I have been biten again by the tsetse fly

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by berridos, Nov 19, 2019.

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  1. Nov 20, 2019 #21

    cheapracer

    cheapracer

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    Yup, but I was suggesting though that the Verhees was particularly nose high, being relative to deltas.



    Exactly, that's why my sentence starts with "I think" ..... i.e. my opinion, and with supporting statement of reason for that opinion.

    Now back to the 'Spanish Inquisition' (the OP is Spanish lol!).
     
  2. Nov 20, 2019 #22

    Doggzilla

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    If the CG can be at 42% then the center of lift is likely closer to 50%.

    Being 25% ahead of that is going to be really funky.

    It would be interesting to load one about 40% and see how it handles.
     
  3. Nov 20, 2019 #23

    Doggzilla

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    Adding a trim tab to the center of the trailing edge would probably help a lot.

    If the control surfaces are facing upwards to trim the aircraft then they really aren’t in a good position for flight.

    Low pressure up front of the wing and high pressure at the rear doesn’t sound very efficient. Sounds like the wing is fighting itself.

    Freeing up the control surfaces will likely change the AoA significantly
     
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  4. Nov 20, 2019 #24

    Doggzilla

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    Or making part of the trailing edge an inverted airfoil would likely help. Pulling down instead of disturbing the top of the wing with high pressure pushing down
     
  5. Nov 20, 2019 #25

    Aerowerx

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    25% 50% 42% OF WHAT???? Please define?

    Typically, the CG and center of lift are referenced to the position of the mean aerodynamic cord (MAC). On a straight wing this is the leading edge. On a swept taper it is typically at 42% of the half span. Regardless, if the CG is 25% ahead of the center of lift (neutral point???), it will be quite nose heavy. Even on tailless flying wings, it is probably no more than 20%.

    All of this has to deal with the static margin, which determines the stability and "ease of control".
     
  6. Nov 20, 2019 #26

    Doggzilla

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    Yes, we are on the same page. That’s exactly my point.

    Regardless of what point of reference you select, it’s still significantly ahead of where it should be using ANY point of reference.

    I’d really like to see how it flies with some balancing weights.
     
  7. Nov 20, 2019 #27

    Aerowerx

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    You threw out some numbers.

    I would just like to know what reference point you are using. A % number with no reference is meaningless. When you used such high numbers, they seemed really strange. And it is highly beneficial to use the same datum as most others use, and is use in most printed texts. Causes less confusion. In this case, CG, center of lift, neutral point, are typically referenced to the leading edge of the MAC (which, as I stated earlier, depends on sweep).

    Now if you had started out saying that it was 25% 42% 50% from the nose of the Verhees Delta, then that would be reasonable I think. But, even for a tailless design, a 25% static margin is quite large.
     
  8. Nov 20, 2019 #28

    berridos

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    Stability
    Unlike conventional aircraft the longitudinal stability has to come from the wing itself.

    This is not as difficult as it seems as long as the cord is large enough. The principle is the same as with any aircraft: the centre of gravity has to be ahead of the aerodynamic centre. So there must always be an aerodynamic moment keeping the tail down. When flying too fast due to an inadvertent descent, the aerodynamic forces will win over the mass forces and put the tail down and visa versa.

    The aerodynamic centre with zero moment is with every section at ¼ cord. The delta has a tapered wing with sweepback, some of them have multiple sweepbacks. So finding the aerodynamic centre is not so easy. If you take the ¼ cord line and take in account every lift distribution along the span as equivalent of the cord, things can go wrong because the lift distribution is not proportional with the cord. There are parts of the wing that have a higher load than others, this depends on the planform.

    Fortunately, because of the wide cord, the centre of gravity is not critical. However it is wise to start with a forward centre of gravity to be sure of positive stability.

    Stall
    Due to the sweepback the delta wing tends to have a higher load at half span. The air going to the half span section is sucked upwards by the air that is already flowing over the nose of the centre part. This causes a higher angle of attack here.

    In a stall this can be a nusance when the wings stalls first half span. Of course this happens on one side first and the result is a spin.

    The Verhees Deltas have counteracted this by making the elevons very wide at this point. When defected up, necessary for a high angle of attack, the wing load will be diminished at this section and a stall is prevented.

    This shape of elevon helps also to make the lift distribution more like an ellips, giving a better efficiency.

    Dutch roll
    The directional stability of a Delta is in fact the same as with conventional aircraft, with the exception that roll stabilty tends to be better as directional stability. This is due to the fact that the aircraft is short and has sweepback.

    This can cause dutch roll: when the aircraft is rolled, say to the right it also slides to the right. The relative wind will “see” a longer right wing and the aircraft starts rolling left before the tail is pushed to the left, so the aircraft rolls left and slides right. This movement will repeat at the other side and the aircraft will make a funny oscillation.

    In airliners this is compensated with a yaw damper, we have to do it in a natural way. The Verhees Deltas have done it by giving the wing anhedral, so on purpose diminishing the roll stability. The positive effect is also that a windgust will push the aircraft with the wing into the wind instead of the other way like with a conventional aircraft.

    Turbulence
    A delta has some particular advantages in flying. First of all the behaviour in turbulence is much nicer. In an updraft most aircraft tend to raise the nose and after that the speed falls down. A delta reacts the other way, the nose is lowered, thus keeping the altitude. This gust penetration can be calculated and it appeares also to be the practice.

    Efficiency
    Another pro is that the propwash will give a benefit. With a normal tractor propeller setup the fuselage will fly in faster air than the wings, this causes extra drag. With a delta the faster airflow over the wing causes extra lift and diminishes the induced drag, so it is a benefit.

    A delta can be efficient but it not a certainty. The wing area has to be bigger than for a conventional airplane (because you can’t use flaps, on the contrary for more lift you even have to put the elevons up), the span is shorter, giving more induced drag.

    The advantage must come from having no fuselage and a lighter structure. Flying wings with fuselage are per definition less efficient than conventional aircraft.

    Furthermore, because of the high induced drag, the delta has to be flown fast for good efficiency. The D2 at 100 km/h takes the same power as at 220 km/h. Because it is a fast aircraft a relative big engine is necessary, the big engine and the light weight compensate for the higher induced drag when flying slow for climbing.

    Because the aircraft needs relatively much power for flying slow, there is less left for climbing. On a hot day or at high altitude the climb performance is relatively more decreased than with fi. a motorglider that needs little power to fly and has the rest to climb.

    Centre of gravity
    The further the centre of gravity is to the rear the less up elevon is needed to keep the nose up, so the wing will give more lift. The D1 has the same performance with almost empty tank and without luggage than with full tanks and luggage due to the tanks and luggage being after the centre of gravity. Of course, a centre of gravity too far to the rear gives an instable aircraft, although the stability is improved with the D1 and D2 by the fact that the elevons are not balanced: flying too slow and the weight of the elevon will win and push the nose down and flying too fast and the aerodynamic trim will win and the nose goes up, kind of artificial stability instead of those computers big aircraft have and are not allowed for us.
     
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  9. Nov 20, 2019 #29

    berridos

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  10. Nov 20, 2019 #30

    Doggzilla

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    Yes, that’s what I’m saying. The numbers look strange because the aircraft CG is strange.

    Look at the pilot and engine position. That’s like 70% of the gross weight and their shared CG is probably 30% from the nose, and 25% forward of the MC.

    It definitely needs some CG work.
     
  11. Nov 20, 2019 #31

    Aerowerx

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    MC==MAC????

    Where is the CG with respect to the MAC? Wouldn't surprise me if it was at the pilot's seat.

    And keep in mind that tailless aircraft, of which the Verhees is one type, typically have larger static margin than "normal". Some people have got in trouble because "the numbers look strange", and then proceed to make the numbers "look normal" by moving the CG back. At which their tailless aircraft goes nose-up, and flips over into an non recoverable tumble.

    just because the numbers don't look like a J-3 cub does not mean they are wrong! You have to judge each design on it's own. And don't forget that you do not have a horizontal tail on a long moment arm to help things out. Since birds have no, or minimally functioning, horizontal tails, one could make the argument that the numbers on a J-3 cub "look strange".
     
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  12. Nov 20, 2019 #32

    Himat

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    Part, optical illusion, part wing planform.
    With a low aspect ratio wing it need more angle of attack to get the same lift as a more ordinary light aircraft. Then the Veerhes Delta have anhedral, a “thick” wing and some built in wash out in the wing. Tractor prop in the nose placed straight in front of the wing leading edge. All together it looks like it fly nose high at all but high speed.
     
  13. Nov 20, 2019 #33

    cheapracer

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    Wow, this is just awesome ...

     
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  14. Nov 20, 2019 #34

    Doggzilla

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    This is already taken into account. The maximum is around 42%.

    So no, that’s not an issue.

    In fact, CG too far forward is what causes tumbling after stalling. As the CG swings downward the inertia flips the vehicle end over end.

    That’s why otherwise stable weight shifters tend to tumble during stalls. The weight sticks out front during stall, then swings downwards and the inertia of the down swing causes it to invert. And repeat.

    This is incredibly dangerous for tailless aircraft where there is a lack of static stability to overcome pitch inertia.
     
  15. Nov 20, 2019 #35

    Doggzilla

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    That’s actually much better looking in motion. The pictures don’t do it justice.
     
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  16. Nov 20, 2019 #36

    berridos

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    When flying too fast due to an inadvertent descent, the aerodynamic forces will win over the mass forces and put the tail down and visa versa.
     
  17. Nov 20, 2019 #37

    Doggzilla

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    Wonder how it would fly with a dogtooth leading edge like an F-4 Phantom. Usually results in better tip control and stall speed.

    Probably make it look more aggressive too.
     
  18. Nov 20, 2019 #38

    berridos

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    It looks super stable in flight in line with the comments of the designer
    Leading edge slats added to very moderate flaps could lower landing speed.
     
  19. Nov 20, 2019 #39

    Riggerrob

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    You can build a delta (flying wing) with a reflexed airfoil or reflexed control surfaces. The end result is the same.
     
  20. Nov 20, 2019 #40

    cheapracer

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    What i would be seriously interested in is that 2 seater as in a high wing version.

    Also, i think he missed a chance for larger wheels, at least at the rear, to get some substantial braking into it.
     

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