what limits an ultralight to a gross weight when it pertains to the frame not the pow

Discussion in 'The light stuff area' started by joshwat1, Dec 6, 2013.

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  1. Dec 6, 2013 #1

    joshwat1

    joshwat1

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    i know it sounds like a stupid question but . im not an engineer . my prob is i have a plane that has a gross weight of 377 lbs . wet with pilot .
    the plane is a maxair hummer vtail . also would like to know is the a way to beef it up to where it would handle more weight . i have seen vids of people with there hummers and they have 503 motors on them and nose cones. i am sure those planes are over the 377 lbs easy . any help would be great thanks in advance

    josh
     
  2. Dec 6, 2013 #2

    TFF

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    They probably just do it. Maybe one size bigger cables; I bet they are not doing any design. How much more weight are you after? How much reserve do you have at the 254 lb mark?
     
  3. Dec 6, 2013 #3

    Dana

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    Impossible to answer without knowing the original design parameters leading to the 377# limit. You'd have to know what design load factor was used, and what the weak point(s) of the structure is/are. Essentially you'd have to redo the entire structural analysis of the aircraft.

    The 254# empty weight is a legal limit for Part 103 eligibility, not a structural limit.

    Dana

    A liberal is someone who feels a great debt to his fellow man, which debt he proposes to pay off with your money.
     
    keith103, 1Bad88 and Topaz like this.
  4. Dec 7, 2013 #4

    TFF

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    I was meaning difference of empty to 254. Then he has that much weight he can can work with.
     
  5. Dec 7, 2013 #5

    joshwat1

    joshwat1

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    The plane weighs 177 lbs dry /w motor. And it is a max air hummer. So there is roughly 75 lbs before it reaches the 254 mark . Like in the opening post I have seen these smith bigger motors on them.
     
  6. Dec 7, 2013 #6

    joshwat1

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    well my plane is 177 and i am 250 . i have room for the 254 when it pertains to aircraft weight . but pilot weight is a diferent story . i am happy at 250 and dont want to drop 50 + pounds in order to fly my plane .and i clearly pass the 377 pounds that the plane is rated at or the max weight it says in the build instructions .
     
  7. Dec 7, 2013 #7

    TFF

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    Your best bet is to see what Lockwood did to the Hummer to turn it into the single seat Drifter. More than likely 0% of the parts are the same. The real answer is different UL.
     
  8. Dec 13, 2013 #8

    joshwat1

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    still would like to know what it is that limits the gross weight of a ultralight . is it the wings or the rigging or what . cant seem to figure it out . for instance a maxair single seat drifter is prety much the same ul but with a traditional tail as where the hummer has a v tail . they use the same 5" boom tube and truss style set up . and the ws is 30 feet i believe and the hummer ws is 34 feet . i maybe wrong but i would assume that the hummer with the longer ws would carry more in terms of gross weight than the drifter . and please keep in mind that i am a newbi as it pertains to ul aircraft thanks
     
  9. Dec 13, 2013 #9

    joshwat1

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    here are some pics
     

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  10. Dec 13, 2013 #10

    joshwat1

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    sorry for the pics being side ways .
     
  11. Dec 13, 2013 #11

    Brian Clayton

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    I understand what you getting at. Gross weight refers to what the airframe will stand before failure, not just what the wings and engine will lift. It includes everything, wing spars, fuselage, etc. The problem you will run into is this..... a lot of ultralights are.....well.....not engineered. In other words, some of this early stuff was engineered by test flights. If it didn't kill the designer, that was good enough. Don't get me wrong, not all of them......but quite a few. Quickest thing to do is find some other folks flying the same model you have. Have you checked for a yahoo group? Don't get discouraged, in the real world, I imagine that a bunch of these were flown over gross weight. No idea if they did mods or not (I would guess not). Or (and I hope you like math) you can dive off into the world of engineering and reverse engineer what you have and see if it is safe to fly at a higher gross weight. The question is not "will it fly at a higher gross weight" but rather "will it fly, break apart, crash and kill me at a higher gross weight". You need to find someone that owns or has owned one of these and talk to them.....that's the fastest route.
     
  12. Dec 14, 2013 #12

    clanon

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    Ohhh my neck ...is killing me!!! :gig:
    20131102_174308 - Copy.jpg 20131102_174149.jpg 20131102_174157.jpg 20131102_174205.jpg 20131102_174217.jpg 20131102_174223.jpg 20131102_174231.jpg 20131102_174256.jpg

    PS: What are the dimensions and wall thickness on the boom ?...:ponder:
     
  13. Dec 14, 2013 #13

    keith103

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    You could add another 75 to 80 lbs and still stay 103 legal.
    But it is important to know the reason for beefing it up:
    is it to go faster by installing a more powerful /heavier engine
    or to get a better rate of climb ?

    You could possibly beef up the structure which may result in the addition of about 45 lbs ( ball park ), and
    then you may need to add a bigger engine - say about 30 lbs heavier than what you have now.

    possible areas to replace / strengthen: Wing spars, ribs, thicker bracing wires all around, replacing structural
    aluminum tubing with bigger / stronger tubing, stronger attachment hardware, bigger tires.
    Basically you may end up strengthening just about everything except the boom.

    You do not have to necessarily increase the span or chord, but to get the extra lift (to offset the increase in weight) you
    could simply increase the camber of the airfoil.

    There is no limit for the gross weight for an UL. It is just that the regulators classify the smallest airplanes
    as UL's. Irrespective of the FAA classification, each plane is designed to certain structural standards and integrity
    to suit a specific purpose.

    FAA classification does not denote a max gross weight for UL. It just says that if the airplane
    weighs in below 254 lb empty, then you can fly it without certification / registration / or a pilot's license.
    But then , a 254 lb empty airplane, can only carry a certain amount of gross weight ( may be 550 to 600 lbs max)
    using current technology, no matter how you design it.
     
    Last edited: Dec 14, 2013
  14. Dec 14, 2013 #14

    BBerson

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    Longer wings lift better. But because of leverage a longer wing is not as strong as a shorter wing with the same diameter spars. These early ultralights are very light duty for lighter pilots.
    Don't assume anything.
    Looks like with your 250 pound body weight and 5 gallons of fuel you are about 80 pounds over the manufacturers gross weight.
    It could be a performance problem (poor climb and high sink rate).
    Probably best to pick another later design made for your weight.
     
  15. Dec 14, 2013 #15

    Dana

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    Gross weight from a structural standpoint is limited by whatever single thing is weakest. For example, you might have an immensely strong wing, but a weak tail boom (or vice versa). Structurally, gross weight is like a chain, which is just as strong as its weakest link. As I said above, there's no way to know where that weakest link is without doing an analysis.

    It could also be a performance limitation. You could have a plane built like a tank, but put too much weight on it and it won't get off the ground, or if it does, won't climb fast enough to clear the trees at the end of the runway. In this case a bigger engine can help, if you're not also limited by the structure as in the above paragraph.

    Aircraft designers try to keep things as light as possible to get the best performance, so in a well designed plane there's not a lot of leeway to go over what the designer specified, unless (and this is not uncommon, but you have to know for sure) he planned for future upgrades.

    The gross weight specified should (must, for certificated aircraft) be the highest weight that the aircraft was tested at. Go over that and you're a test pilot.

    Ultralights are also limited by the stall speed limit specified in Part 103. This doesn't take into account gross weight, but uses the empty weight, a "standard" 170# pilot, and a full fuel tank. Increasing the empty weight may put you over the limit if the wing isn't big enough.

    Dana

    A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.
     
  16. Dec 14, 2013 #16

    Vipor_GG

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    Just an observation, the wood prop in these pictures is vertical. From what I've been told it should be kept horizontal when not in use.
     
  17. Dec 15, 2013 #17

    Dana

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    That's been believed for years, but one prop manufacturer tested it at one point and concluded there was no difference in how the prop is stored.

    Perhaps in the old days, with badly finished props and planes parked in wet fields, there might have been an issue, but not with a well cared for prop stored inside

    Dana

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  18. Dec 15, 2013 #18

    Turd Ferguson

    Turd Ferguson

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the

    Be hard to determine just by looking. You'll need to do a complete structural analysis on the airframe - both mathematical and static testing. The principles of aerodynamics are not going to stop working because of overload. However, at some point, loading from additional weight will cause the structure to fail. Most prefer a margin of safety in the structure, that is once you reach the design limit, you want an additional 50% or so in strength before the structure actually fails.
     
  19. Dec 15, 2013 #19

    keith103

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    Re: what limits an ultralight to a gross weight when it pertains to the frame not the


    I mentioned a few things in my earlier post. I add a few more:

    For your airplane to carry more gross weight, you need to produce more lift to carry the increased weight:
    Beefing up the fuselage structure alone, will not necessarily increase lift, unless you modify the airfoil.

    We could increase the lift by:

    1. increasing wing area ( increasing span and / or chord )

    2. increasing the camber of the airfoil

    3. increasing the angle of incidence of the wings ( there are limitations and consequences to this )

    4. increasing the operating speeds ( without changing the airfoil structure ) : this needs a more powerful engine to increase the speed, means a higher stall speed because the plane is now heavier, longer take off / landing distances etc, and also the possibility that you may go over Pt103 speed limits for stall (27 mph) and level max ( 63mph). ( this particular course of action is mentioned here, more to explain the issue, but usually not the first course one should attempt, unless the increase in gross weight is marginal. )

    When we attempt one or a combination of the above ways to increase the lift, we will usually find our empty weight going up. To handle this increase in weight of the structure itself, and also to support the additional weight of the increased payload and the bigger engine, we will have to strengthen other non-aero-dynamic structural items. So beefing up the structural items is just a consequence made necessary by the all the other changes you make, (not the enabling factor to make more gross weight possible).

    Changing the gross weight significantly ( not just minor changes ) is changing the basic design of an airplane. It will need changes to almost everything else on the airplane. Confronted with an issue like this, I will probably select a different design altogether that meets my needs of carrying more gross weight.


    As mentioned in the above paragraphs, wing span is just one aspect that determines how much lift is generated by an airfoil. If the other plane is carrying more weight, its wings are obviously producing more lift than your plane to support that extra weight. Shorter wing span and more lift ?? Yes possible.. May be the other one has more wing area ( more chord length ), or more camber in the airfoil. Basically, if it is flying safely with more weight, its wings are definitely producing more lift. So to match that performance, you must first increase the lift generated by your own airplane, for which you have to do one of the 4 factors mentioned above.. (There may be additional ways to increase lift , these 4 readily come to my mind..) After you have modified your aero-dynamic design to match the lift produced by the other plane, you will have to modify your design structurally, to support the increased weight due to changes made, and also to support increased payload and increased engine weight.
     
    Last edited: Dec 16, 2013
  20. Dec 15, 2013 #20

    clanon

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