Prop jet.

Discussion in 'Firewall Forward / Props / Fuel system' started by pictsidhe, May 21, 2018.

Help Support HomeBuiltAirplanes Forum by donating:

  1. May 26, 2018 #41

    pictsidhe

    pictsidhe

    pictsidhe

    Well-Known Member

    Joined:
    Jul 15, 2014
    Messages:
    5,889
    Likes Received:
    1,497
    Location:
    North Carolina
    Another reason for combustion at the tips. Sealing compressed air at low temps is a whole lot easier. New issues arise, instead...
     
  2. May 26, 2018 #42

    bmcj

    bmcj

    bmcj

    Well-Known Member HBA Supporter

    Joined:
    Apr 10, 2007
    Messages:
    12,562
    Likes Received:
    4,584
    Location:
    Fresno, California
    Perhaps if the rotational inertia DRAWS the fuel through the blades, it will create a low pressure area in the hub which might make a perfect hub seal unnecessary.

    otherwise, maybe you can guard against leaking by insulating the flow with a high speed air dam (a blanket of high pressure airflow).
     
  3. May 26, 2018 #43

    Victor Bravo

    Victor Bravo

    Victor Bravo

    Well-Known Member

    Joined:
    Jul 30, 2014
    Messages:
    5,755
    Likes Received:
    4,621
    Location:
    KWHP, Los Angeles CA, USA
    OK, I'll remove my aluminum foil hat for a second and throw one SWAG suggestoin into the mix:

    Rotating propeller with hollow blades

    Rotational inertia draws air in from the root (spinner area) and centrifugal / centripetal forces create spanwise flow toward the tip.

    SOMEWHERE inside the blade (wherever works) is a venturi and fuel nozzle, such that the air flowing through the venturi draws a fuel vapor into the airstream, like a carburetor.

    SOMEWHERE else inside the propeller (again, wherever works correctly) is a reed valve system that makes it work like a reed pulsejet. But the pulsejet system is operating inside the propeller blade as a gas generator, not a pulsejet engine "mounted to" the propeller tip.

    The jet exhaust is ducted out through the trailing edge of the tip of the propeller, to create thrust that moves the propeller blade forward, allowing the propeller to generate thrust for the aircraft.

    Unlike ramjet tip propellers, the presence of essentially a "compression stroke" as part of the power cycle creates more BMEP or combustion pressure at lower speeds, not relying on ram air compression. Meaning you don't have to get the whole thing to a high RPM / mach number before it can start to work, it is more efficient at low speeds.

    No high pressure or high temperature sliding seals needed between the spinner and prop blade, all the pressure is kept in the combustion chamber (outboard half of the blade) by a reed valve. The only seal is for the fuel to get into the rotating propeller/spinner, but it is not highly pressurized and it may well be at below ambient (because it is being "sucked" into the blade by rotational inertia, not pumped).

    ANOTHER potential idea for this is to use Peroxide, and have a reactive catalyst inside the blade, venting the high pressure steam out of the trailing edge of the prop tip.
     
  4. May 26, 2018 #44

    henryk

    henryk

    henryk

    Well-Known Member

    Joined:
    Mar 8, 2010
    Messages:
    4,544
    Likes Received:
    417
    Location:
    krakow,poland
  5. May 26, 2018 #45

    ARP

    ARP

    ARP

    Well-Known Member

    Joined:
    Nov 24, 2009
    Messages:
    301
    Likes Received:
    34
    Location:
    Darenth, Kent / England
    Many years ago I saw in the Science Museum, London a propeller that had an air intake at the hub and the air went through the hollow blades where fuel was added, burnt and exhausted out of the tip. The video posted by henryk looks like the same concept. Whether it worked or not I cannot remember but the idea is very old.
     
  6. May 26, 2018 #46

    pictsidhe

    pictsidhe

    pictsidhe

    Well-Known Member

    Joined:
    Jul 15, 2014
    Messages:
    5,889
    Likes Received:
    1,497
    Location:
    North Carolina
    Jets run way lean. Combustion is with a small portion of total air. Mixing fuel with all thr air won't work. You could split the airflow into combustible and plain flows, though.
     
  7. Aug 6, 2018 #47

    Sockmonkey

    Sockmonkey

    Sockmonkey

    Well-Known Member

    Joined:
    Apr 24, 2014
    Messages:
    1,566
    Likes Received:
    402
    Location:
    Flint, Mi, USA
    I had this exact same idea years ago. Channeling the thrust through a long prop blade is a no-go.
    However...
    Put only the burners and nozzles at the tips. Not the intakes. Deliver compressed air to them through a tube in the blade from a roots blower or some other kind of positive displacement pump geared to the prop shaft. The long tube also acts as an intercooler so the air is cold and dense when it get to the burner. A tank of compressed air fed by that same positive displacement pump can be used as your "starter battery".
    Getting electrical power to the igniters at the tips is no problem at all. An induction coil works without physical contact so add one to the shaft. Hit the air tank to get the prop tuning and hold a magnet next to the induction coil.
     
  8. Aug 6, 2018 #48

    markaeric

    markaeric

    markaeric

    Well-Known Member

    Joined:
    Aug 1, 2015
    Messages:
    346
    Likes Received:
    83
    Location:
    Wichita
    I too thought about this many years ago, but for a heli rotor system which then morphed into something quite different. So the thinking went there was a mast-mounted centrifugal compressor (find me a centripetal compressor!) that was geared up and driven by the rotors and the torque counteracted by the mast. The compressed air was piped through the blades to burners and nozzles at the tips. As it turned out, I discovered online that someone had done just that back in ye olden days. There was even a picture. However, there was very limited info as I recall.

    Then, if memory serves me correctly, there was something similar to that, but designed much more recently (some heli UAV), where the compressor (also mast-mounted) was electrically driven. Same principal otherwise.


    I could have sworn I've also seen an old patent or something where this system was integrated into a propeller. It could have been my imagination, but I'm pretty sure it was real. Not sure if one was ever operational, and the "compressor" may have been designed to rotate at the prop's speed. Also quite old-timey.

    I have little doubt it could be made doable. Should be better than a pure turbojet of similar PR and mass flow rate, as the thrust is used to turn a prop, but jet velocities would still be high. There might be benefit to coupling the jets to ejectors (or airflow amplifiers, or one of the many other names they fall under). Figuring the efficiency of the system becomes a bit of a brain bender, as they can increase thrust, but their kinetic energy conversion is poor. I do have some further thoughts on this design if anyone cares to discuss it further.

    Unlike a piston engine where detonation is a concern, you would want to maintain as much of the temp of the post-compressed air as possible, because that corresponds to the lesser amount of fuel you need to add in order to reach your desired temps. The only benefit would be the smaller air passageway that would be required for the cooler air. I doubt it would be worth it. I like your thinking otherwise.
     
  9. Aug 6, 2018 #49

    pictsidhe

    pictsidhe

    pictsidhe

    Well-Known Member

    Joined:
    Jul 15, 2014
    Messages:
    5,889
    Likes Received:
    1,497
    Location:
    North Carolina
    I think I need to run the numbers on this.
     
  10. Aug 6, 2018 #50

    henryk

    henryk

    henryk

    Well-Known Member

    Joined:
    Mar 8, 2010
    Messages:
    4,544
    Likes Received:
    417
    Location:
    krakow,poland
  11. Aug 6, 2018 #51

    pictsidhe

    pictsidhe

    pictsidhe

    Well-Known Member

    Joined:
    Jul 15, 2014
    Messages:
    5,889
    Likes Received:
    1,497
    Location:
    North Carolina
    That combustion chamber is sized decidedly optimistically.
     
  12. Aug 6, 2018 #52

    Sockmonkey

    Sockmonkey

    Sockmonkey

    Well-Known Member

    Joined:
    Apr 24, 2014
    Messages:
    1,566
    Likes Received:
    402
    Location:
    Flint, Mi, USA
    Huh, I though cold, dense air = more air fitting into the combustion chamber = more oxygen =better burn.
    I guess I'm thinking in terms of piston engines. Ah well.

    Yeah, this is one of those ideas where you know a dozen guys had to have thought of it before, but you can't let it go until someone can show you exactly what the fatal flaw is.
    Time and tech march on, so what not might have been possible/practical/affordable in the past might be so now.

    Anyhoo, picture time because I loves muh 3-D mockups.
    [​IMG]

    I have another wacky notion involving airscoops on the cylinders of one of those radials where the whole engine rotates.
     
  13. Aug 10, 2018 #53

    Sockmonkey

    Sockmonkey

    Sockmonkey

    Well-Known Member

    Joined:
    Apr 24, 2014
    Messages:
    1,566
    Likes Received:
    402
    Location:
    Flint, Mi, USA
    The helicopter version looks like it would still need a tail rotor because turning the compressor is still going to exert torque on the body. I think it would need two rotors. Separate shafts of course. I wouldn't want to try routing fuel and air through two sets of nested concentric shafts. Sticking a compressor between a set of counter-rotating props on the same shaft solves the air issue, but you still need two sets of rotating seals for the fuel lines.
    Adding additional air entrainment ducts around the burners probably won't help much in terms of power, but they would do a good job as Merideth-effect cooling ducts for the jet nozzel.
    I would appreciate that.
    If we start with how much air a roots blower of a particular size can put out at a reasonable RPM, then figure how much fuel we can burn with that in a jet-engine fuel-air ratio, we know how much thrust it can exert on the end of a lever-arm of a certain length, which let's us calculate what the best prop length would be, Longer props tend to be more efficient, so that's a good thing.
    Revised slightly more sensible version.
    [​IMG]
    I think I've got the essentials covered. Having the compressor in front means we don't have to worry about getting the rotating seal of the fuel line past it, and the ram air being fed in as speed increases will somewhat compensate for the fact that the compressor can only turn at a fixed rate relative to the prop rather than seeking the ideal speed freely the way a turbo would.

    Anyhow, revisiting some of the "been tried, didn't work" ideas does have merit since tech marches on solving problems, providing new data, and making high-tech parts and materials affordable.
     
  14. Aug 10, 2018 #54

    Tiger Tim

    Tiger Tim

    Tiger Tim

    Well-Known Member

    Joined:
    Apr 26, 2013
    Messages:
    2,776
    Likes Received:
    1,538
    Location:
    Thunder Bay
  15. Aug 10, 2018 #55

    markaeric

    markaeric

    markaeric

    Well-Known Member

    Joined:
    Aug 1, 2015
    Messages:
    346
    Likes Received:
    83
    Location:
    Wichita
    Did you find it? I do recall the old one claiming no tail rotor was required, but that didn't make sense to me since as far as I knew, it had to apply some torque to the mast for the compressor gearbox. Now, that could be less torque than would be necessary to drive the rotor on a conventional heli, as powering a compressor will be some fraction of the power a rotor would require.
     
  16. Aug 10, 2018 #56

    Sockmonkey

    Sockmonkey

    Sockmonkey

    Well-Known Member

    Joined:
    Apr 24, 2014
    Messages:
    1,566
    Likes Received:
    402
    Location:
    Flint, Mi, USA
    Sadly, no. I was working it out in my head. Maybe the torque was low enough that an angled tailfin or something was enough to counter it?
     

Share This Page

arrow_white