Posts: 1106
Joined: 1/8/2002 From: Erie, PA, USA Status: offline
Why not try repacement blades made for variable pitch hubs? Placing small vanes in the exhaust tubes should help direct the spiraling exhaust from the turbine. They may also help in cooling the exhaust-more surface area. Basically, sparingly ribbing the inside of the exhaust tubes.
Just some thoughts.
Jeff
BTW, this thread has been/is extremely fascinating! Great workmanship!
Posts: 584
Joined: 3/14/2002 From: green valley,
AZ, USA Status: offline
Great project, But I think you should consider using the Futaba gv-1 rotor speed governor. This is a variable torque electronic device tht measures rpm's and holds them there with amazing accuracy. so in your case you would flip a switch and the propeller rpm's would go up to a preprogramed value and would be held there regardless of propeller pitch, just like the big boys. The unit has worked flawlessly for me (in heli} and turns a mixed system into a variable torque system. You just need to install the small magnet for the hall sensor in the prop blade, and mount the sensor as close as possible. The control unit can be setup for any rpm you would want to turn the prop at.
Posts: 435
Joined: 12/11/2001 From: oberndorf, AUSTRIA Status: offline
hi jazzy,
>Why not try repacement blades made for variable pitch hubs?<
good idea, but they have to fit to the selfmade bladeholders (right term?) and additionally to the two screws (distance from one to the other) they are fastened with. not easy to find.
cause of our different languages i did not understand what you want to achieve with your second thought, the vanes. do you want to direct the exhaust gases more backwards to achieve more thrust? heat wasnt a problem. please explain.
Posts: 435
Joined: 12/11/2001 From: oberndorf, AUSTRIA Status: offline
hi rpmodeltech,
thanks for ideas,
the gv-1 rotor speed governor is well known for me, cause i started modelling with a heli (however i used a simpler speed governor)
our system is working similar, the difference is that rpms are measured directly at the turbinewheel. decreasing rpm are answered by higher fuel voltage and more fuel and vice versa. the optical sensors notice if there are 80.000rpm or 80.500rpm. due to the delay of the fuel flow and combustion pressure this system manages to maintain a constant turbine rpm between 78.000 and (if you put back pitch immediately) 86.000rpm. i think, if you take the propspeed as the input, you will reduce the accuracy of the system by the factor 1:16 (80000turbine : 5000prop). do not know if i am right, however. btw: with the slide-potentiometer (term?, the two slideswitches on the mc 24) we can vary turbine rpm between idle (40.000rpm) and full throttle.
I've been following this thread with great interest. I am going to ask what may be a stupid question.
Is your turbine unit a turbo-jet or a turbo-shaft engine. If's a turbo-jet engine, isn't the sole purpose of the turbine to power the compressor and create a high velocity exhaust flow. If it's a turbo-shaft engine, there should be a power turbine for the shaft output and a compressor turbine to power the compressor. For example, the famous PT-6 turbo-shaft engine used on many full scale turbo-prop airplanes has a large power turbine section that supplies torque to the propeller. The compressor turbine only supplies torque to the compressor.
I have to wonder how much power is going to the shaft and how much is going out the exhaust and if you simply don't have enough power going to the prop.
BTW, I know zero about model turbines. All my knowledge is from the full scale helicopter heavy lift industry.
Posts: 435
Joined: 12/11/2001 From: oberndorf, AUSTRIA Status: offline
hi ilikeplanes,
I know zero about full size turbines. and a little bit about modell turbines .
the turbines in modeljets are turbo-jet engines creating a high velocity exhaust gas flow. for modellhelis (and now for turbo prop planes) this unit has been altered in that way, that the turbine main shaft drives a gear with several gearwheels, a flywheel clutch, and the prop shaft. this unit produced 12kg thrust on a test bench. including the residual exhaust gas flow-thrust approx 2kg (however due to the y-exhaust pipe, this trust was eliminated to not affect the flying characteristics of the plane). weight of the plane 12kg with full tanks.
in theory: power enough, although not for verticals.
Posts: 493
Joined: 1/10/2002 From: minneapolis, MN, USA Status: offline
[QUOTE]Originally posted by mec @KingAirJockey: Mil is right. the AECU is able of maintainig the proper rev, 80.000rpm, of the turbine. at field tests rpm data showed 77.800rpm at max pitch (a 2.5% loss resulting in decreasing the prop rpm from 5200 to ~5100) acording to imperfect AECU setup. the new Orbit AECU has the possibility to limit the max fuel pump voltage as an additional safety factor for not overreving the turbine. at max load you read the actual fuelpump voltage and setup the limit at approx 10% more (in case of later coming fuel filter or fuel clunk problems). obviously we had this limit a little bit too low (but not effecting the thrust remarkable).
questions to the best prop shape, profil, wash out a.s.o.: weeks ago i sent an email to Hartzell to find help for these unknown (for me) parameters. unfortunately no response till today. [/QUOTE]
Very interesting project. Full reverse thrust using the heli blades could be a lot of fun. Imagine a straight verticle dive coming to a complete stop and then reversing a bit to a backwards vertical climb until the plane falls off the prop. Of course full reverse flying would not be possible unless you were also able to shift the C.G. back a foot or two in flight. Maybe if you had gyros on the elevator and rudder they would be fast enough to stablize reverse flight without a weight shift. Just don't turn on the gyros in forward flight if they are set up for reverse or you will have big trouble.
I see some unique problems for you if prop rpm stays constant and throttle is used only to vary the pitch of the prop. You will have good static thrust (zero airpeed-not flying speed) at full throttle as long as the maximum lift AoA of the symetrical heli blade is not exceeded. Blade thrust and rpms will decrease and blade drag will increase if the max lift AoA is exceeded. This is probably what you encountered when the rpms dropped during your static thrust tests.
However, setting full throttle maximum pitch for static air speeds may not give you enough pitch to produce thrust at flying speeds. As airspeed increases, prop pitch can and must be increased to produce meaningfull thrust. The problem with setting max prop pitch for good thrust at flying speeds is that you will be able to stall the prop at low speed. This is not good if you are low and slow flying at the stall speed of the plane. Instinctively you might give max throttle to get out of the situation, stall the prop and loose airspeed instead of accelerating.
The only solution I can see is to limit the max blade pitch based on the airpeed. I have seen small airspeed indicators discussed here on RCU (ask Dustflyer) for model aircraft but, I don't know how you would be able to set that up to limit blade pitch. Otherwise, you would just have to estimate airspeed visually and find out how much throttle to give for a given airspeed by trial and error. If you have a good ear you can probably tell when the blades are stalled because of decreasing rpm.
If you are going to design your own more conventional prop, a Clark-Y airfoil is good. There is some prop designing information here http://mitglied.lycos.de/MartinHepperle/Airfoils/ . Some full scale aircraft designers are using a program called "X-Plane" to design aircraft and props http://www.x-plane.com/http://x-plane.org/ . X-plane has a lot of aircraft design tools. You won't be able to put a heli setup on a plane, but you could modify a heli to work somewhat like a plane. The program is free to download (70mb), costs $80 us, but the only limit of demo mode is 5 minute flying sessions. That is plenty of time to test the flight characteristics of an aircraft. Here are some screen shots.
Posts: 1106
Joined: 1/8/2002 From: Erie, PA, USA Status: offline
Mec, I thought you might be able to drill new holes or modify as necessary. My thinking is efficiency of the prop.
{>Why not try repacement blades made for variable pitch hubs?<
good idea, but they have to fit to the selfmade bladeholders (right term?) and additionally to the two screws (distance from one to the other) they are fastened with. not easy to find.}
Posts: 354
Joined: 1/9/2002 From: Ossining, NY, USA Status: online
[QUOTE]Originally posted by Craig However, setting full throttle maximum pitch for static air speeds may not give you enough pitch to produce thrust at flying speeds. As airspeed increases, prop pitch can and must be increased to produce meaningfull thrust. The problem with setting max prop pitch for good thrust at flying speeds is that you will be able to stall the prop at low speed. This is not good if you are low and slow flying at the stall speed of the plane. Instinctively you might give max throttle to get out of the situation, stall the prop and loose airspeed instead of accelerating.
Great project. Beutifully done. As the quote I clipped from Craig mentions I have heard about a variable pitch do-dad for model engines. Problem other than weight was slow down for approach it went to full pitch, add power to make the field and the plane fell out of the sky due to stall prop. Hope you get this worked out because it looks too cool.
Anthony
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Anthony Romano Wrong rudder is the most powerful force in the universe.
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