George Buckley's Experimental Ornithopter Design

Email gbuckley@rosenet.net

This is my radio controlled ornithopter. I call it Hula. It first flew in Sept 2003 and has had many successful flights. It differs from the usual "bird" type ornithopter in several ways. Its wings are not attached to the "body", but are supported on hinged struts half way out on each wing. The two wing panels rock on their struts and they appear to undulate when it's airborne. Hence the name "Hula". The wings are twisted by the crankpin rather than depending on air pressures and inertia as the usual "bird" wings do. In my opinion that makes its flight characteristics predictable over a large range of sizes, weights and air speeds. I have a computer program that predicts performance, but without a wind tunnel or flight data recorder, I can only say that it seems to perform as predicted.

First Hula Video 11/01/2003...10 seconds If it will not load or run, read the notes at the bottom of the page.

A rocking wing has some advantages over "bird" type wings. Some are evident in the video and construction: Balanced and lower wing inertia, less spar weight, lighter support structure, handles higher "g" forces, wings are supported by struts instead of the crank, no engine torque is needed to support the wings, better stability, little danger of wing tips striking the ground during take-off and landing, balanced average lift as downflap and upflap occur simultaneously and there is a lot less shaking. There is less chance of rolling inverted at slow flap rates because its flaps at twice the rate of a bird type wing and the wing is well above the center of gravity. Some of these features may not be important for a small model but would be on a full size piloted ornithopter.

Specifications: Span= 25", cord= 5", weight= 2.35oz, motor= DC5-2.4, gear ratio= 32 to 1, battery= (2) Kokam 140, flap angle= 26 degrees up and 18 degrees down, radio= SH&R RX72 hybrid with esc, servos= (2) Cirrus CS10BB.
It was built using 1/16"square and smaller basswood, balsa, 1/32"ply and 1mm carbon rod. The covering is undoped Airspan.
The 6% airfoil has a balsa arc at the front and 1/16"x 1/32" basswood to the rear spar. No attempt was made to reduce drag. The spars and struts have unsanded corners. The L/D is around 4 to 1. The longest flight was 12 minutes and it thermals easily. Other designs are under construction.

The basswood structure was intentionally built to have minimum strength so that it would reveal a weak point if a part failed. The front of the curved fuselage broke during a trim glide. I should have wet the longerons before bending them. I replaced them with carbon rod so I could do hand catch landings. I learned not to let it contact the ground while the wings were flapping. The crank became tangled in weeds and broke two struts and a wing leading edge. I patched them with wood splints and thread. Despite its fragile appearance, it has resisted all attempts to overstress it with loops from a dive, hard pull ups and steep banks. The wings bend but do not break. Except for its slow climb, it handles as well as a propeller model. That's good to know if you want to pilot one.

Hula Video 12/17/2003...41 seconds If it will not load or run, read the notes at the bottom of the page.

Last year, my growing interest in ornithopter flight prompted me to purchase an "Air Hog". It had lots of power and it climbed rapidly. It was a wild thing but it would eventually go into cruise mode as the air pressure dropped and the speed increased. Then it would glide like a rock and crash. I wondered if it could be tamed. I wired the wings level and tried glide tests. The wing membrane distorted so much that it had little useful lift and a high stall speed. I also tried using the wings with a geared electric motor and radio control without success. At that point I realized I was working with too many variables and I should try a more predictable wing. I abandoned membrane wings, with their unsupported trailing edges, and built a rubber band powered model with a spar at the trailing edge. I had read "Man Powered Flight" by K.Sherwin Copyright 1971 and in the chapter on flapping wings, he wrote about having the inboard part of a wing move in the opposite direction of the tip as a way to smooth out the lift. I don't think he meant hinging the wings half way out to achieve it but that came to mind. The 16" span, 7gm model flew well. Its crank bent the flexible 1/32" x 1/16" front spar to achieve flapping. It flew slowly and could do 3 laps in my living room. When the rubber band ran down, the spar would often automatically straighten out into gliding position. Seeing that the inertia-driven wing twist was greatest at the top and bottom of the flap instead of mid-flap where the flap velocity was greatest, I realized that I should control the wing twist otherwise the twist would only be partially correct at one flap rate, weight and foward velocity. I wrote a computer program that allowed me to change variables and compute the wing twist needed at all flap angles, flap rates, span positions and foward velocities of interest. However It did not tell me how to achieve the required twist. I decided to try various amounts of crank-driven twist and then change the other variables until a satisfactory solution had been found that matched the motor and battery power I wanted to use.

One my goals was to determine if an Ornithopter with an L/D of 4 would fly without needing to use vectored thrust to augment the lift. If it would, then it would be flying faster than its gliding stall speed and it would not stall if it stopped flapping. Hula flys in a level attitude and transitions to gliding without elevator input.

Although my analysis is incomplete and much larger models must be built, I believe the chances are good that Hula can carry a man. The main problem will be wing inertia. Although the inertia is balanced, it takes a large amount of power to accelerate and decelerate a big wing twice during each flap cycle. In the nineteen-seventies I designed, built and flew two single surface airfoil and four double-surfaced rigid wing hang gliders. Three were flying wings and three had conventional tails. Three of them were foot launched from flat ground with 12 hp engines using 24" to 28" propellers. They all had an L/D of between 8 to 10 and weighed 60 to 70 lbs. The engine and gas added up to 30 lbs. I am confident I can achieve an 8 to 1 L/D with a strong and lightweight structure. Flight attempts will be made into a smooth sea breeze to reduce ground speed and enhance pilot safety.

In the 100 years since the Wright brothers invented the airplane, much theoretical work on ornithopters has been done but no one has achieved a flapping take off and sustained flight with a pilot aboard. Is it really that difficult? The principles of flapping flight are not trivial, but have been understood by many who expended the time and effort. Of those, only a few have attempted to build and fly their flapping machines. I believe the Wright brothers could have built and flown an ornithopter if they had tried. They invented wing twisting, their propeller formulas would have told them how much to twist and they owned a wind tunnel. That was 100 years ago. Birds didn't evolve to perfection without mothers and fathers. More experimental projects are needed. Lets get to work!

Notes:
These videos are encoded using "Mjpg" also known as "Motion jpg" or"Mjpeg". I know they can be viewed with QuickTime, Windows Media or Realone players if you have their recent software upgrades. I check the videos daily to see if they are downloading correctly.

This is my first website and I am learning as I go. Constructive comments are welcome.

Better quality photographs and videos are comming soon.

Email gbuckley@rosenet.net

Links:
Ornithopter pilot and builder Patricia Jones-Bowman http://www.ornithopter-pilot.com/

Copyright 2004 by George Buckley. All rights reserved