Downwash Angle

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by berridos, Jan 12, 2011.

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  1. Jan 12, 2011 #1

    berridos

    berridos

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    According to NACA report 648 I have made a little excel sheet to cuantify the downwash angle on the tail for a plane with 30% slotted flaps over 70% of the semispan.

    I got the following numbers:

    No Flaps AoA=0º (Cl=0,1) Downwash Angle 1.18º
    No Flaps AoA=12º (Cl=1.2) Downwash Angle 7.1º
    Flaps Extended 40º AoA=12º (Cl=2.4) Downwash Angle 14.21º

    Do you think I have interpreted the paper correctly?
     

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  2. Jan 18, 2011 #2

    berridos

    berridos

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    Well the post doesn´t seem very succesfull.
    One reason I posted it, is because if I use the polynomial fit used by Roncz/Dommash I get 8,5º for deployed slotted flaps on a standard high wing airplane.
    Somehow 14º sounds a little too much but it would be wind tunnel data according to the NACA paper.
     
  3. Jan 19, 2011 #3

    HumanPoweredDesigner

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    Wow! Thanks! That is just the paper I needed. It is now saved on my hard drive.
     
  4. Jan 19, 2011 #4

    berridos

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    Here you have the equivalent downwash paper in ground effect.

    This one I still havent digested.

    But sniffing into it, the ground effect really starts just before touch down (the effect starts at 50% of the semispan). Lower than I thought. 40% is really the highwing waterline height plus landing gear.

    On a high wing I still dont know if the ground effect is so relevant regarding the problem of running out of elevator according to the semispan-% heights used in the paper.

    I am working on all this stuff to size my U-tail aproximately in the right range.

    if you use the papers could you please share your results just to compare our numbers?
     

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  5. Jan 19, 2011 #5

    c p skeates

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    down wash :whistle: dosent exist ..nor dose dynamic lifting of birds at sea ..nasa is natorious for believing in perpetual motion .. check out navier-stokes manuals before taking any thing nasa says as correct they will kill you before admitting they were wrong .. how many shuttles have crashed .. so get navier-stokes manuals and books ..i have them all and not a word about down wash and dynamic lifting even though nasa employes their manuals ..:tired:
     
  6. Jan 19, 2011 #6

    wsimpso1

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    c p skeates,

    "Down wash does not exist" According to all classic airfoil theory, the foil changes the momentum of the air, and the forces for doing so get imposed upon the foil. The momentum changes necessarily drive, through Newton's laws of motion, a downward component to the change in momentum of the air exactly with the lift, in the fore aft direction exactly with the drag, etc. Since you assert that it does not exist, you really are going to have to explain the mechanism of lift without vertical momentum changes.

    Billski
     
  7. Jan 19, 2011 #7

    topspeed100

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    How does 14 degs affect the ground effect will it multiply the lift if the wing is 30 cm above the ground ( compared to 1.5 meters ) ?
     
  8. Jan 19, 2011 #8

    berridos

    berridos

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    Apparently in ground effect, the downwash disapears and without the downwash, carrying the large flaps with their pitching moment (-0,3) becomes a problem. The tail has to be sized for this situation without the help of downwash.

    I am looking to quantify how much of the downwash dissapears in stall speed in ground effect with full flaps and forward CG.
     
  9. Jan 19, 2011 #9

    Topaz

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    It's not that downwash disappears in ground effect, it's that the ground gets in the way and straightens out the flow. You really only need to do a trim check at the most-critical ground-effect condition: On the ground. Assume no downwash effect on the tail at all under this condition, and you'd have a conservative bound for tail power required in ground effect. The downwash effect on the tail will slowly increase with added altitude until you get to the free air (no ground effect) value. Estimating anywhere in-between doesn't really gain you much in terms of the engineering required, at least for an airplane.
     
  10. Jan 19, 2011 #10

    berridos

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    Ok Thanks
    With the prop windmilling do I still have some adittional propwash over the tail?
    Or to stay conservative should I ignore propwash in this situation?
     
  11. Jan 19, 2011 #11

    orion

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    Good answer Topaz - exactly. The critical condition is that the plane must be able to flare so the design condition to check is to make sure the tail has enough authority to bring the plane to full stall, with flaps down, in the proximity of the ground. This is actually the condition that sets your forward CG limit.

    As such, there is one other influence to consider - the ground plane affects the horizontal tail in a way that's similar to the upper wing's affect on the lower wing in a bipe. In other words the lift curve slope is "squashed down" so the CL per any angle is reduced, as is CL.max.

    Regarding propwash, this disturbs and somewhat blocks the air flow so this too reduces the horizontal tail's power. This condition is critical on pushers. The only way the prop's detrimental effect will be reduced is to stop it completely or keep a bit of power on at all times.
     
  12. Jan 19, 2011 #12

    topspeed100

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    So is it very practical to keep the horizontal tail high at ground effect ? I assume so ?
     
  13. Jan 19, 2011 #13

    orion

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    The only real benefit is the lack of the ground's affect on the lift curve. The downwash issues are still pretty much there so for that condition you'll still need to run the numbers as for a low mounted tail.
     
  14. Jan 20, 2011 #14

    berridos

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    Orion, to account for the mentioned biplane effect on the tail, is the following 2D to 3D formula for a all moving u-tail still valid when considering that the Downwash reflects on the ground?
    dCL/dAoA=0.10966 x AR / (2 + sqrt(4 + (AR x Comressibility Correction)^2 x (1 + (Sweep Correction/ Compressibility Correction)^2))) ?????

    Is the theoretical dCL/dAoA=0.10966 really a constant for a Riblett GA37A315 or a NACA 63A012 ???
     
  15. Jan 20, 2011 #15

    orion

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    Well, you really don't need to take it out to five decimal places - sort of goes back to the old Boeing saying: Measure with calipers, mark it with a crayon and cut it with a hatchet. It's good to be accurate in your design but also understand that in many of the variables there is a level of uncertainty so a bit of rounding wll not affect the answer to too a significant degree. But to answer your question, yes, the dcl/dalpha for section families is pretty constant. True, there are some variances but for the most part, they're few and relatively minor.

    The ground effect primarily deals with the forward CG limit so in the program that I use the analysis looks strictly at that (coupled with the flap deflection of course). A number of the variables are defined earlier in the program so hopefully what I'm attaching is sufficient.
     

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  16. Jan 20, 2011 #16

    PTAirco

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    See my signature.....
     
  17. Jan 21, 2011 #17

    berridos

    berridos

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    Thanks a lot Orion
    I tried to write it down in excel but had a couple of issues with the notation.
    Several doubts and no idea what the term j stands for.
     

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  18. Jan 21, 2011 #18

    orion

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    Many calculations in this program are in matrix form - they examine a number of numerical values for particular variables in one shot. As such, the term "j" is simply a suffix referring to the variation of values for different incidences of the horizontal stab (which is listed earlier in the program).
     

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