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Thread: Design question


  1. #1
    iron eagel's Avatar
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    Question Design question

    What type of G load would a 3 lb airplane need to withstand to be able to make a turn with a radius of 150' when traveling 350 Mph?

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    I come up with about 5.5g, but physics was a really long time ago. Have a look at:

    http://en.wikipedia.org/wiki/Centrifuge

    Hope this is helpful.

    Ken

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    speedracerntrixie's Avatar
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    That seems a bit low. I was guesstimating closer to 25 G.

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    iron eagel's Avatar
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    Thanks guys.
    Looks like I'm going to have to do some math despite my wish to avoid it.

    Ken I know what you mean about Physics being a long time ago.

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    the g's in the circle are v^2/r. v in feet per second, r in feet. 350mphx88/60=513 feet/sec (I can't image a model going that fast) So v^2=263169 ft^2/sec^2 and v^2/r=1754 ft/sec^2 . Convert to g's by dividing by 32.2 give 54.4 g's

    Not likely to happen in the real world.

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    iron eagel's Avatar
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    Your right it is not that likely , but remotely possible. Thanks for taking the time to reply. I just want to build the model with that particular consideration in mind. The way the german speed guys have their rules setup you only have 50 meters to enter the 100 meter course for the speed run. While pylon planes and gliders can handle it with out burning off too much speed, a plane that is designed for real speed will have problems with small radius turns like that. 513 feet/sec may be possible with the right design given enough power, but that turn will scrub off a lot of speed.

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    Did the math again and came up with about 55g. That's what I get for doing some of it in my head. I guess math skills get dull just like physics.

    Love to see the model with that kind of performance.

    Ken

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    iron eagel's Avatar
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    Quote Originally Posted by kenlowe View Post
    Did the math again and came up with about 55g. That's what I get for doing some of it in my head. I guess math skills get dull just like physics.

    Love to see the model with that kind of performance.

    Ken

    It's just a design limit at this point, worse case type of thing.
    Right now the number to beat is 304 set by the German speed cup guys and the FAI, and they are calling a measurement at the end of a dive straight and level powered fight.
    Some say real straight and level would be more like 260-270 MPH.
    Anyhow right now I am in the process of developing a design that will have pretty high performance, perhaps even close to that 300 figure I just want to be sure it's not going to fold or explode from the stress. I was afraid it was going to be around that high just as a guess.

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    Jetdesign's Avatar
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    I'm gonna call it zero. When it disintegrates the mass of the individual pieces will be negligible.
    Joe Marri
    Enjoying all things aviation.

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    iron eagel's Avatar
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    The German speed cup guys do a pull up from a dive as an entry into the timed run that is limited to 50 meter's, last year their high AR planes were doing 304 MPH with that type of a maneuver. A high AR plane will pretty much bleed all of its speed off trying to maneuver in something that tight. I was just curious as to just how high the G's were within that tight of a radius. I don't think the li-pol battery would survive that type of maneuver.
    Last edited by iron eagel; 03-01-2014 at 07:42 AM.

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    Lnewqban's Avatar
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    Cool

    Quote Originally Posted by iron eagel View Post
    What type of G load would a 3 lb airplane need to withstand to be able to make a turn with a radius of 150' when traveling 350 Mph?
    Each of the components of your airplane will become 54 times heavier than it is when moving on a rectilinear trajectory.

    During those moments. all those components will hang from the wings, which will need to generate a lifting force of more than 164 lbf (164 lb of dynamic "weight" + negative lift exerted by tail to compensate stability margin) before reaching the critical AOA.

    That centrifugal force equals the product of the mass of the model times its centripetal acceleration.

    The mass of the model = 3 lbm / 32.17 ft/s-s = 0.093 slugs

    The centripetal acceleration = V * V / r = (513 ft/s * 513 ft/s) / 150 = 1754 ft/s-s

    The centrifugal or g-force = 0.093 slugs * 1754 ft/s-s = 164 lbf

    Quote Originally Posted by iron eagel View Post
    It's just a design limit at this point, worse case type of thing.
    Right now the number to beat is 304 set by the German speed cup guys and the FAI, and they are calling a measurement at the end of a dive straight and level powered fight.
    Some say real straight and level would be more like 260-270 MPH.
    Anyhow right now I am in the process of developing a design that will have pretty high performance, perhaps even close to that 300 figure I just want to be sure it's not going to fold or explode from the stress. I was afraid it was going to be around that high just as a guess.
    That is a difficult design because the airplane needs to be enough aerodynamically clean in order to obtain a high terminal velocity and robust enough and lightly wing loaded in order to survive those g-forces without folding or stalling the wing.
    Last edited by Lnewqban; 03-02-2014 at 11:58 AM.
    Lnewqban - "God will not look you over for medals, degrees or diplomas, but for scars. He has achieved success who has worked well, laughed often, and loved much." - Elbert Hubbard

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    iron eagel's Avatar
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    It just reinforced the fact that the plane I am building will not be capable of flying that type of a course it doesn't have a tail.
    The Germans are doing this with high aspect ratio wings, I know a low aspect plane is able to do this tight of a radius, it could be engineered to withstand the G forces but it would have to trade a lot of it speed for lift which would sort of defeat the purpose. I think their rules are aimed more for DS type of planes which will limit the actual speed they can produce.
    Bottom line is this is not really a competition which gives you the real performance limit for straight an level flight, just the speed out of a dive.


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