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Aerospace Aircraft Propeller Blast velocity while in motion

  1. Nov 22, 2016 #1
    I am working on a range extender for full size electric aircraft. It is a turbine located under the cowling, protruding only 8 inches. On my full size static model, the turbine can produce 600 watts with the prop blast v measuring 38 mph. If we could fly the plane (we can't, no wings) at 80 mph, would the prop blast increase (38 + 80 mph), stay the same, or maybe decrease? There would be all sorts of mitigating factors -- headwind, angle of attack, ground effect, etc., but as a foundation theory -- any ideas? Thank you so much.
     
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  3. Nov 22, 2016 #2

    russ_watters

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    That is the way it generally works if you have a variable pitch or speed propeller. if not, your propeller no longer produces any thrust once you get to 38mph, which limits the speed to under 38mph.
     
  4. Nov 22, 2016 #3
    Hmmm ... that sounds difficult to get right in detail.

    I expect that turbine outlet speed minus the turbine inlet speed is not constant. The delta v probably fall as the inlet speed grows. On the other hand the turbine is probably optimized for a particular speed, so the delta v may increase before it starts falling. However I feel confident that above a certain speed it will start falling.

    I also expect the inlet speed is not the air speed. The inlet speed will grow monotonically with airspeed but it won't be equal or probably even linearly proportional.
     
  5. Nov 22, 2016 #4
    prototype-front-posterized-cage-retracted-300x279.jpg

    Thank you both. I see the light in both responses. I don't think I've explained this very accurately. I've added an illustration to clarify the invention.
    What I'm doing is using the propeller blast, the rush of air coming off the propeller as the prime mover for the centrifugal turbine you see above the nose gear. In the shop right now, I have a Cessna with a 3 blade sport prop spinning at 1100 rpm, with a modest pitch (to limit load on the electric motor). When I measure the prop blast at the turbine with an anemometer, it reads 38 mph. So I am using the prop blast much like a stream of water. Now I want to put the whole thing in motion -- get the airplane flying at 80 mph. Same prop pitch (enough thrust for such a slow airspeed), same rpm, just the additional factor of the aircraft moving forward at 80 mph. If I mount the same anemometer in front of the turbine, will the slipstream (the rush of air over the whole plane in motion) add to the prop blast, leave it alone, or subtract from it? To my knowledge, no one has ever measured what the prop blast is doing under an aircraft cowling while the plane is flying. The only way I have come up with to test this is to mount my fuselage on a truck, prop spinning by DC motor at 1100 rpm, and drive 80 mph. :) Before I do that, and I might, I thought I'd 'check around'. Thank you so much for answering.
     
  6. Nov 22, 2016 #5

    cjl

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    Even then, that's not usually how it works. If you assume fixed prop efficiency with variable pitch (sort of an ideal case), you'd get approximately fixed power delivered to the air, so as speed increases, dV across the prop decreases (since the same kinetic energy gain to the air produces a smaller velocity increase, and the mass flow rate of the air is also increased).
     
  7. Nov 22, 2016 #6

    cjl

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    Wait a second - are you trying to extend the range by using a turbine in the prop blast to partially offset the power draw of the motor? If so, this will never work. The additional drag caused by the turbine will always outweigh the extra power you obtain by it, so the overall efficiency and range will always be reduced by this kind of a setup.
     
  8. Nov 22, 2016 #7

    Baluncore

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    I think the OP should better define the use of the term “turbine”. Is it a RAT ?
    https://en.wikipedia.org/wiki/Ram_air_turbine

    It is not the prop-blast that propels an aircraft, it is the lift of the propeller blades as they move through the air. The aircraft is pulled forward by the propeller blades applying tension to the propeller drive-shaft.
     
  9. Nov 22, 2016 #8
    Thank you CJL. I'm going to have to talk to an engineer friend who is a pilot, to help me apply the pitch/flow rate equation.
    I get that a lot --the thermodynamic oversight. No, we've taken drag into consideration. The turbine is a rotating squirrel cage, as in the illustration above. It only protrudes from the fuselage about 8 inches. A good way to think of its action is to liken it to a conveyor for your luggage at an airport -- all those little rollers. Very little friction. We've tested how the prop blast reacts to it, and it 'sees it' as a bump in the fuselage. Even better, it is eating some of the prop blast and spinning it out beneath the fuselage. The blades of the squirrel cage blower are an airfoil, also. The nose wheel landing gear actually creates more drag than the spinning cylindrical squirrel cage. When Cessna converted their 172 from IC to electric, there was 13% less drag than when the cowling had it's cooling openings and protrusions. So that's probably about the net loss. To apply the power from the turbine to the battery pack, I have a switching algorithm that allows the discharging batteries to be used sequentially, while the charging batteries are done so simultaneously. You can use smaller batteries (fewer amp hours) so you don't create more mass. But of course, there are efficiency losses along the chain of components, and the net gain diminishes as the batteries cycle. There will still be a net gain. This is why I need to know what power the flying aircraft will be capable of providing to the turbine. I would love to chat with you more about this -- here is the patent application url. Good for a long winter's night reading! http://www.google.com/patents/WO2015195856A1?cl=en
     
  10. Nov 22, 2016 #9

    cjl

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    Prop wash is a perfectly fine way to look at it though - just like you can measure the lift of an airfoil by looking at the downwash after it passes.
     
  11. Nov 22, 2016 #10

    Baluncore

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    If such a squirrel cage turbine could generate more energy than it costs in drag, then why not cover the entire surface of the aircraft with those magic turbines?
    The closed loop of electric propulsion generating prop-blast, then converting that to electricity, to be re-invested in propulsion, appears to qualify as perpetual motion.
     
  12. Nov 22, 2016 #11

    russ_watters

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    Right.
    Shoulda realized that -- it's the same issue as driving a car faster; the road is moving past faster, so the same force takes more power.
     
  13. Nov 22, 2016 #12

    russ_watters

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    Agreed. Sorry, but this is a straightforward perpetual motion machine and we don't allow that here. Thread closed.
     
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