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Thread: Race 39 Gear Leg Drag

  1. #1

    Default Race 39 Gear Leg Drag

    During Saturday's sport gold heat race I rememebered that Jeff Lavelle's Glasair had had problems with its landing gear during the final race of 2018, and I started checking my pictures to see if the tell-tale shadow appeared. Sure enough there it was. My pictures aren't as clear as some of those posted on the forum but they were clear enough to show Jeff and his team. (I borrowed WJ Pearces photo for this post, I hope that's OK.)

    Here's one of my pics:
    Click image for larger version. 

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    I got to wondering how much of an effect the gear being out had.

    My first cut at it is based on the assumption that both the qualifying run and Saturday's race were run flat out. (Footnote 1)

    The power required to for an airplane to fly in a given condition is simply Drag times Velocity:

    Pr = D * V

    If Pr is the same for qualifying as it was for the heat race then:

    Pr = D_qual * V_qual = D_race * V_race

    Since V_race is slower than V_qual, we know the D_race must have been bigger than D_qual.

    Drag can be calculated from speed (V) , air density (rho) , wing area (S) and a drag coefficient (Cd):

    D = 1/2 * rho * V^2 * Cd * S

    Pr is just D*V so it is:

    Pr = 1/2 * rho * V^3 * Cd *S


    Now, we don't know Cd for the airplane. But we can say that during the heat race Cd was increased by some amount by the gear leg hanging out, which we'll call deltaCd.

    Since we assume Pr is the same in both conditions we have:
    1/2 * rho * V_qual^3 * Cd * S = 1/2 * rho * V_race ^3 * Cd * S + 1/2 * rho * V_race^3 * deltaCd * S

    With a little algebra we can find the ratio of the change in drag to the basic drag:

    deltaCd/Cd = (V_qual^3 - V_race^3) / V_race^3

    V_qual = 396mph = 578 ft/sec
    V_race = 373mph = 544 ft/sec

    deltaCd = 19.6% of the basic drag. Pretty big!

    Another way to think about this is to consider the "Drag Area" of the airplane. This is the area of something with the same drag as the airplane, but that has Cd = 1.

    We can write Drag the same way as above, but use a shorthand for 1/2 * rho * V^2 above.
    We'll call that 'q'

    So:

    Drag = q * Cd * S

    If we pretend Cd = 1, then

    Drag = q * S

    and the drag area is:
    S = Drag / q.

    We'll call Drag Area Ds for short.

    Using the same idea of letting Pr be the same for both qual and racing we get

    Ds_qual / Ds_race = V_qual^3/V_race^2 = 1.196 - which not too surprisingly is 19.6% more than the basic drag area.

    But what *is* the basic drag area?

    From the Glasair website we learn the following:
    Glasair III HP Max speed at Sea Level Wing Area
    300 290mph 81sq ft.

    We can rearrange our power equation to solve for Cd based on wing area

    Cd = 2 * Pr / (rho * V^3 * S)

    We have to adjust the engine power by the efficiency of the propeller get Pr - Pr is probably 85% of 300hp, or 255hp.
    which gives us and estimate of Cd = 0.0192.

    We can then use Cd to calculate Pr at 393mph: 650 hp
    And adjust that by the prop efficiency to get the engine power: 765 hp. (Note that I did this at sea level)

    Remember that Pr = Drag * V. And we can get the drag area with Pr / (V * q) = Ds

    Ds = 1.56 ft^2.

    Now I'm not all that great at estimating height by shadows, but I remember that if you can measure two shadows, and know the height of one of the things casting a shadow, the height of the other thing is in proportion to the ratio of the shadows. The flap bracket casts a shadow roughly the same length as the gear door. The flap bracket looks a lot taller in the shadow photo than it does from the wingtip (WJ Pearces picture is much sharper than mine), and with
    a little kentucky windage, I'm going to guess that the gear leg as up about an inch along about 36 inches of wing.

    Click image for larger version. 

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    Click image for larger version. 

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    This gives a frontal area of 36 sq inches.

    I'm going to estimate that the drag coefficient is about that of a bluff body or Cd = 1, giving a drag area of 36 sq in, or 0.25 sq ft

    0.25sq ft / 1.56 sq ft = 16%.

    Close enough. =)

    Footnote 1:

    We assumed that Pr was the same for both qualifying and racing.
    One might assume that means that the engine was producing the same horsepower, but that's not quite true.
    At a constant airspeed, the Drag is equal to the thrust, so we can also think of Pr like this:

    Pr = T * V
    The engine produces power at the crankshaft; the propeller applies that power via thrust. If V is lower, then T must be higher to keep Pr the same. This could be done by spinning the engine faster or through prop pitch being a little different. Without knowing the engine parameters I can't say for sure, but I think for this estimate assuming that Pr is constant is reasonable.

  2. #2

    Default Re: Race 39 Gear Leg Drag

    Quote Originally Posted by L.E.D. View Post
    I borrowed WJ Pearces photo for this post, I hope that's OK.
    Absolutely is!

    Am I understanding this correctly in that, by your estimation, the open gear increased the Drag Area (Ds) by 16%? Sorry if I got lost in the calc. At around 395 mph, what would be the approximate speed penalty for such an obstruction?

    Also, I want to point out that the gear issue was NOT just for 2019. The below image is from 2018, and the gear shadow looks about the same to me.


    Going back further is problematic because the image quality (and photographer enthusiasm) drops off. But things look pretty good in 2017. Note that the left side wheel door seems to have a slight shadow, but all appears flush on the right.
    Click image for larger version. 

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    Bill Pearce

    Old Machine Press
    Blue Thunder Air Racing (in memoriam)

  3. #3

    Default Re: Race 39 Gear Leg Drag

    Has anyone sent the crew of #39 these photos? It certainly wouldn't be the first time the photographers helped a team out by showing them something their airplane was doing that they weren't aware of. A high quality photograph, or video, of an airplane at speed can show a ton of things that the naked eye and human brain can't perceive in real time.
    Last edited by knot4u; 10-05-2019 at 01:38 PM.

  4. #4
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    Default Re: Race 39 Gear Leg Drag

    Quote Originally Posted by knot4u View Post
    Has anyone sent the crew of #39 these photos? It certainly wouldn't be the first time the photographers helped a team out by showing them something their airplane was doing that they weren't aware of. A high quality photograph, or video, of an airplane at speed can show a ton of things that the naked eye and human brain can't perceive in real time.
    They know it was happening. The shape of the gear doors on the Lancairs makes it tough to get them to seal at really high speeds. Jeff has worked hard on improving it, but it still lacks a little.

  5. #5

    Default Re: Race 39 Gear Leg Drag

    Quote Originally Posted by W J Pearce View Post

    Am I understanding this correctly in that, by your estimation, the open gear increased the Drag Area (Ds) by 16%? Sorry if I got lost in the calc. At around 395 mph, what would be the approximate speed penalty for such an obstruction?
    That's correct. A speed penalty of 23mph (qual speed vs race speed), at the same thrust horsepower, would indicate increase in drag area of 19%. The pictures indicate roughly 16% more drag area based on geometry so that tends to confirm the estimate.

    I did show them my pics after the race, and I heard second hand that they adjusted the uplocks but I don't have any other info on how well it worked.

  6. #6
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    Default Re: Race 39 Gear Leg Drag

    Quote Originally Posted by knot4u View Post
    Has anyone sent the crew of #39 these photos? It certainly wouldn't be the first time the photographers helped a team out by showing them something their airplane was doing that they weren't aware of. A high quality photograph, or video, of an airplane at speed can show a ton of things that the naked eye and human brain can't perceive in real time.
    Right you are. I'm up to 3 different teams that I've shared my shots with who have found speed from them. (Dusty Dowd, Full Noise and this year, One Moment)

    You don't even necessarily have to have the sharpest picture in the world, it just needs to show something out of place be it a gear door, flaps, exhaust plume extending 4 feet below the plane, or a separation of the spray bar water as it comes off the tail. There is much that can be learned from pictures if you know what to look for.

    Will

  7. #7

    Default Re: Race 39 Gear Leg Drag

    Love stuff like this, thanks for showing all your work.

    The gear is at the minimum pressure area on the airfoil. Even though they are on the "high pressure" side, the gear is only exposed to the bottom surface. So with all that high velocity air moving by it creates a lower pressure area than the (hopefully) stagnant air inside the gear well and tries to suck the door open. Once the door is cracked open and air starts getting into the gear well, things get messy.

    And Will your photos have messed with my OCD for the past year. So much so that I had to find a different picture for my desktop background. So thanks for that....
    "young" Thomas

  8. #8
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    Default Re: Race 39 Gear Leg Drag

    Quote Originally Posted by CubersWrist View Post
    And Will your photos have messed with my OCD for the past year. So much so that I had to find a different picture for my desktop background. So thanks for that....
    Hahaha, now that is funny. Next year I'll be able to get even more detailed shots, I just bought an even higher resolution camera body.

    Will

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