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Thread: Total drag vs speed

  1. #1
    skywagon8a's Avatar
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    Total drag vs speed

    Courierguy's post in the 2018 Texas STOL thread triggered a memory. So I thought that my response would be better brought up in a separate thread.
    Quote Originally Posted by courierguy View Post
    Here is the punch line: as near as Joel and I could determine, the damn plane flies BETTER with the pods then without. It got off a bit quicker, and landed a bit shorter, not a lot, but for sure not slower to get off, or any longer to land. We reluctantly came to the conclusion that just possibly, some kind of black aero magic was going on, similar I think maybe to a lot of you Cub guys reporting a bit faster cruise with a belly tank. Maybe they act like little wings, or at least produce enough lift to offset their drag and weight, beats me. NO discernible difference in cruise.

    Based on my earlier testing last year when I flew at one point with just one pod, (no yawing problems, couldn't tell any difference) when I bought recently I bought just one, and have about 25 hours on it so far. Maybe I'm just clueless but I can't tell it's there in flight, (including stalls) the only disadvantage seems to be it looks weird, having two would look better, but I really don't need all that extra baggage area two would offer.
    Long ago I recall seeing a drag vs speed curve which I have not been able to locate to help with my response. This curve showed the rate of total drag increase from zero speed up through supersonic. Basically there is a gradual drag increase up through just above 100 mph when the rate of drag abruptly goes upwards to maybe 120 mph where the rate again returns to a gradual increase until reaching the speed of sound where there is an abrupt increase in drag until above the sound barrier where the increasing rate again becomes gradual. If anyone can locate this graph, please post it.

    Long ago I owned a Lake LA-4 which has a pair of very aerodynamically clean floats/sponsons hanging under the wings. I hit a bird with one sponson which produced a dent. So the sponson was removed for repair followed by a test flight to see how it flew with just one. What do you know, the cruise speed was 5 mph slower with just one. So me being the curious type, I removed the other sponson and went flying. Guess what, the cruise speed went back to the same as it was with both sponsons installed. My conclusion was that the amount of rudder displacement to offset the drag of one float was equal to a 5 mph speed loss. With both installed the rudder was not offset. The cruise speed of the Lake is approximately 118 mph. This is within that first abrupt drag increase on the curve which I mentioned.

    How and why does this differ from courierguy's observation? I don't know his cruise speed though I suspect it to be in the 80-90 mph range which is well below the abrupt rise in drag above 100 mph. His drag is in the lower range so the the rudder offset with one pod would be minimal. At least small enough so a speed loss with one pod would not be noticeable. In his case I would be concerned if the airplane entered a spin. It will spin at a higher rate in one direction vs the other. This could result in a nonrecoverable situation. I flew the spin tests for Lake when the sponsons were certified to carry fuel. The testing was done with one full and the other empty.

    Now as far as some Cubs going maybe a little faster with a belly tank installed. The tank is directly behind the landing gear V/shock strut drag mess. The tank streamlines the turbulence which is generated thus reducing drag a bit. The tank is essentially a landing gear fairing.
    N1PA
    Thanks Steve Pierce, DENNY, courierguy, Farmboy, cubpilot2 thanked for this post
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  2. #2

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    It is hard for me to fathom a fixed speed that drag rises in between the wide variety of aircraft. Basically I have not experienced this in my learning and design studies.
    Food for thought, an RV-8 or one of the a variety of Lancairs compared to most any of the pre-40s designs such as the Cubs and others, or for this matter anything in between.
    From what I have observed the clean fast planes, at least the small ones in this example exhibit a drag reduction through the 100-120 range where the airframe and wing are just starting to get leveled off and "in stride". It is much later as the speed climbs that they start to exhibit a rise in drag.
    As I see it all planes have a sweet spot where they are the most efficient and that varies quite a bit when it comes to speed.
    I am not going to touch on larger planes that are barely even fling without lift aides at 120.

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    Eddie Foy's Avatar
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    Click image for larger version. 

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    "Put out my hand and touched the face of God!"
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    skywagon8a's Avatar
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    That's one Eddie, but not the one I'm thinking of.
    N1PA

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    RVBottomly's Avatar
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    Quote Originally Posted by skywagon8a View Post
    That's one Eddie, but not the one I'm thinking of.
    I'll confess to looking pretty hard for a curve like you described, but I only found smooth parabola-like curves like Eddie posted.

    I wonder if you were looking at data points for a particular airframe?

  6. #6
    skywagon8a's Avatar
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    Quote Originally Posted by RVBottomly View Post
    I'll confess to looking pretty hard for a curve like you described, but I only found smooth parabola-like curves like Eddie posted.

    I wonder if you were looking at data points for a particular airframe?
    This is from very distant memory and was not for any particular airplane. Just basic aerodynamics.
    N1PA

  7. #7
    Gordon Misch's Avatar
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    Sky - I don't think I've seen any low speed discontinuities in drag graphs, and it's hard for me to imagine any such generalizable discontinuity dependent only on a common specific speed, other than perhaps for a specific application.

    For a specific application I can imagine a transition in flow, e.g. a transitional Reynolds number, that is purely speed dependent. But I'd expect that to vary dramatically between shapes.

    I've done some looking also, but haven't found what you're referring to. Here's one "older" (1973) publication that might hold a clue. But forewarned, it's 486 pages!

    http://www.dtic.mil/dtic/tr/fulltext/u2/771572.pdf
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    The classic drag diagram refers to the total drag of powered sustained flight. The graphs like that cut off at stall speed. The parasitic drag is what goes upward from zero at zero speed. There are lots of diagrams on the internet - here is one of them.
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  9. #9
    skywagon8a's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    Sky - I don't think I've seen any low speed discontinuities in drag graphs, and it's hard for me to imagine any such generalizable discontinuity dependent only on a common specific speed, other than perhaps for a specific application.

    For a specific application I can imagine a transition in flow, e.g. a transitional Reynolds number, that is purely speed dependent. But I'd expect that to vary dramatically between shapes.

    I've done some looking also, but haven't found what you're referring to. Here's one "older" (1973) publication that might hold a clue. But forewarned, it's 486 pages!

    http://www.dtic.mil/dtic/tr/fulltext/u2/771572.pdf
    Thanks for this Gordon. I down loaded and reviewed this pub but didn't see anything which resembles my memory. It's beginning to look as though the chart which is in my minds eye as common knowledge apparently is not. Having read seemingly hundreds of aviation volumes over my lifetime it will be a miracle to find this. It does seem as though most certified low horsepower (65, 75, 85 hp) airplanes (Aeronca 11AC, Taylorcraft BC-12D, Cessna 140, Luscombe, J-4 etc) seem to have a high cruise speed in the 100-105 mph range. Lower drag due to the wider fuselage. The tandem seat planes (J-3, 7AC, Porterfield) seem to cruise 10 -15 mph slower than the side by side seats. To get speeds up to about 120 mph it seems to be necessary to increase the power by a factor of about 2-1/2. Even the retractable geared Swift with 125 hp would only cruise in the 125 mph range. Above this speed it seems to require a smaller percentage of power increase for higher cruise speeds. This has just been my observations over time with nothing scientific to back me up. However this does fall in line with the drag wall which I mentioned above.

    I'll keep looking for this chart. In the meantime it's food for thought for all.
    N1PA

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    Ha! You pretty much summed up the difference between the RANS S-20 (side by side) and the the S-7S that I fly. You can imagine how irritating it is for us tandem S-7 pilots to find out that the newer (the 7 has been around for 30 years, the 20 is still "new"), though it uses the EXACT same wing, cruises faster! We don't even want to talk about the huge baggage area it has, due to that added fuselage width. We console ourselves with the best in class viz we S-7 pilots enjoy, and the fact that real pilots fly tandem aircraft of course. And thanks for helping me avoid thread drift by posting a new thread on the subject.

    I pay attention to any new lighter then air ship, powered ones that is, dirigible shaped, like most pilots, and have noted how part of their performance is the lifting moment of their "fuselage". I am somewhat convinced, that Milloway's pods, are producing lift, (being extremely "dirgible shaped", a bit unlike the Lake's sponsons which have to also address the hydro issues) and offsetting their drag and empty weight by just a bit, enough to make them a win/win. Most importantly, they allowed me to quit lusting after the greater baggage capacity of the 20, and stay with a real aircraft, a tandem of course. Former T-Cart pilot here BTW, and yes the thing cruised a bit faster then my 7, on less power to boot.

    Hal's day job is a top of the line, very heavy equipment mechanic, multi million dollar gold mine type equipment, that costs big bucks when broke down. We Rotax 912 pukes are extremely lucky to have him in our camp, his engine mods (the Zipper BigBore) for us offer more torque, lower the CHT's by 30 degrees, and LESS weight, and at least for me with my low compression version combined with my field elevation, the ability to burn REGULAR/low octane mogas, another win/win. I fly 200 hours a year on average, he makes me look like a total slacker!

  11. #11
    skywagon8a's Avatar
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    Quote Originally Posted by courierguy View Post
    I pay attention to any new lighter then air ship, powered ones that is, dirigible shaped, like most pilots, and have noted how part of their performance is the lifting moment of their "fuselage". I am somewhat convinced, that Milloway's pods, are producing lift, (being extremely "dirgible shaped", a bit unlike the Lake's sponsons which have to also address the hydro issues) and offsetting their drag and empty weight by just a bit, enough to make them a win/win.
    Totally agree, when you look at the Lake's sponsons you will see that they are shaped like your pod. The difference being that the top is extended up to the wing and the bottom extended down and flattened.

    As to providing their own lift, I also agree with that. Seaplane floats provide their own lift as well. This is noticeable during a power off landing approach where you will see about a 5 mph lower glide speed with the floats installed vs when they are not. They also have considerable drag with all of their struts and wires. This may not have the same effect on all planes but that is what I observed with a 7GCBC.

    I chased a blimp once with a Comanche. I was amazed at how fast the blimp was moving. When you are on the ground and they are flying overhead, it seems that they are barely moving while making lot's of noise. When I was a kid they were overhead all the time as I lived near the South Weymouth Naval Air station where they were based.
    Last edited by skywagon8a; 10-03-2018 at 05:01 AM.
    N1PA

  12. #12
    TurboBeaver's Avatar
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    Just seams like old news Pete! Girls and PA-12 pilots have know this for years, wider is better. LoL
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