The V22 tilt rotor Osprey is in the news.
This class of aircraft has a bright future. Man has dreamed of taking off from his back yard and flying someplace at high speed for a long time. Tilt rotor VTOL has made that dream come true. In my opinion the tilt rotor will replace the helicopter and take some conventional aircraft sales. This will be due to a higher cruise speed at a the same fuel burn. The helicopter will go the way of the Dodo bird IMHO. It won't happen overnight of course. It will take many years.
SCALED DOWN TILT ROTORS
The high power to weight ratio of the all aluminum Mazda 16B Wankel rotary engine and low fuel consumption compared to a small turbo prop engine combined with carbon fiber aircraft construction has made practical scaling down old vertical take off aircraft designs that did not enter production in the 1960's. The all aluminum Mazda 16B engine introduced in October of 2007 weighs about 185 pounds and is capable of about 285 HP at 7500 RPM.
Certain very fortuitous things happen when you are able to take off vertically. The first thing that happens is; it is no longer necessary to have a lot of wing area to take off and land. Wings are heavy as the bending load on the wing spar is high. A lot of weight can be taken out of the wing as the span and wing area are limited to only enough to keep the vehicle airborne at high speed. The cost of the aircraft compared to conventional aircraft is reduced due to less raw materiel required. Read carbon fiber.
Here is what one of the old tail sitters looked like.
Landing control can be enhanced with modern low cost electronics. Here is a low cost back up camera sold for use with cars.
A delta version is more stable when parked vertically compared to a conventional aircraft configuration.
Web sites for history back ground.
"EzyCopter employs a Coaxial Drive System (CDS) coupled with twin rotary engines and linked with innovative collective and cyclic controls. With the benefits of simpler design, lighter weight, more balanced and torque-free hovering capability, safety features of EzyCopter will exceed all expectations. While compact in size, the EzyCopter is the only recreational coaxial helicopter equipped with twin rotary engines, twin articulated rotors, four rotor blades, and correlated cyclic and collective controls, making EzyCopter easier and safer to fly!"
THE PERSONAL VTOL CONCEPT
The two rotor Mazda Wankel engine is behind the pilot and passenger and drives the coaxial props with a carbon fiber drive shaft removed from a Mazda RX8. The Mazda Wankel rotary engine is the only intermittent internal combustion engine with a power to weight ratio of well over one and durable enough to use in an aircraft. It has a 20% lower fuel burn per HP generated than a small turbo prop engine at only a slight weight penalty. It has a 35 year history of being used in aircraft and over two million engines have been produced for the automotive market. Dozens of light aircraft are now flying powered by the Mazda Wankel rotary engine. The RX8 version is currently selling at the rate of 65,000 engines a year. It is also very low cost for an aircraft engine. A Mazda rotary powered four place airplane achieved 205 MPH average in a recent 500 mile air race while burning only twelve gallons per hour.
The gear box is in the nose. Motor cycle passenger configuration is used to save length and weight. The delta has only 10 foot span. It will fit in a one car garage. It would still be twice as fast as any gyro or helicopter and have three times the range. No problem getting the structural weight under 300 pounds in a two piece blended wing body female mold. This will be real easy and quick to build as well.
Nothing new here. This was done back in the 60's. The 60's VTOL pilots had problems with landing because they did not rotate the pilot and he was ahead of the wing so visibility on landing was poor. It is easier to rotate the pilot for helicopter mode and place a window in the floor. Low cost closed circuit television systems were not available in the 1960's so they could also be used. People park their cars by looking in the rear view mirror. The main requirement is good control when in the helicopter mode.
Here is a chart from a Boeing technical paper on various load ratios on successful VTOL aircraft. Any new design should follow these guide line ratios.
I have added power to weight ratio rows on the bottom of the Boeing VTOL table.
This design is much easier to build than any VTOL aircraft design I have seen so far. The beauty about being able to use turbulent sections for the wing, is you can use 15% or better airfoil's (airfoil thickness is 15% of chord) This gives you the ability to make a really strong wing with minimal blended wing body structure because of spar depth and shortness. It also gives you more volume for fuel. This is much easier than building a scaled down Osprey.
I chose 50 square feet of wing area. Wing loading is 24 pounds per square foot. Way less than most tilt rotor VTOL aircraft which average 100 pounds per square foot. Tilt rotor VTOL aircraft do not need a lot of wing area because at 200 mph the dynamic pressure is 100 pounds per square foot. That is 5000 pounds of lift with a 50 square foot wing at a C sub L of one. 1200 pounds gross will give us a C sub L of only .24 which is no problem. The high wing loading compared to conventional aircraft result in a better ride while in turbulence.
At the target speed of 300 MPH the dynamic pressure is 234 pounds per square foot and 11,700 pounds of lift at C sub L of one. The required C sub L would then be only .1 (Point one.) Way down in the minimum drag region of the L/D curve.
The HP consumed by the small wing alone is .01 C sub d times 50 square feet times 234 pounds or 117 pounds of drag at 440 FPS. That is 51,480 ft pound second or 94 HP at 300 MPH. We have 285 engine HP available. A rotor eff. of .8 gives us a net thrust HP to work with of 228 HP. Here is a chart from Abbott & Von Doenhoff "Theory of Wing Sections that illustrates the advantage of small wing area at high speeds.
The gross weight needs to be less than 1200 pounds for available HP and rotor disk loading reasons. That gives us a .237 HP per pound installed HP to gross weight ratio. Right in the ball park with other designs. Payload and fuel must then be 534 pounds. Two 175 pound FAA people are 350 pounds. That leaves 184 pounds of fuel or 31 gallons. At 10 gallons an hour 200 MPH cruise that gives us 3.1 hours or 600 mile range. That beats a Robinson R22 helicopter in speed and range by a lot.
We need to get the empty weight down to 666 pounds. 16B p-port engine with aluminum end housings is 185 pounds. Gear box and rotors are 100 pounds. Thirty pounds for the gear box and the rest for rotors and hubs. Drive shaft and accessories are 100 pounds. That leaves only 280 pounds for carbon fiber structure. No allowance for anything other than an optimized design in carbon fiber.
WHY WE DID NOT SELECT A DUCTED FAN
This is a chart from Aerodynamics of Propulsion. D. Kuchemann & J. Weber. As you can see the drag of the duct becomes a significant factor at the speeds we are looking for.
MAZDA 16B PERIPHERAL PORT ENGINE
The stock Mazda 16B is not a P-port engine. It is a side port engine. However it is fast and low cost to modify it as a p-port engine by boring holes in the rotor housing and welding in tubing. This improves the volumetric efficiency across the RPM range of interest by 20% and so too the HP. The engine is capable of a BSFC of 0.47 which is in the range of most light aircraft engines. Perhaps 5% worse than the best. In turbo compound form it is theoretically capable of a BSFC of 0.4 or below. It is also capable of running in the range of 18:1 to 20:1 air fuel ratio at high RPM and low load. Our theory on why this is possible is due to the fact the engine is a rotating combustion chamber engine. This forces the heavier fuel molecules out near the spark plugs. In effect it is naturally stratified charge. The red line on the stock 16B engine is 8500 RPM and a P-port 16B engine will be making close to 300 HP or more at 8500 RPM. Consequently it is a better high RPM high HP aircraft engine than a low RPM low HP car engine. It's exceptional power to weight ratio more than compensates for the slightly higher fuel burn of a piston engine. This is a picture of such an engine installed in a light aircraft.
VTOL GEAR BOX
This gear box is stunning in it's simplicity. Since we are proposing using the A/C proved Ford eight pound A4OD six pinion planetary in a differential mode the total gear reduction is 6.34:1. See http://www.rotaryeng.net/psrus.html. The ring gear is rotating in one direction and the planet carrier is rotating in the other. As long as both props have the same drag the RPM's of each prop will be equal. With an engine RPM of 8000 the rotors will be turning 1262 RPM for take off. I would use a 13 foot diameter rotor at this RPM.
There is a possibility that the gear box could start up with both blades rotating in the same direction. One blade would be going backwards so that might be sufficient torque load difference to force the unit into counter rotating mode.
The over all diameter of this gear box is only about 6.5 to 7 inches. It should have it's own oil supply as it is on the very front of the airplane and the engine is at the very rear for weight and balance reasons. A simple mechanical pitch change design is used. The size of the aft shaft is the same size as the ring gear. Namely five inches. A simple butt weld is all we need. One of the results of this is the gear case now becomes a simple piece of thick wall aluminum tubing about six inches in diameter and eight or ten inches long. This will allow increasing the diameter of the front shaft connected to the planet carrier to four inches. A front shaft needs a lot of torsional and bending stiffness as it is cantilevered a fair distance out of the gear box.
I would use plain bearings, oil filter and an oil cooler with a dedicated high pressure stock Mazda 13B automatic transmission oil pump driven off the input shaft. The remaining problem would be high speed shaft seals.
The gimbal is centered on the U joint which is a requirement. The gimbal allows the props to be pointed in any direction within limits. The same as the Air Scooter. This will allow maneuvering in landing and take off mode and help with the transition to horizontal flight and back to vertical flight. Here are some Air Scooter pictures
Once they are rotating the highest speed rotor tips will encounter a transonic brick wall transferring the remaining torque to the slower rotor.
Here is a chart of drag coefficients of an airfoil as a function of angle of attack as it approaches the speed of sound. Small changes in pitch make large changes in tip drag. It is essential that they be twisted correctly.
Top speed would be limited by the rotor tips going supersonic. As a compromise we have chosen a 13 foot diameter rotor.
We are compromised somewhat by the 6.24:1 gear ratio. Increasing it would add weight to the gear box which is probably unacceptable for weight and balance reasons. That gives us a nice conservative 15 pounds per square foot disk loading in helicopter mode. Right in the range of other tilt rotors.
Rotor tip speed will be limited to 590 MPH or there abouts. Engine RPM will be limited to about 7000 and HP about 233 with a Ford 2.85:1 planetary at cruise. A turbo compound would make a large difference and give us about 325 HP at 7500 RPM on the top end.
All this assumes we can get the total drag down including the cooling drag down to a flat plate equivalent of one which will be a challenge. Its been done however with airplanes with a longer wing span than ten feet.
VTOL TAIL SITTER COOLING LAYOUTThe fuel tanks are 20 gallons each. I was stunned and pleased on how much volume I had for fuel with only a 15% thick airfoil and a ten foot span. I actually had to cut them down from 35 gallons OR 70 gallons total as there was no way we could lift that much fuel. If one wanted longer range without a pax one could increase the total size of the fuel tanks by 30 gallons for a 70 gallon total. At 15 gallons an hour, 180 HP, 0.5 BSFC and 300 MPH that would be a max range of 1400 miles. Amazing! This dramatically illustrates the huge advantage of a tilt rotor VTOL with a high wing loading.
The two pass rad is 20" by 30" by .75" thick from a late model Dodge pickup mounted in the aft portion of the wing. The oil cooler is in the other wing. The cooling air scoops are in the front part of the wing tip landing gear pods. The air comes in the bottom of the rad with a Kays & London wedge diffuser and is sucked out the top with the engine driven fan during takeoff mode. This config is ideal for an exhaust ejector which is also implemented as it does a good job of quieting the engine with little or no weight penaltiy.
Here is a 3D of the proposed hub and blade arrangement.
Since we will not be flying forward in helicopter mode for long or at high speed the leading and lagging hinges can be done away with. Same goes for the flapping hinges so we can use Glass Reinforced Plastic for the hub and outer blade skin. The same as several European helicopters use. GRP has high tensile strength and a low modulus of elasticity allowing it to flex without failing. The innovative feature of having a one two piece carbon GRP blade with internal carbon fiber torque to allow twisting the blade is a break through. More about the reason for this shortly. The outer bearing race is also an innovative feature. The connection between blade and hub is achieved by the bearing balls and they are fed into the races by one access hole on each side. We have used this feature in the past on an aircraft propeller design and it works well.
Here are a couple of FEA on the GRP hub thanks to H & J Johnson. Here is what Jarret said about it. "I loaded this face up w/ 1800lb's [it's 10" from the ball CL] It's plenty strong.. has a base FOS of 3+. I also did a model w/ a section view to show internal stresses. This was w/ 1020 steel."
VORTEX RING STATE
No discussion on the controversial V22 tilt rotor would be complete without a discussion on Vortex Ring State or VRS for short. An issue has been made about the V22 Osprey's 47 degree blade twist and how it affects the Vortex Ring State. Unlike a helicopter high blade twist is necessary in a tilt rotor to get good propeller efficiency at high speed. This high twist can cause blade stall in the vertical landing mode. I also suspect it affects the ability of the V22 to auto rotate.
The V22 can roll over if one rotor stalls and the opposite side rotor does not stall. Special landing techniques have been devised to overcome the problem namely by limiting the decent rate to 1000 FPM.
What the V22 Osprey critics don't realize is an off the shelf variable twist rotor blade such as we are proposing above is already on the market and has been proved to work. Namely the IVO. Thousands have been sold over the past ten years.
Ivo prop cross section drawing.
Crashed IVO prop showing the internal construction. The crash had nothing to do with prop failure.
Contact IVO and see if they will build a variable twist rotor for your needs. http://www.ivoprop.com/ if I were Bell/Boeing I would be looking long and hard at this option for the V22 rotors.
We are not affected to the same extent as the V22 is to VRS as we have only one rotor axis. However blade twist negatively affects the efficiency of the rotor in the vertical take off mode. Never the less Boeing is working on their own system of variable blade twist. Here is a graphic that gives some estimated performance improvements.
AIR BAGS SAFETY DEVICES
No vertical takeoff design would be responsible without some consideration of saving the pilot and passengers in the event of a mechanical failure during the landing and takeoff modes. If you used two air bags shaped like this mounted on a person's shoulders and it was 24 feet in diameter the frontal area would be 452 square feet. Given a drag factor of approximately one and a person weight of 250 pounds the free fall speed would be drag = V^2 x 452 x .0026 = 250 = man's weight.
V^2 = 250/ (452 x .0026) 'MPH
V= (250/(452 x .0026)^.5
V is roughly 15 MPH.
You don't need a parachute. The person will land upright as his CG is below the center of the sphere. AND it will be a HIGHLY cushioned impact.
Probably 24 feet diameter is over kill. Inflation is near instantaneous, unlike a parachute, with two explosive charges such as blank shotgun shells. One needs about 500 feet to make a parachute work. Many Hollywood stunt men have repeatably jumped off ten story buildings and landed on air bags.
Some of the Mars probes used them for the final descent phase -
Sprit in 2004 - http://space.about.com/cs/nasanews/a/spiritlandingst_2.htm
Beagle 2 in 2003? http://www.space.com/missionlaunches/missions/beagle_lander_000522.html
If you get the bag pressure just right you can come to a complete stop when your feet touch the ground.The bag will be distorted by dynamic pressure as it is falling but the max pressure is pretty low at about 2.34 pounds per square foot at 30 MPH or .0163 psi. Testing this idea is a simple matter of jumping at high altitude WITH the air bags and with conventional parachutes as back up timing the rate of decent while using the air bags.
My wife Robin suggested using a third smaller air bag under the pilots seat to eject him from the aircraft. This amounts to a super light weight ejection seat. Much safer than a rocket powered ejection seat.
"Up to about 1000 HP the rotary is a better choice for aircraft use."
"The turbo compound rotary could be as much as 20% more economical than a piston engine and half the fuel burn of a comparably sized turbine with only a slight penalty in power to weight ratio."
Fantastic! So, if a 13B p-port engine with aluminum end housings weighs 165 pounds, how much would the same engine with a turbo compound weigh? And, though it is heavier than just the 13b engine, would the turbo compounded 13b yield enough extra energy to lift the VTOL design into the air and push it through the sky at 300 mph?
Maybe your favorite Gnostic,
Les Nordman :-)
Yes if based on a P-port Mazda rotary engine.
A TO4 turbo charger weighs 16 pounds. A gear box and belt drive for it weighs about 3 or four pounds. In fact the turbo compound engine really pays off big time at high altitude. My guess is 350 MPH. Which will beat any personal jet on MPG by a factor of five or more if not on speed. Here is a chart for a R3350 turbo compound engine fuel consumption as a function of altitude. As you can see both the power is up and the fuel burn per HP generated is down while so is the aerodynamic drag. This is due to less back pressure on the turbo compound turbine.
We could go even faster if we upped the wing loading to a typical tilt rotor VTOL wing loading of 100 pounds per square foot. That would reduce the wing area to 12 square feet and the wing drag HP to about 25 at 300 MPH. It would also reduce the wing weight and the interior volume for fuel and cooling system. Perhaps to an impractical level.
In this airplane with the engine running there is no such thing as a stall. As the aircraft slows down the angle of attack automatically increases up to 90 degrees.
Paul Lamar ...No rotor no motor.
While it might not technically stall, if the engine isn't making very much power, an unsurvivable sink rate will develop when the airspeed is reduced. Or, do you anticipate the ability to auto rotate. I realize landing while flying on the wing will destroy the rotor system.
Not necessarily Matt. The original Pogo had the ability to jettison the lower rudder in order to land conventionally.
If you stop the blades by feathering them in a horizontal position and use a gadget like a parking brake to lock them in place you could glide at high speed. Pitching the gear box down will negate the additional lift from the blades. They would then act sort of like a canard.
Delta type airplanes stall at very high angles of attack. On the order of 45 degrees. If done right just before touch down this will slow them greatly due to the high drag at the high angle of attack. Probably no harder to land than the space shuttle.
The problem is the tail hits first so the aircraft pitches forward slamming the nose down on the ground. Depending on the surface it could flip over or cart wheel. An emergency air bag activated by a blank shotgun shell under the nose could mitigate this somewhat allowing the air plane to rotate more slowly and remain up right and slide along on its belly and what is left of the air bag. I am not about to volunteer to test this hypothesis however :)
The obvious thing is to use an off the shelf parachute like the Sirius in the cruise mode.
I think the jury is still out on the auto rotate bit. See the variable twist blade below.
Paul Lamar ...No rotor no motor.
You've said it at least once before, but I think expecting VTOL to not spend much time hovering is unrealistic. Before I'd consider flying it, I'd want to hover the thing for several hours - just to figure out how.. And then, people are going to hover taxi from where they start up to a safe location from which to depart. Vertical departures from the tie down are going to blow the neighbors around - unpopular. Finally, I wonder about the approach method. Do you expect that a normal approach will be flown (with a traffic pattern)? In what configuration? Translation from wing lifting to rotor lifting at low speed close to the ground? The Ospreys transition from forward configuration to vertical while still on the downwind - was watching them at Kirtland AFB a couple of weeks ago..
Testing is another matter. One might have to limit the time one tests in the vertical mode to insure the engine does not over heat.
If it is a heliopad vertical departures from the tie down blowing the neighbors around is normal. You can not taxi a skid helicopter with out first lifting off a few feet.
I envision a normal high angle of attack approach with a complete transition to vertical on short final. A modified flare if you will. Previous delta wing airplanes approached at a high angles of attack anyway. Lacking adequate thrust and control they could and did get in the back side of the power curve mode which generally resulted in a crash. Particularly the jet powered deltas as they had little or no roll control. They had no prop wash over the tail.
We have the thrust and we have the pitch control at angles of attack all the way up to 90 degrees. Roll control is provided by both the variable thrust vector angles (both lateral and fore and aft) and the prop wash over the elevons.
The V22 Osprey was sold as both a helicopter AND a normal airplane.
I think that is unrealistic for what I have in mind. What I have in mind is super efficient high speed cross country travel with VTOL only. It is not an adequate helicopter IMHO.
Paul Lamar Editor
Thanks to all the people that uploaded ideas and suggestions. Especially Roger Parker for displaying his STOL airplane at the Camarillo air show and inspiring me. This is even cheaper and easier than building a scaled down Osprey.
Honda is getting into personal jets. Mazda could build something like this almost anybody could afford. People have dreamed about an airplane that can take off in their back yard and fly for a 1000 miles for the last 100 years. The Mazda rotary engine and carbon fiber could make that dream come true.
Paul Lamar August 2007