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Achieving 2200lbs Thrust

  1. Feb 16, 2008 #1
    hi
    what is the easiest way of achieving 2500lbs+ of thrust, is it by using a ducted fan/propellors or turbofans?
    first i also considered valveless pulse jet engines but i don't know if you can get enough thrust from them.

    also are there better ways than the ones i mentioned?

    help and advice will be appreciated

    thanks
     
  2. jcsd
  3. Feb 16, 2008 #2

    Danger

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    Welcome to PF, Jezym.
    The easiest way would be a solid-fuel rocket. No moving parts.
    What sort of project do you have in mind? 2,500 lbs is a hell of a lot of thrust to be playing around with if you don't know what you're doing. It could be hazardous.
     
  4. Feb 16, 2008 #3
    oh yeah
    i completely forgot about the rocket but once the reaction starts you cant control it can you?
    so im basically looking for something that can be turned off and wont disintegrate the surrounding area

    how much power would you need to produce 2500lbs of thrust with 70 inch blades in a ducted fan?
    are there any formulas i can use to work that out?

    i'm not doing it but the thrust is to generate lift for a VTOL aircraft
    do you know how this could be done without helicopter rotors?
     
  5. Feb 16, 2008 #4

    Danger

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    I don't. Fred Garvin and Astronuc are the resident experts in propulsion technology. Probably moreso Fred in this case, since he works with jet engines daily. Astro's a bit more of a space engine guy.
    There is also the possibility, though, of using a hybrid rocket. That is a solid fuel unit, so still fairly simple, but utilizes a throttleable liquid or gaseous oxydizer. You can therefore adjust the thrust as needed.
    Cruise around the web a bit and look for the Mueller Sky Car as well. (I'm not sure that I spelled the name properly.) It was supposed to have hit the market by now... for over a million bucks... but apparently there have been some hang-ups. Anyhow, it uses 2 Wankel motors in each of 4 pods driving ducted fans. Maybe studying it can help you a bit.
     
    Last edited: Feb 16, 2008
  6. Feb 16, 2008 #5
    i thought about that as well
    but for a vtol craft it wouldnt be safe would it because wont it just melt the tarmac

    i thought that ducted fans are the best option becuase the thrust coming out from them would be cool enough for you to land on something soft like grass
    thats why id like to know how about any equations where i can work out power needed if i know blade diameter and thrust

    i dont think even a jet engines exhaust would get as a hot as a rocket's
    i know rolls royce did an engine specifically for vtol:
    http://en.wikipedia.org/wiki/Rolls-Royce_RB.108
    and the size seems to be pretty good as well
     
  7. Feb 16, 2008 #6

    Danger

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    You do seem to have a point about the thermal management. Direct jet-wash doesn't seem to be detrimental to tarmac over short periods; a rocket's power is a function of the exhaust speed/temperature, so they probably run them as hot as possible. I know that the old NERVA series nuclear rockets pumped the liquid hydrogen through a coil wrapped around the nozzle to keep it from melting (and to pre-heat the fuel).
    Seriously, pal, I'm out of my depth here. I'm enjoying the conversation, but I'm afraid that I really can't be of any help to you. I don't even know if it would be advantageous to use variable-pitch blades in a ducted fan. Anything that I ever flew relied on wings for lift, not VTOL stuff.
    I just realized that Russ Watters can probably help you out as well. While not in the field of aeronautics, he knows one hell of a lot about fans.
     
  8. Feb 16, 2008 #7
    well im glad you have enjoyed the conversation
    im going to contact frd garvin and russ watters
    should i just search for them on the forums with the names you provided?
     
  9. Feb 16, 2008 #8

    Danger

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    Yeah. Fred is off-line right now, but Russ is on. I just saw a new post by him up in the Physics section. If you're not in too much of a hurry, just hang around and wait; they'll show up here eventually. Otherwise, try Private Messaging them.
     
  10. Feb 16, 2008 #9
    ok danger
    thanks for all your help

    i'll give it till tomorrow and then PM them
     
    Last edited: Feb 16, 2008
  11. Feb 16, 2008 #10

    Danger

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    Cool. I really have to log off now, anyhow. I'm at work, and so far haven't done any. Since there's only an hour and a half left in my shift, I probably should make some sort of attempt. I'll be back later today or tomorrow.
     
  12. Feb 16, 2008 #11

    russ_watters

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    Ducted fans are good for VTOL applications because of the high volume and low velocity. You can calculate pounds of thrust with Bernoulli's equation and power with newton's laws.

    For example, say you pick your rotor/fan diameter to be 10'. Via Bernoulli's equation, p= 1/2 rho V^2, 2500 lb of thrust requires a velocity of 261 ft/sec. And that equals 652,500 ft-lb/sec or 1,200 hp. Then assume a 50% efficient fan and call it 2,400 hp.
     
  13. Feb 17, 2008 #12
    thanks for that russ
    so if this was to be done you would have to use a turboshaft to power the blades

    but this would be safer than a VTOL craft with jet thrust because the air coming from the ducted fan will be cool compared to the hot gases from a jet wouldnt it?
    like wouldnt a harrier jump jet burn the ground when its taking off vertically?

    are there any ways the power figure can be reduced
    ie. increasing blade diameter

    also this is a stupid question but what does 10' mean is that 10 feet or 10 inches?
     
    Last edited: Feb 17, 2008
  14. Feb 17, 2008 #13

    Danger

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    ' means feet; " is for inches.
     
  15. Feb 17, 2008 #14
    i thought it was that but i just had to be sure

    so danger do you agree that a turboshaft powering blades in a ducted fan is safer than normal jet thrust because of the hot gases coming out as thrust in a jet?
     
  16. Feb 17, 2008 #15

    russ_watters

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    With a high bypass turbofan, the velocity and the temperature are both lower than with, say, a turbojet. But still, I think the Harrier did damage some runways (not sure, though).
    Yes. With larger blades you get more lift with less power.

    Basically, what you are now describing is a helicopter, though. (you wouldn't duct a fan that big)
     
  17. Feb 17, 2008 #16

    Danger

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    Absolutely. Although you'll still have that jet exhaust coming out somewhere, it's certainly easier to manage with a turboshaft.
    Keep in mind, however, that the power figures from Russ are based upon that 10' fan. There's no way in the world that you would be using something that size as a single unit. You'll want a few smaller ones working together. The fans on the Sky Car, for example, are maybe 2' at most. (I'm going by the pictures, rather than a spec sheet.) It requires only 1 Wankel motor per fan (the second is a backup in case of failure). That's another aspect that you'll have to address in your design. Anything that flies without wings has to have multiple redundancy. An aeroplane can glide to safety if an engine quits; a VTOL can't. I think that the Sky Car also contains a large parachute as a last resort. (Helicopter rotors actually are wings, so they can autogyrate down.)
     
  18. Feb 17, 2008 #17
    yeah i figured as much that the blades will be too big for a duct

    for the shape of the craft we were considering a using something aerodynamic
    i had an idea of doing something like this:
    http://www.smartfish.ch/index.cfm/fuseaction/show/path/1-2.htm

    so danger are you saying we should use around 2 smaller fans powered by the 1 turboshaft engine (there shouldnt be too much weight in the craft)

    russ if i had 2 50 inch diameter blades in ducted fans how much power would i need to lift 2700lbs?
    (i tried the equation but i dont get it can you break it down and explain it to me?)
     
  19. Feb 17, 2008 #18

    Danger

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    That's a sweet-looking ride.
    You really should stop asking my opinion about this stuff, because I don't know what I'm talking about. It's way over my head. In keeping with your basic ideas, my approach would be to run two turboshafts with cross-linked power trains to run at least 2 fans per side. That way, if either an engine or a fan packs it up you stay in the air. Again, though, this is just a hobby for me. Listen to the guys who know what they're talking about.
     
  20. Feb 17, 2008 #19
    hey
    i'm just running ideas by you and you seem to be coming up with some good ideas
    and im just glad people are replying to my questions

    so run two turboshafts running 4 ducted fans? with that the engines alone will weigh about 500kg
    (also i dont know if there will be enough space for 4 ducted fans) so what about 2 turboshafts powering 2 ducted fans?

    i think overall the aircraft will have 3 engines, 2 for lift and one for moving forward, i wonder how much power will be needed to go 350mph+
    (it would be great if fuel economy was over 10mpg)
    know how this could be done?

    i love the smartfish design it looks amazing
     
    Last edited: Feb 17, 2008
  21. Feb 17, 2008 #20

    Danger

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    If the safetly factor isn't of great significance, then just go with the one engine and a couple of fans. The reasoning for the set-up that I suggested is that if one engine quits, the other one can still drive both fans. The two fans per side aspect was to ensure symemtrical lift even if you happen to shell a blade on one of them. If this is purely a design excersize that's never going to get off of the ground, then you don't need any safety factors at all. That would be like putting airbags in a concept car; what's the point? Of course, that opens up a whole new world of possibilities, such as electrically-driven fans tapping into an isotope power supply.
    Maybe it would be a good idea to walk us through exactly what this thing is supposed to be. I've got the basic concept, but I don't know what the rules are.
     
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