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Thread: Vertigo componant of lift and droop tips, recovering this winter, keep tips or not

  1. #11

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    Default Re: Vertigo componant of lift and droop tips, recovering this winter, keep tips or no

    The other thing to consider, with squared off and/or droop tips, the wing will produce greater lift for any given AoA, and an associated reduction of induced drag for the wing. That said, for a given weight and airspeed, the fuselage will be at a lower angle, and most likely an angle that actually creates more drag than that saved by the wing. Increasing the efficiency of the wing most likely would need an adjustment in angle of incidence to really take advantage of the lower AoA of the wing. Thatís one of the reasons why droop tips seldom result in an increase in cruise speed.

    Like all modifications, everyone needs to evaluate what you want to do with the airplane, and pick and choose what will help the airplane fit the performance you want.

    The most efficient wing plan form is an elliptical wing. The Culver Dart and Cessna Airmaster most closely utilized elliptical platforms, followed by Monocoupes and the Howard. A rectangular wing with round wingtips closely mimics an elliptical wing in the lower speed ranges, and is much less expensive to produce. The Airmaster was touted to be one of the most efficient airplanes, could carry 1 pound for every pound of empty weight, and go 1 mph for each HP under the cowl. The Piper Clipper is the next closest airplane for efficiency, only missing the weight carrying capability by about 50 lbs.


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  2. #12
    Old3pacer's Avatar
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    Default Re: Vertigo componant of lift and droop tips, recovering this winter, keep tips or no

    No aeronautical engineering although I did take an intro engineering class once. Anecdotal evidence tells me most
    pa22 or 20 pilots who have flown with modified wingtips want
    to keep them. For my money and downtime, I would keep what Iíve got and FLY.
    If I were H&!!bent on doing something I would put VGs on the wings. Mine has them
    & I love them! Added benefit- one or two day downtime

  3. #13

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    Default Re: Vertigo componant of lift and droop tips, recovering this winter, keep tips or no

    I have to google vortexas generators, I have seen them up close, flown planes with them, but never understood how something that sticks up into the air can help it stay flowing over the surface of the wing delaying the breakup and stall? Imagine a world where we could see everything..

    I just realized more about lift from reading Larrys replay, I always thought induced drag was only the drag caused by the wingtip vortices tilting the lift vector backwards so it is not acting perpindicular to the relative wind. But even without the wingtip vortices if you can visualize a wing that goes on infinitely and visualize the airflow moving over the upper surface of the wing and being shoved downward and off the trailing edge of the wing, this downwash is always going to be tilted backward more than 90 degrees from the actual upward direction of lift we are trying to move the plane, so there always is a rearward componant of lift , thats induced drag, its not just the vortices at the wingtips curving up and around and hammering the downwash down even more tilting the lift vector backward even farther back than the required downwash to make the lift.

    So the droop tips can stop the wingtip vortices from hammering the downwash down even more than required to make lift, that stops the lift vector from being tilted backward significantly and from pulling the plane backward instead of upward and allows the plane to develop lift equal to weight at a slower speed and liftoff and fly sooner than a stock bow tip wing.

    Now I am thinking about adding vortex generators to keep the airflow sticking like glue to upper surface of the wing at the criticle AOA to keep my wash down instead of breaking up. Get the reduced drag from the droop tips allowing me to liftoff slower and also the same affect from the VGs by keeping the air being shoved downward and shoving my plane upward.

    Shoot, if the VGs stop the airflow from breaking up.. wouldnt that change the critical AOA to even a higher AOA?
    Last edited by PA-16; 11-21-2020 at 08:53 AM.

  4. #14

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    Default Re: Vertigo componant of lift and droop tips, recovering this winter, keep tips or no

    I am not saying it is the case with the wingtips but there are 2 problems with anecdotal evidence; if someone goes through the time, expense and hassle to make a modification, they are more likely to exaggerate success than admit defeat. Frequently information presented after the mod doesn’t include verified numbers from before the mod.

    The easiest example I can think of is gas mileage driving a pickup with the tailgate up or down. Up has been proven to give the better mileage but there are still people driving around with tailgates down to save gas.
    "Good judgement comes from experience, and experience comes from bad judgement." - Fred Brooks

  5. #15

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    Default Re: Vertigo componant of lift and droop tips, recovering this winter, keep tips or no

    The airlines use both wingtips and VGs so that indicates they probably are doing something. I have just always wondered how those little VG blades sticking up into the wind on the leading edge help the flow instead of hinder it?

    With the truck tailgate, I am sure that changes depending on if you have a long 8' box or a short 6' box. Just like the VGs, BUT A TAILGATE IS A LOT BIGGER THAN a little VG balde, I will have to test the gas milage with my gate up and down, I have a 96 Ram 4x4 with an 8' box and a 08 F150 with a 6' box, tiny little baby box.. cant find a truck with a nice 8' box anymore If the gate is close to the cab and blocked than it will not cause drag, but if the gate is farther back and catching the air it could cause drag... after you made me think this thru I now am thinking that the gate would probably have to be back 20' to not be blocked by the cab.
    Last edited by PA-16; 11-21-2020 at 09:15 AM.

  6. #16

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    Default Re: Vertigo componant of lift and droop tips, recovering this winter, keep tips or no

    Quote Originally Posted by dgapilot View Post
    The other thing to consider, with squared off and/or droop tips, the wing will produce greater lift for any given AoA, and an associated reduction of induced drag for the wing. That said, for a given weight and airspeed, the fuselage will be at a lower angle, and most likely an angle that actually creates more drag than that saved by the wing. Increasing the efficiency of the wing most likely would need an adjustment in angle of incidence to really take advantage of the lower AoA of the wing. Thatís one of the reasons why droop tips seldom result in an increase in cruise speed.

    Like all modifications, everyone needs to evaluate what you want to do with the airplane, and pick and choose what will help the airplane fit the performance you want.

    The most efficient wing plan form is an elliptical wing. The Culver Dart and Cessna Airmaster most closely utilized elliptical platforms, followed by Monocoupes and the Howard. A rectangular wing with round wingtips closely mimics an elliptical wing in the lower speed ranges, and is much less expensive to produce. The Airmaster was touted to be one of the most efficient airplanes, could carry 1 pound for every pound of empty weight, and go 1 mph for each HP under the cowl. The Piper Clipper is the next closest airplane for efficiency, only missing the weight carrying capability by about 50 lbs.
    In terms of lift distribution and reduction in induced drag *for a given aspect ratio* - the elliptical planform is the most efficient wing planform. However it's very expensive to produce given the curved leading and trailing edges and the very large number of unique ribs.

    The closest economical approach to that is a semi-tapered like the one used on the second generation PA-28s like PA-28-161 Warrior, etc. That semi tapered wing gave a good boost in performance over the older hershey bar wing.

    However, there's also a lot more involved in wing planform. A short aspect ratio generally gives superior climb performance, and higher roll rates. A good example is the Grumman F8F Bearcat, it was more than just lots of horsepower in a fairly light airframe that gave it excellent performance as a dog fighter. The short aspect ratio wing was intended to provide a low aspect ratio wing optimized for climb and high rates of roll. That however came at the the expense of increased induced drag. It also used a double tapered wing planform which also economically approximates the elliptical wing planform. Still, if you look at the loft distribution over the wing it isn't linear, you'll a reduction as you approach the tip, and also a reduction near the fuselage due to interference from the airflow around the fuselage.

    At the other extreme, you'll see sailplanes with very high aspect ratio wings, and more often a than not with either semi-tapered or double tapered wing planforms. A high aspect wing comes a lot closer to approaching an infinite span wing where tip effects and spanwise flow are not an issue. Induced drag is much lower on a high aspect ratio wing than on a low aspect ratio wing. You get a slight increase in parasitic drag, but that increase is very small in comparison to the reduction in induced drag. It's not all positive however, as roll response is poor and there are a lot more bending stresses in the long wing which have to be accounted for in the structural design.

    That's all intro the concept of the Oswald efficiency number which is a correlation representing the change in the lift and drag in an ideal wing having the *same aspect ratio* and an elliptical lift distribution. The use of the same aspect ratio is key as aspect ratio has a significant effect on efficiency. 1.0 is the value assigned to an elliptical wing, while other wing shapes have lower numbers, usually in the .7 to .85 range for moderate span subsonic aircraft.

    So...the elliptical wing is efficient, but aspect ratio also plays a huge role, and in fact a much larger role than the wing planform alone.

    ----

    In that context the short wing Pipers are seriously aspect ratio challenged. The reason they were developed with that short wing was to reduce the weight, frontal area and wetted area of the wing to get faster cruise performance than is the case with the long wing pipers. If you compare a 4 seat PA-14 to a PA-16 with the same O-235 engine you'll get an good idea of the overall effects. The Clipper is only a couple miles faster in terms of top speed, but the cruise speed is 7 mph faster. It has a GW 200 pounds lower, but it also has an empty weight that is about 170 pounds less so it's more or less a wash. The climb rates are similar as are the useful loads, so the big differences are in the Clipper's 6-7 mph faster cruise speed and a stall speed that is also about 6-7 mph faster for the Clipper.

    ----

    I guess you can argue that the rounded tip approximates the elliptical profile, but even with the addition of a rib in the middle the airfoil section is at best an approximation and then really only on the inner half of the tip. In terms of lift production it isn't very efficient.

    If you look look at wind tunnel data for round, Hoerner and square wingtips shapes on wings with the same chord and aspect ratio, you'll find the round tip produces a significantly lower maximum lift coefficient. For example, on a low aspect ratio wing using the NACA 64(1)212 airfoil, a round tip will produce a Cl max of 1.10 compared to 1.17 for a Hoerner tip and 1.19 for a square tip. That's a 15% to 16% reduction in the max coefficient of lift with the round tip.

    To be fair, the hit you take with a round tip decreases at higher aspect ratios. However, the reverse is also true, which means the round tip works a lot less well on a Vagabond, Clipper, Pacer or Tri-Pacer, than it does on the longer aspect ratio Cub, Supercub, Super Cruiser, or Family Cruiser.




  7. #17

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    Default Re: Vertigo componant of lift and droop tips, recovering this winter, keep tips or no

    Quote Originally Posted by PA-16 View Post
    The airlines use both wingtips and VGs so that indicates they probably are doing something. I have just always wondered how those little VG blades sticking up into the wind on the leading edge help the flow instead of hinder it?
    They add energy to the boundary layer and help keep the flow attached at higher angles of attack. That keeps the wing producing lift at higher angles of attack before the flow separates (when the wing stalls).

    Even in a laminar flow wing the flow over the wing doesn't stay laminar over the entire wing chord, it just stays laminar longer. If you remember back when people smoked in-doors, the smooth portion of the smoke coming off a cigarette resting on an ash tray is laminar flow, and the turbulent portion of the smoke is turbulent flow. On the USA 35B airfoil used in the short wing pipers, the flow doesn't stay laminar much past the leading edge, so you'll see VGs pretty close to the leading edge, near the maximum thickness of the wing. On a high performance aircraft you'll usually see VGs farther back on the wing, where the laminar flow becomes turbulent.

    In either case, the VGs add energy to the boundary layer and keep it attached both at higher angles of attack and farther aft on the wing before it does separate.

    On some aircraft you'll see VGs installed in specific areas where flow separation was a problem. The A-4 Skyhawk for example has VGs out near the wing tips that were added to keep the flow attached at high AoAs. Some airliners will similarly, have short sections of VGs that are meeting a similar specific need.

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