Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Aerospace engineering

  1. Mar 26, 2009 #1
    Hi, i dont really know anything about physics and engineering... i wish to learn alittle..... but i am building something and i dont know what i need... its more like a helicopter to lift close to 75lbs. i would like to use a 4blade or 5blade fan system but do not know the measurments i need them to be to be able to lift 75lbs. i need to also know how fast the fan needs to spin RPMs and the diameter of the fan and how much torque i need for an ELECTRIC motor to lift 75lbs i would like to use no bigger than a 2ft diameter fan.

    I NEED TO LIFT THE MASS 1ft TO 1.5ft OFF THE GROUND
    please can you help me?
     
  2. jcsd
  3. Mar 26, 2009 #2
    I'm sorry, that isnt going to work. You have no idea what it would require to lift 75lbs. The helicopter could kill a person. Please do not even think of attempting this.
     
  4. Mar 26, 2009 #3
    appearently you didnt read my post...


    can someone help me??? << insult deleted by berkeman >>
     
    Last edited by a moderator: Mar 26, 2009
  5. Mar 26, 2009 #4

    Danger

    User Avatar
    Gold Member

    Eskimo, that was an unnecessarily rude response to a legitimate answer. There's no damned way in the world that you're going to lift 75 lbs. with a 2' diameter fan. Also, helicopters use rotors, not fans. How on Earth do you think that you could control the cyclic and collective on a fan? You should listen to advice given by someone like Cyrus who knows what he's talking about.
     
    Last edited by a moderator: Mar 26, 2009
  6. Mar 26, 2009 #5

    berkeman

    User Avatar

    Staff: Mentor

    Eskimo, Personal insults are not allowed here on the PF. Cyrus and now Danger are pointing out that your initial proposal is not feasible. Perhaps you should ask what would be needed in general to perform the lift you want. And you should let us know what safeguards you will use to be sure to not get hurt in building and testing your device.
     
  7. Mar 26, 2009 #6
    ok sorry bout the insult.......im just sick of people wont work..... i said i want a 2' diameter dont mean i cant get bigger.... tellin i need to go up and down no other directions.... it is not a helicopter i said it is LIKE a helicopter....... i got a 10000rpm electric motor..... think that might help me? and what voltage do i need to get it fo max preform???

    look i know this is possible..... i just need to pretty much know how big of an electric motor(torgue/rpm) and how small i can go on diameter of lift blades.... i can take care of acceleration with a power control switch/remote.... im just in a hurry to get this done... dont mean to be insulting.... sorry

    please help me
     
    Last edited: Mar 26, 2009
  8. Mar 26, 2009 #7

    Danger

    User Avatar
    Gold Member

    Apology accepted on my part; it's up to Cyrus whether or not he concurs.
    The rpm of the motor is irrelevant in this situation. Your available torque is what matters. That in turn has to be factored in with how much your rotor assembly weighs and how it's balanced. As a rough guess, I say that you need at least a 2 metre diameter rotor set to lift 75 lbs., considering that you also have to lift the mass of the machine itself. An even rougher guess is that you would need somewhere around 2-3 hp, which you are not going to get from your electric motor. I admit that these are just intuitive figures, but others here can do the math and figure out the reality. Hang on for them to respond.
     
  9. Mar 27, 2009 #8

    FredGarvin

    User Avatar
    Science Advisor

    Whoa. Someone does have a deathwish. That's OK though. I doubt you'll get far enough to cause any real damage.

    You're sure that this will work? Let's look at some very basic reasons why it won't:

    1) At 10,000 rpm, a 2ft diameter fan would have a tip speed of 1047 ft/s. That's M=.94.
    A regular industriual fan will work like crap at those tip speeds.

    2) An industrial fan at those speeds will most certainly be overstressed at the hub. Structurally the fan will not survive.

    3) Even if you could get it to structurally survive, the bearing and structure are not designed to go at those speeds. To produce 75 pounds of thrust (if you were 100% efficient) the bearings would have to be able to support that 75 pound thrust load. You'll probably toast those cheap Chinese bearings with that load and at that speed.

    4) For argument's sake, since I don't know what fan you have, let's look at the airflow requirements for a 2 ft diameter fan to produce 75 pounds of thrust:
    - If you assume an exit velocity of 50 ft/s (M=.45), you would have to produce a mass flow rate of 4.8 Lbm/sec. That is HUGE for a fan.

    - If you assume a reasonable mass flow rate of 1 Lbm/sec (still pretty friggin big), that equates to an exit velocity of 2415 ft/sec (M=2.2)

    I think this is a pretty good start. This doesn't even begin to talk about what Danger mentioned
    in regards to controlling the thing.
     
  10. Mar 27, 2009 #9
    ok i got another qed out of a question.... if the airflow is forced out of a smaller hole the pressure will be stronger..... will that help me out??? can it carry 50 lbs?
     
  11. Mar 28, 2009 #10
    Ugh, no eskimo.......no. You will not Carry 75lbs. Not 50lbs, not even 10lbs. I would google the lifting capaibilities of an RC helicopter to get an idea of what you are trying to accomplish here. You are in fantasy land.
     
  12. Mar 28, 2009 #11

    russ_watters

    User Avatar

    Staff: Mentor

    A fan has a certain performance range and restricting it's flow will hurt, not help. Google "fan curve" and check some out...
     
  13. Mar 29, 2009 #12
    All depends on torque, rpm, and prop type,

    on http://www.jetcatusa.com/spt5.html it's specs are

    Thrust: 55 lbs with 27" prop at 7,000 RPM

    Though this is using 27" prop, also I would research more on this subject.

    If you ask questions in a logical a manner, and check your grammar (so that it doesn't feel like it's coming from a teenager) I'm sure you will get better answers to your questions in the future.

    "im just in a hurry to get this done" is a red flag, beware accidents usually happen when your in a hurry. BTW people have died with props at this size going that fast, to things like someone else mentioned earlier "bearing failing" i.e. your motor might work at those speed, but can it handle the pull/push on the motor rotor?
     
  14. Mar 29, 2009 #13
    Jesus christ, do you have any idea how much that turoprop motor costs? You're easily talking in the 4-5 thousand dollar range.

    That is one serioussss turbine you linked to. I'm not too sure about turbines because I don't fly them, but I know you need special permits and training. You cant just buy one, slap it into an airframe and fly around with one. That thing could very easily kill you, or someone standing next to you.
     
  15. Mar 29, 2009 #14
    No need to curse.

    Example I linked, just demonstrates that many pounds (>40lb) of thrust can be generated with an smaller prop 27" and under 10k rpm with the proper torque, as that was my only point.

    btw the permits you talk about are at --> http://www.modelaircraft.org/documents.aspx and look under 'Turbine' section, it's really doesn't appear to be that special about it.
     
  16. Mar 29, 2009 #15

    FredGarvin

    User Avatar
    Science Advisor

    That's way outside what the OP was talking about. He was asking about an electric motor being the prime mover. Besides, I doubt the OP has the technical background for running a turboprop. I am assuming they had one too many zeros in the final prop rpms.

    One does need to be turbine rated if you get a license, but if you throw the experimental tag on the aircraft, I'm pretty sure it is wide open.
     
  17. Mar 29, 2009 #16
    Curse? :confused:
     
  18. Mar 29, 2009 #17

    russ_watters

    User Avatar

    Staff: Mentor

    Omg, I want one! I don't know why, I just do. Too friggin cool.
     
  19. Mar 29, 2009 #18
    My apologies, you didn't curse, just the words were used in a manner that I thought it was a curse.
     
  20. Mar 29, 2009 #19
    I do not think it was outside the OP question,

    The link I provided shows acuall data with thrust of 55 lbs with 27" prop at 7,000 RPM, which is relevant to the question above, as it shows feasibility.

    BTW I'm not suggesting he buy a turboprop, or turbine engine.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Aerospace engineering
  1. Engineering Advice (Replies: 3)

  2. Clearly not an engineer (Replies: 15)

Loading...