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What is the disadvantage to a round (disk) shaped aircraft

  1. Mar 29, 2008 #1
    For some time I've been thinking of making a round disk shaped model aircraft that would appear to have low drag, maximum internal carrying space, and maximum load carrying ability. As you can guess, there is little available on such a design, other than the Pancake, built at the end of WWII. Which seemed to fly fairly well from the web reports.
    I was thinking of using a laminar flow airfoil, as thin as possible that would create the require lift. And yet, there seems to be a serious problem with this round aircraft design, as the design hasn't been further developed, or for that matter researched, since slightly after WWII.
    However flying wings are in great favor now, (apparently after control problems were solved) after being ignored for 40 years. Can anyone tell me what the drawback is that I'm not seeing, or aware of to a round shaped aircraft? You know, so I don't totally waste my time trying to make something that doesn't work, yet should. Any insight would be appreciated.

  2. jcsd
  3. Mar 29, 2008 #2


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    Staff: Mentor

    Take a half circle with the diameter parallel to the horizontal axis, and with the reference direction (0°) in the vertical. Now look at the chord length with respect to the vertical orientation as one goes to -90° or 90°. There is quite a variation. The lift effect becomes much less as the chord length decreases, and approaching -90° or 90°, the lift (pressure differential) drops of dramatically - due to leakage over the edge.

    Ideally the chord length should be equal, but mechanically this is undesirable because the wing is usually a cantilever and for a given vertical force, the moment increases with the moment arm, to the designer would wish to have greater lift nearer the root of the wing and minimize the lift toward the wing tip.
  4. Mar 29, 2008 #3


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    The efficiency of a wing is better with a high aspect (long wingspan) ratio than a low aspect ratio (short wingspan). This efficiency also includes drag - there is no drag advantage to a saucer-shaped aircraft. And you mentioned making it thin - what would the fuselage look like?
  5. Mar 30, 2008 #4
    PF MENTOR, thanks for the reply. I recall that one of the advantages of the WWII Spitfire fighter was its elliptical wing because it gradually stalled. While the ME-109 and FW-190 stalled suddenly if over controlled in a high G turn for example, and became instant victims due to the Spitfire still having some control.
    I was thinking that a circular wing would be even more manuverable, stronger in high G loading, and though it would may have more drag over the center section of the circular wing, it would be less so, (hopefully) than a aircraft with a fuselage and tail surfaces. Also the entire aircraft would be a lifting body, not just parts of it.
    The thickness of the wing would follow the chord of the laminar flow circular wing, and would open up the center interior automatically for the engine space, RC controls, and fuel without having any bumps to cause air separation.
    So there would be no fuselage as in the B-2 flying wing. The old XB-49 flying wing of the 40's was a very clean design, but because it did not have a fuselage, the thickness of the wings was very high to accommodate the engines and load carrying capability. It would seem that a circular wing would avoid some of these problems.
    Another reason(s) I was looking at a circular wing model aircraft design was the simplicity in construction out of foam and fiberglass, strength to weight ratio, and of course, it would look different than the norm. However, I gather from your reply that the aspect ratio effects are so bad, that they over come any advantages over a flying wing for example?

  6. Mar 30, 2008 #5


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    One has to look at the aspect ratio (length vs width (chord length)) in addition to geometry. Shorter wings would be beneficial for maneuverability and are certainly stronger - BUT that benefit comes are the cost of reduced lift and greater energy consumption, and consequently shorter range.

    There also other considerations with respect to fuel storage and armaments (in the case of a fighter) or passenger volume (in the case of passenger transportation).

    These might be of interest -




    Commerical aircaft for passenger transport from the larger 737/747/777 to the smaller regional jets are using winglets to improve wing performance. This reduces the losses at the wingtips.
    Last edited: Mar 30, 2008
  7. Mar 30, 2008 #6


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    One of the main advantages of a round aircraft (as a fighter) is that it would not have to turn (change direction) to fire on any target along its own longitudinal plain. However, solving the problems with the flying wing was an enourmous problem that was only recently solved at tremendous expense. I odn't think the cotnrol problem has been resolved for a disk.

    Mind you, I sometimes suspect the Avrocar has been developed and put into the air, and is responsible for some UFO sightings. I don't really believe it, but then again, I wouldn't be all that suprised, either.
  8. Mar 30, 2008 #7


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    Huh. You learn something new every day.
  9. Mar 30, 2008 #8
    Thanks PF MENTOR for the 3 references, 2 of which were very good. Actually I now see part of your argument, and yet I'm still curious as to how bad, bad is in relation to other to the other aspects I see as advantages.
    I have a large irrigation canal in my backyard, and as the weather warms up, (it is currently snowing) I think I'll do some water flow testing of the design with food coloring to show the flows. I've always kind of wanted to do that, but never had enough motivation to actually go to the trouble.
    The circular shape would be something totally new, and interesting to try out. Beyond the norm you might say. Undoubtably the results will show why circular shaped aircraft are not filling the skies, as much smarter people than I have undoubtably thought about it, and rejected the idea.
    But as you say, "understanding how to solve the problem (i.e. how you get the right answer) is just as important, if not more so." Besides I will learn first hand, why PF MENTOR was right. To the lathe we go. Thanks for your time and effort.

  10. Mar 30, 2008 #9


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    While it may have been an improvement over designs at the time, it is still quite primitive compared to current aircraft. And it doesn't bear much resemblence to your idea of an actual circular shape (it was quite an eccentric elipse). But yes, tapered wings are gentler in stall than straight wings. There aren't many stright wing craft out there anymore though (mostly just small prop planes).

    Most planes at the time had straight wings and poor stall characteristics is one of many problems with straight wings. But then, you mentioned you would do a laminar flow airfoil and poor stall characteristics is also a characteristic of laminar flow airfoils.

    Regardless, jet fighters today have the ability to fly at absurdly high angles of attack, and that's tough to do with a low aspect ratio wing. Check out what the F-18 uses for flow control. The wing root extensions create vortices that keep the flow attached to the wing: http://www.globalaircraft.org/planes/f-18_harv.pl
    Well, certainly strength would be an attribute of such a low aspect ratio wing, but that's really about it. The wing itself would be much less efficient than typical wings. Aerodynamically, there really are no benefits to what you are suggesting.
    Yes. Aspect ratio is extremely important for aerodynamic efficiency. The round profile causes other problems. You wouldn't want to fly at high speed in such a plane either. With a thickness that varies across the profile (and gets really big), you'd start getting shock waves in the center at relatively low speed compared to other aircraft. And without anything for the main shock wave to attach to (something wedge-shaped), you couldn't go supersonic - drag would be enormous.
  11. Apr 15, 2010 #10
    Um, you guys are posting a real mix of fact and fancy here.

    Tapered wings, if other remedies are not applied, stall worse than straight wings. There's all kinds of research behind that. They can be more efficient structurally and aerodynamically, but for a wing with no twist, untapered ones will have less trouble with sharp stalls. Assuming no sweep.

    Really low aspect ratios do not docilely obey the mathematical approximations which were originated for wings with larger aspect ratios. When you get down to an aspect ratio of one or two, it's a different story. I suggest you go over to ntrs.nasa.gov and look up two of Zimmerman's classic papers on low aspect ratios. The first is "Characteristics of Clark Y Airfoils at Small Aspect Ratios". The second is "Aerodynamic Characteristics of Several Airfoils of Low Aspect Ratios". I also suggest you go to facetmobile.com and read what's there, plus the PAV report for NASA.

    The structural weight advantages for low aspect ratio aircraft can make up for some of the induced drag problem. Also, for some low aspect ratio configurations, induced drag is not quite as bad as classic theory suggests. And the increased Reynolds number can make up for some of the drag problems. Handling can be less critical as well, since there is less change in lift per unit change in angle of attack. As observed by someone above, many drag producing things can be hidden inside the wing.

    Obviously a sailplane isn't going to be terribly low aspect ratio, but there are other mission where the low aspect ratio works.

    Models tend to be overpowered, so there's even less reason not to go with low aspect ratio.
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