On the left, the death of “Pee Wee” Judge in a Wallis WA-117 gyroplane at Farnborough, England in September 1970 could have been avoided. The CAA Wallis claimed it was pilot error. It was not. This accident would repeat itself many times, in other gyroplane accidents, world wide. It was a condition of negative “gs”. Infact the accident in England were so horrific that gyros have been grounded and not allowed to fly a number of times. It is important to understand this accident and let other gyroplane pilots know what causes it so they can avoid repeating it in the future. It is also important to understand what “PPO” and “Gust Induced PIO” or just PIO is in gyroplanes and how to avoid designing gyros which are susceptible to this often catastrophic condition.
The reason for the many, many fatalities in England can be attributed to two key people, Ken Wallis and John Kitchen, who to this day still claim that gyros, with teetering rotor blades, do not need a horizontal tail. Both of these pilots have no technical understanding of the difference between static stability (which most gyros have) and dynamic stability which gyros (without a horizontal tail) do not have. They both preech, that gyros do not need a horizontal tail and many people have listened to them and have died as a consequence. In the USA, we know the truth.
Recent articles in various magazines have recorded the large number of recent accidents in gyroplanes because of PIO and PPO. My son, Eric M. Hollmann Ph.D. and I have developed a computer program that predicts this behavior and allows you to tailor and design the gyroplane to minimize PIO and prevent future PIO accidents from occurring. The program allows changing the gust intensity and duration and calculates the dynamic response of the gyroplane as a function of time with a very small and with a large horizontal tail plane of any size or location. This program is now available with the new landing gear and new rotor loads in our new edition (Edition 3) of MODERN GYROPLANE DESIGN. This book is the only book in the world which tells you how to design gyroplanes. Let us all work together by preventing future gyroplane accidents. The gyroplane is a safe aircraft if it is designed properly. The one on the left, designed by Ken Wallis, was not.
A good friend, John Tempest, in England just wrote that the Wallis WA-117 Autogyro Accident, Report No: 7/1974
from the British Department of Trade, Air Accident Investigation Branch is now available on the internet as a .pdf file:
This accident, in which Mr. J W C Judge, age 48, was killed in a Wallis, WA-117 gyroplane (G-AXAR), at the Royal Aircraft Establishment Aerodrome at Farnborough, England occurred on 11 September 1970.
Ken Wallis sent me a copy of one of the videos that was taken of the accident in 1971 and I sent him a report of what
I thought had happened. My conclusion was that it was a case of “negative gs,” just as the conclusion of this report.
It It was I that had coined this word. It is most probable that the reporter had read my report in 1971
and had come up with the same conclusion based on his own flight tests and observations.
This report is very well and carefully done and concludes what “I and my friends have been preaching for the
past 40 years.” By the way, two excellent videos were shot recording this tragedy.
The accident report can be summarized as follows.
1. G-AXAR just before the accident was flying at some 92 kts EAS and at that speed a relatively small relaxation of the push force required to maintain level flight such as to allow the control column to move aft one inch, could account for the last steep portion of the climb. To recover, the aircraft could have been flown out of this manoeuvre by increasing the bank angle and executing a “wing over” to maintain positive “g”. In the event the pilot moved the control column rapidly forward.
2. Once full forward control had been applied and sustained for approximately one second the aircraft was pitched rapidly nose-down the pilot would expect to try to counteract the motion as soon as the aircraft appeared to him to have recovered from the steep nose-up attitude which had caused him to move the control column forward in the first place. Some 4.76 seconds before impact the rotor head did, in fact, tilt fully back indicating that the pilot had moved the control column fully aft. However, the rotor rpm had by this time started to fall, negative “g” having unloaded the rotor, and as a result of the consequent reduction in control effectiveness the aircraft did not respond, continued to pitch nose-down until it reached the vertical and the rotor blades came into contact with the aircraft structure.
Theoretical instability analysis of the aircraft, without a tail plane showed a marked instability “stick fixed” as speed increased. This instability was not so noticeable in practice below 65 kts because of a contribution from the aircraft’s markedly stable “stick free” characteristics in which forces present at the control column tended to move the pilot’s hand in the correcting direction.
4. Some quantitative improvement in “stick fixed” stability resulted when the aircraft was fitted with an experimental tailplane.
5. Cause: The accident resulted from a loss of control due to the effect of negtive “g” when the pilot attempted to control a nose-up pitch which occurred during a manoeuvre when the aircraft was flying at a speed in excess of the authorized maximum.
6. With no tailplane, flight tests indicated that the aircraft was unstable in pitch when the control column was mechanically locked (stick fixed).
7. With tailplane, the aircraft appeared to be stable “stick fixed.” For other conditions, stick free and no tailplane, the aircraft appeared to be stable with possibly slightly increased damping of the short period oscillation. Martin Hollmann
Letter from Ian J. Lawson to M. Hollmann:
Dear Martin, I own an Air Command 532, which I built and flew whilst I was resident in the States. I fly both Fixed wing and Gyro, and have some little time in both, but when I first flew the Air Command, it scared the pants off me. I eventually modified the machine by fitting a Ken Brock type stick, and a Horizontal Stabilizer, which “Tamed the Beast” and it is now quite easy to fly. At least it doesn’t scare me anymore.
In its original configuration, I found it to be extremely unstable in pitch, and in my opinion, dangerous to fly.
I returned to the UK in June 91 to find that the CAA had “Grounded” all Air Commands here (There are 65 on the Register) because of the high accident rate. In the year prior to April 91 there were 6 accidents involving 5 single seaters, and one dual seat machine, resulting in 7 fatalities. The dual seat machine was flown by a CFI with a passenger.
As of now, the Machines are still Grounded. The investigations into the accidents have now been concluded, and in all but one case, the cause has been put down to “Pilot Error”. Reservations were made as to the airworthiness of the machines, but no definite conclusions drawn. However, the factory now produces its kits with a “Pod”, KB type stick, and a Horizontal Stabilizer. The CAA has indicated that provided all machines are brought up to current factory ‘spec’, they will authorize a test flying programme to prove whether or not in their opinion, the machines can be considered safe to fly again, failing which, they will permanently “ground” the Gyro.
No one seems to have made any investigation into the aerodynamics of the machine, and indeed, when it was first authorized to fly in the UK, neither was any research done then. Permission was given on the basis of “in service” experience in the States where it was claimed a total of 20,000 hours had been flown with no accidents due to air-worthiness. Incredibly the CAA accepted the statement!
I am therefore trying to look at the machine to try and find out why it is so unstable, but there is a lack of data available for the machine, and the manufacturers don’t seem to have any either, both airframe and Rotors. I’m not an engineer or aerodynamic expert so I am finding the task very hard.
I have a copy of your book “Modern Gyroplane Design” which I have owned for some years, and also a copy of your paper “Aerodynamics of the Gyroplane”, both of which are very helpful, but which don’t answer all of the questions I have, mainly due to my lack of expertise.
Referring for instance to the info in “Modern Gyroplane Design”, is it possible to calculate (say) the rotor drag without knowing the airfoil section ? I know the solidity ratio of the McCutchens, which is 0.035, and have come up with a figure of 60 lbs at 60 mph. I obtained this by working thru the equations in the book, but am doubtful of the accuracy, as the rotors are semi-symmetrical.
Also referring to your 3 degree “Rule of Thumb” for placement of the rotorcenter in relation to the CG, I find the Air Command does not obey the “rule”. When standing level on its wheels, the CG is 53″ below and 11.25″ ahead of the rotor center, which makes the rotor center 12 degs aft of the CG. To obey your rule, the gyro has to fly in a nose down attitude of 8 degs to put the rotor center at 3 degs. Before fitting the horizontal stab my machine did in fact fly nose down for best speed, but by how much I don’t know. With the mast raked aft at 9 degs (the Torque Tube has an 18 deg arc of movement in pitch) an 8 deg nose down flying attitude would surely mean that in neutral the stick would set the rotordisc at only 1 deg to the Horz., which wouldn’t be enough to fly straight and level. I have always understood you need about 7 to 9 degs to maintain this. This would suggest therefore that to fly straight and level there would only be about 2 to 3 degs left on the arc of travel in which to maneuver before the torque tube hits the back stop.
Would that be a reasonable assumption ? if so, might that account for the sensitivity in pitch? Or do we need to look at all the various moments such as engine thrust, CG, drag etc around the Rotor Hub ?
My Gyro “hang tests” with the stick neutral at a torque tube angle of 2 degs nose down, which I understand is within the generally accepted range. Therefore it seems to me that to obey your 3 degree rule, the convention on the “hang test” should be modified.
I have a host of other questions, which hopefully will be answered in the two publications I have asked for.
I would therefore be grateful if you can let me have a copy of your early paper “The Longitudinal Stability of the Gyroplane” and your Book “Flying the Gyroplane”. May I write to you again if I have any unanswered questions? I enclose a check to cover cost of book and airmail.
Sincerely, Ian J Lawson
Reply: Dear Ian,
Thank you for your very informative letter. I have put you on the mailing list for AIRCRAFT DESIGNS with a complimentary subscription. Following are the answers to some of your questions. The rotor drag does not change very much for different airfoils and the equations in Modern Gyroplane Design will apply to most rotors even though the airfoil shape may be different.
The angle that the gyro hangs under the rotor head is not that important as long as the blades do not hit the propeller or anything else. The rotor will fly at the required angle of attack to produce sufficient lift regardless of the hang angle. The horizontal stabilizer and fuselage will add damping in pitch and should help the pitch oscillations. The Air Command has a bad accident record here in the US also.
Many people have been killed when, after landing, the Air Command rolls over with the blades still turning. The high inertia McCutchen blades, which weigh about 77 lbs, snap off the mast exposing the pilots head to the ground. Usually the pilot’s neck is broken and he is killed. With the lighter blades this does not happen. If a pilot rolls over in a Bumble Bee for example, the blades are destroyed and the mast is bent but it does not break. The Bumble Bee blades weigh 28 lbs total. The pilot is not hurt. Good luck and my best wishes.
Sincerely yours, Martin Hollmann
A video of a landing accident of Ken Wallis is shown. Ken Wallis, the pilot, survives this unbelievable accident. The landing gear tread is obviously too small on this gyro designed by Ken Wallis.