The key to rotary wing aircraft – is the “wing”
Gyrocopter rotor blades are not only the most intriguing part of any rotorcraft but they are also the most important for flight safety and performance. About 37 years ago, 1 set about designing a two-place gyroplane, which I called the Sportster and realized that the blades are the key to making a ship fly. As a result, I spent three years designing and building the rotor blades for my two-place gyro.
We wrote a gyroplane-performance computer program to optimize the size of the blades, determining that for a two blade teetering system, the maximum disc loading should not be over 1.8 lbs/sq. ft. and that the optimum solidity ratio is 0.035.
The disc loading equals the aircraft weight divided by the rotor disc area and the solidity equals the blade area divided by the disc area.
For the 1,100 lbs gross Sportster, this meant using a diameter of 28 feet and a blade chord of 9.00 inches. These blades were built out of aluminum; first flown in 1968 and worked well.
When in 1970, the 1,500 lb gross McCulloch J-2 gyroplane came on the market; I was surprised that Mr. Jovanovich had designed this aircraft around the Hughes 269 helicopter rotor system, which had a diameter of 25.3 ft and three blades with a chord of 6.83 inches each. Given this size, the disc loading is almost 3.0-lbs/sq. ft. There is no way this gyro could perform well and of course, it did not.
The J-2 was a very attractive, little, two-place gyro but its performance, even with a 210 hp engine, was poor. Flying around the pattern with my friend Jerry Breuner in his 210 hp J-2, we barely made it to 200 feet altitude. We made one circuit and landed. In the Sportster we had no performance problems flying with the both of us even thought the Sportster is only powered by a 130 hp Franklin engine.
My friend, Dr. Igor Bensen had experimented with four blades on his Gyrocopter instead of the usual two. This only increased the solidity ratio and it does nothing to the disc loading. As is the purpose of most experiments, Dr. Bensen gained some information about the problems with four bladed system and the design was abandoned.
More recently, Vittorio Magni experimented with four blades on his gyro and to his fortune, quickly abandoned the design. He had not incorporated a lead lag hinge in his four bladed rotor system.
When using more than two blades, the blades must have a lead and lag hinge in addition to the flapping hinge to prevent rotor blade fatigue since the blades accelerate and decelerate in the plane of rotation as they move 360 degrees around the azimuth. The acceleration is the same for opposite gyrocopter rotor blades so there is no need for the lead lag hinge with the two-blade system as we use on most sport gyroplanes.
Frank Courtney, Cierva’s test pilot, had warned Juan de la Cierva about incorporating a lead lag hinge into the four-blade C19 rotor but Cierva failed to listen. One day. flying at about 250 feet above the ground in a CI 9, one blade fatigued and separated. Luckily, Frank Courtney survived but he ended up in the hospital and left the Cierva Autogyro Co., never to fly gyroplanes again.
I met Frank Courtney in Chino, California at an airshow and offered him a ride in my Sportster but he turned
me down. He was 85 years old at the time. However, from the day of that accident, lead lag hinges were used on all rotor systems with more than two blades. Even in rigid rotor systems in which the blades are supposed to be mounted rigidly at the hub, a small amount (0.5 to 1.0 degrees) of lead and lag must be allowed. The French discovered this when using redesigned Bo 105 helicopter blades on their SA-340.
Another not often known secret in gyrocopter rotor blades design is to incorporate a small amount of tip weight into the blades. In the Sportster blades I use a 2.6 lb steel rod. This tip weight does wonders.
It reduces the coning angle, which increases the performance of a helicopter or gyro.
It increases the inertia of the rotor for better autorotation. The early Robinson R22 helicopter did not have tip weights and pilots complained about poor autorotation (2 seconds to put down the collective for autorotation when the engine stopped) and high control sensitivity. I met Frank Robinson one day and told him about rotor tip weights and asked him why he did not use them on his little helicopter. He said that he simply did not think of it. Two weeks later a bulletin from the Robinson Helicopter Co. came out announcing that all R22 blades would be retrofitted with tip weights.
It increases the inertia mass of the disc for lower control sensitivity.
It reduces vibration.
GYROCOPTER ROTOR BLADES :: THE CLEAN AIRFOIL
The airfoil shape of the blades is also important for optimum performance. In the early years, I selected a NACA 8-H-12 airfoil for all my blades because it resembled the shape of the blade sections used by the early gyroplanes.
Max Munk designed this airfoil also known as the M7, at NACA in 1923. Furthermore, Figure 9-11 in the book “Aerodynamics of the Helicopter” by Gesso and Myers, shows that for this airfoil, operating at low section lift coefficient as in a gyroplane, a significant drag reduction is realized over other airfoils.
If you are designing a gyro or helicopter, I strongly recommend getting a copy of Gesso and Myers. It has been a bible to me and many other rotorcraft designers.
Consider that the airfoil and blade drag reduction could only be realized if the blades are clean and free of bugs. Exposed rivets, on the outside of the gyrocopter rotor blades, eliminate this drag reduction, which comes about from laminar flow.
The early Sportster blades were riveted aluminum and did not achieve the low drag that they could have. Although adhesive was used in addition to rivets, I did not trust the bonding. It is extremely difficult to bond aluminum and stainless steel. My opinion reflects the attitude of many in the aerospace industry and the “glue and screw” philosophy is still used today when joining metal by most aerospace companies. However, bonding composite materials such a fiberglass and graphite (carbon) works well.
ABOVE: Ken Brock and Hollmann at El Mirage Dry Lake, with the HA-23 Rotor. 1981.
In 1980, I designed composite rotor blades using the NACA 8-H-12 airfoil for the larger, 750 lbs, single seat gyros that were using converted auto engines (VW, Revmaster) and aircraft engines such as the Barnett. The rotor diameter of the HA-23 rotor was 23 feet, to give a disc loading of 1.8 psf and a chord of 8.00 inches for solidity of 0.037. An extruded aluminum leading edge was used with a styrofoam trailing edge. The gyrocopter rotor blades were covered with two layers of fiberglass with a smooth outside surface.
I telephoned Ken Brock to test them on his glider. He had a spring scale attached to his tow cable so we could read the drag during towed flight. We were all amazed at the increase in performance over existing metal blades. See chart I for the performance comparison. I also called my good friend, John Bond, to test the blades on his Sky Dancer. Again a significant performance increase was realized as shown in table 2.
Back then we realized that a rotor blade with good and clean airfoil can achieve laminar flow. Bell Helicopter had refuted that laminar flow was possible on helicopter blades but then you can see why. When flying at low altitudes within the bug layer in Texas your blades will become bugged very quickly destroying that precious laminar flow. That is one reason I always clean the leading edge of my blades before every gyro flight.
For a jump-start gyroplane, you may want to use a special blade airfoil that we have designed and it is better suited than the NACA 8-H-12 for this purpose. This airfoil has the Jones-Joukowski parameters of Xc=-0.08, Yc=0.028, Xt= 1.025, Yt=0.02, D=0 and it can be viewed using the Oshkosh Airfoil program which can be downloaded for free from our web site at www.aircraftdesigns.com. Pressure distribution on the top and bottom of the airfoil and stream-lines can be viewed for various angles of attack.
Table 1. Tow Force Measurements by Ken Brock on Gvroglider with a Gross Weight of 350 lbs
|ROTOR MAKE||HA-23||Bensen||Rotor Hawk||Rotordyne|
|Tow force at 12mph||185||Not flying||Not flying||Not tested|
|Tow force at 35mph||85||125||125||Not tested|
|Tow force at 60mph||110||135||140||Not tested|
Table 2. Engine Speed Required to Fly. Tests Performed by John Bond on the Sky Dancer. Engine: 2100 cc Revmaster at 3200 rpm
|Flight speed of 15mph||2200rpm||3200rpm|
|Flight speed of 40mph||2500rpm||2800rpm|
Over the years the structural design of rotor blades for gyroplanes and helicopters has changed considerably. Early rotorcraft used wood blades as shown in figure 3.
Bensen Gyrocopter Rotor Blades
The first blade section shown is for a Bensen blade that was glued together out of wood. A steel strap ran the length along the bottom of the blade and it was designed to take the centrifugal load. The steel strap was attached with wood screws. At about the 75% radius of the blade length, a lead weight was extended past the leading edge to balance the outboard section of the blade. A small trim tab attached to the trailing edge was used to help reduce the pitching moment. These blades were very flexible but many people, including me, learned to fly with them. Not shown are the Bensen metal blades, which used an extruded leading edge.
A flat thick bottom skin ran the length of the blade and was riveted to the leading edge. The thin top skin was segmented and shaped to provide an airfoil. This skin was made in segments so it could be formed by using the rollers of a washing machine in which the cloth were rung out after washing. Again rivets were used to attach the top skin to the leading edge and bottom skin. These brazier head rivets did not help the airflow. Neither did the gaps in the top skin since centrifugal pumping and the negative pressure on the top of the blades forced air out on the top surface. However, the blades were much stiffer and a big improvement over the wood blades.
Rotordyne Gyrocopter Rotor Blades
One of the early metal rotors were and still are the Rotordyne blades. My two friends, Steve and Bud Phanuef. made these blades. A 6061-T6 aluminum leading edge extrusion is used and covered with a 0.02-inch thick skin, which is wrapped around the leading edge and bonded to the extrusion and at the trailing edge. The skins are continuous and only a few rivets at the trailing edge are used to prevent peeling. Early blades did have bonding problems and a large void existed where the skins wrapped around the leading edge.
As such the leading edge was made blunt which helped the bonding but degraded performance. However, the blades were inexpensive and they became very popular. A large 9-inch chord Rotordyne blade was built as shown in figure 3. However, there was no ribbing in the T.E. section and the blades would deform under load changing their shape. These blades are no longer produced, (ed: Tracy Hansen has been producing the current 8-inch chord Rotordyne blades using bonding techniques currently employed by the aircraft industry).
Sportster Gyrocopter Rotor Blades
The early Sportster blades used ribs in the trailing edge made out of 0.040 thick aluminum. These ribs were spaced every 7 inches to keep the T.E. for deforming from airloads. 0.025-inch thick skins were riveted and bonded to the top and bottom of the L.E. These blades worked well and I estimate that about 20 Sportsters flew using these blades. However, they were labor intensive and the rivets did not allow laminar flow.
The blades were redesigned using ribs made out of 6061-T6 aluminum extrusions cut into one-inch length. The ribs are riveted to the leading edge extrusion and fiberglass skins are bonded to the upper and lower surface. The skins are made using temperature cured fiberglass/epoxy. These blades are aerodynamically smooth and wc arc presently building these blades for people who want a rotor that can lift up to 1,400 lbs. The HA-28 Sportster blades have a disk diameter of 28 feet and a chord of 9 inches and each blade weighs 34 lbs which includes the 2.6 lb tip weight.
Sky Wheel Gyrocopter Rotor Blades
After reading about the high performance of the HA-23 rotor, Mr. McCutchen designed and built 8-inch chord blades that used a L.E. extrusion made out of 6061-T6 covered with 0.09 thick wet layup up fiberglass using Derakane resin. No ribs were used and the blades were extremely heavy. Although the 8-inch chord is not large enough for a two-place gyro, many of these blades were used for those aircraft.
On single-place gyroplanes many accidents occurred which have led pilots to believe that, there is a dynamic coupling between the rotor and the airframe. The blades are no longer being produced, (ed: Although the McCutchen blade manufacturing has closed its doors, it is quite common to see 8-inch Sky Wheel blades on both two place and single place gyros at various fly-ins. – Canadian Home Rotors purchased the manuafacturing rights and equipment for the helicopter profile of this blade to fit on their Safari Kit Helicopter).
Dragon Wings Gyrocopter Rotor Blades
Ernie Boyette of Rotor Flight Dynamics produces an all aluminum-bonded blade, which has a chord of 7 inches and a diameter from 22 to 25 feet. The leading edge is made of an extruded 6061-T6-aluminum section. The blades are light and balanced about the 25% chord at the tip and they are ideal for the single place, ultralight gyroplanes, such as my Bumble Bee. For his price of $1,195 for a set of blades, I cannot fabricate composite Bumble Bee blades. As such I recommend contacting him.
Rotor blades extruded out of one piece of aluminum without a built up trailing edge and constructed out of 6063 architectural aluminum are very low in strength and completely unsuited for this application, (ed: According to Mc Master-Carr catalog page 3104, the yield strength of 6063 is listed at 21ksi while the yield strength of 6061 is 40ksi) The wall thickness is 0.09 inches thick for the T.E. and as such, the blades do not balance about the 25% chord. Recent improvements have seen the use of a 6061 spar though the is no difiniative reports of how or if it has improved the design.
ABOVE:John Bond inspects the Hub of the HA-23 Rotor Prior to Test Flights as Rick McWilliams looks on.
Gyrocopter Rotor Considerations
If you are going to design your rotor blades or just purchase a set, you should consider the following items. The section located at the outer 50% of the blade radius should balance about the 25% chord of the blade section.
The stresses in the blade section must be determined and shown to be less than the maximum strength of the blade material. My book “Modern Gyroplane Design” tells you how to do this and there are a number of computer programs in it that can help in determining stress and loads in the blade. See my web site at www.aircraftdesigns.com if you wish to purchase this book. If you have any doubts about the strength of the blades you are buying, ask the manufacturer for a stress report.
Bonding to aluminum or steel is difficult and requires an oven and aluminum hot phosphoric etching process such as specified by Boeing Spec. 555 or equivalent, (ed: Jim Vannick of Sport Copters uses such a process in the manufacture of his blades and he agrees that the Boeing Spec or it’s equivalent is required for the construction of quality bonded blades).
Calculate the blade frequencies as a function of rotor speed and generate a Spoke Diagram (called a Fan Diagram at Bell) to assure that the blades are not operated at one of the rotor speed harmonics.
Blades must balance within an ounce about the teetering hinge line.
Blades must be rigged so that a string passing thru identical points on the tip of each blade will pass thru the center of rotation within 0.03 inches.
The blades must be tracked within a 1/4 inch. This may mean you will need a pitch adjustable hub so you can change the pitch of each blade until they track.
When buying blades, make certain that the blades are designed for the weight of your aircraft. A number of blades are being used on two place aircraft and they have not been designed for this purpose.
Buy blades with tip weights. But if the blades do not have tip weights, do not install them since the weights will significantly change the stress in the blades.
For a two bladed teetering system the undersling should be set properly for your blades to minimize stick vibration.
Fly safely and have fun. Martin Hollmann.
TOP IMAGE: Ken Wallis rotor blades at top of page…, first a .125 inch birch ply skin is laid. Then a strip of Ultra High Tensile (UHT) strength steel is glued to the plywood skin with Aerodux 185 resorcinol-phenol-formaldehyde resin adhesive. Then multi layers of Hydulignum (a Horden-Richmond Aircraft Company product of heat pressed thermoplastic resin laden plywood that was used to make propeller blades for British Aircraft during World War Two) laminates that are laid up to form the “D” structure spar, followed by one more layer of .125 inch birch ply skin. The outside of the blade is then covered with Madapolam, a cotton linen fabric and aircraft grade dope is used to glue down and seal the Madapolam.
It is to my understanding that the wooden rotor blades on the WA-116 gyroplane “Zeus III” that Ken Wallis often seen flying in were manufactured by him well over 30 years ago! Now that is unlimited life rotor blades!