’d sell his molds if someone wanted them,” I said. “Today, similar RAF 2000 horizontal stabilizer kits are sold by Retro Composites in New Zealand and by RAF-AfterMarket in Arkansas.”
“Just this year American Autogyro Inc., a Groen Brothers aviation company, developed a stability augmentation kit for the RAF 2000. It includes a center-line thrust kit, a suspension and landing gear kit, a horizontal stabilizer, and a tall tail kit,” I said.
“It takes about 40 hours to install, and at additional cost, RAF 2000 owners can have the factory in Arizona install the upgrade kit. They also offer a double-bearing rotorhead upgrade designed for the RAF 2000, which also works on gyros with a gross weight of 800 pounds or more.”
“What about the old Bensen GyroCopter? Does anyone offer any upgrades for it to reduce gyrocopter PIO?” my friend asked.
“Bensen closed its doors in 1988, as I remember,” I said. “Larry Neal’s company, New Horizons Components, offers a horizontal stabilizer and center-line thrust upgrade kit for the Bensen B8M GyroCopters, complete with an engine mount for the Rotax 503 if the owner wants to replace his old McCulloch engine.”
The PRA’s Recommendations for gyrocopter PIO
“Does the Popular Rotorcraft Association have anything to say about unsafe gyro designs?” my friend asked.
“Definitely,” I said. “In August 2002, the PRA Board of Directors said ‘the following five items may be helpful in making many, if not all, gyroplanes more stable: horizontal stabilizer that is installed parallel to the keel, horizon indicator, center-line thrust aligning the prop thrust-line more closely to the aircraft vertical center of gravity, proper amount of rotorhead travel per inch of cyclic travel, and proper disc loading.”
“What do you think of the PRA recommendations?” asked my friend.
“The PRA leadership took its responsibilities seriously and made smart recommendations that can make gyros safer to fly,” I said. “Applying the PRA recommendations is an insurance policy to make your gyro flying safer.”
“Some gyro kits were originally designed with center-line thrust and horizontal stabilizers,” I said. “One of the first, and the best-known, is the Rotor Flight Dynamics Dominator, designed by Ernie Boyette. He is one of the first designers to put a tall tail on his gyro, too.”
“There are several other gyro kits with center-line thrust and horizontal stabilizers, like the H-1 Racer, the 3D-RV, and maybe others,” I said. “All tractor autogiros, like Ron Herron’s Little Wing Autogyros and the Pitbull autogyros, have center-line thrust and horizontal stabilizers. Almost every commercial gyro kit on the market today has a horizontal stabilizer.”
“But the old gyro designs don’t,” my friend noted.
“Right, and it’s the old gyro designs I worry about most,” I said. “When a low-time gyro pilot, flying a gyro with a high thrust line and no horizontal stabilizer, gets into the deadly gyrocopter PIO or something he can’t handle, I’m not surprised that he crashes.”
“Flying a gyro with a high thrust line and no horizontal stabilizer is perfectly legal. Unfortunately for some pilots, it can be perfectly fatal, too,” I concluded. “A gyro upgrade could save your life.”
About the Major Contributors To This Gyrocopter PIO and PPO Story
Gyrocopter PIO Upgrade Kits – (Ed. details as per article original date “2003”)
Air Command International. P.O. Box 1177, Caddo Mills Municipal Airport Bldg. B, Caddo Mills TX 75135, 903-527-3335. E-mail:
American Autogyro Inc.. 3000 South Palo Verde Rd,, Buckeye, Arizona 85326,623-393-9451. E-mail
New Horizons Components. Larry Neal, P. O. Box 927, Boyd TX 76023, 940-627-9887 Retro-Composites, 1 Precelly St, Allanton, RD2 Mosgiel, Otago 9032, New Zealand. Phone: Country code 3 489 8222, Fax: Country code 3 489 8129. Email: RAF – AfterMarket. Larry R. Martin, Cabot AR 72023, 501-988-1345
Retro-Composites, 1 Precelly St, Allanton, RD2 Mosgiel, Otago 9032, New Zealand. Phone: Country code 3 489 8222, Fax: Country code 3 489 8129. Email:
RAF – AfterMarket. Larry R. Martin, Cabot AR 72023, 501-988-1345