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Aerodynamics of a concrete glider vs a brick (MythBusters)

  1. Apr 2, 2014 #1
    I have just watched a MythBusters episode investigating whether a concrete glider could fly. They conclude that it was plausible on the basis of a model concrete glider 'flying' 34 feet while dropping 9 feet, giving a glide ratio of nearly 4:1. Now if I recall correctly, the launch speed of the glider was 22 mph. Would I be right in thinking that a brick launched horizontally at 22 mph would achieve the same sort of results, making the conclusion inconclusive? (I am ignoring the fact that the presenter appeared to continue towing the glider on a line after the so called launch.)

    Also, someone in another forum said the lift/drag ratio of a glider is independent of the weight of the glider, (all else being equal). Is that true? If it is, does equal lift/drag ratio translate to equal glide ratio?
     
    Last edited: Apr 2, 2014
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  3. Apr 2, 2014 #2

    AlephZero

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    I haven't seen the program, so no comment on that.

    You are right the lift/drag ratio is independent of weight. It only depends on the shape of the plane.

    The glide ratio is defined as flight in a straight line (but not a horizontal line, of course) at constant speed. In that situation, the resultant of the lift force (normal to the flight path) and the drag force (backwards along the flight path) exactly balances the weight (vertically down). So the L/D ratio is the same as the glide ratio.

    For a given shape, a heavier glider has the same glide ratio but has to fly faster, to generate more lift to balance the heavier weight. That is why gliding competitions specify the maximum weight of the plane, not the minimum, and the competitors usually load the plane up to the maximum permitted weight with sandbags, so they can glide as fast as possible.

    It sounds like the MythBusters experiment was poorly designed. The speed along the glide path is not arbitrary, it depends on the weight of the plane. Towing the plane is obviously adding another force. A glide ratio of 4:1 is terrible (a good sailplane design would be more like 50:1) so a likely guess is that they made a poor model (either the wrong shape, or with a poor surface finish), or they didn't fly fast enough to generate enough lift.

    Using the above definition of glide ratio, a brick would "glide" almost vertically downwards, at its terminal velocity. Launching it with a forward speed of 22 mph is irrelevant.
     
  4. Apr 2, 2014 #3
    OK, I think I have worked out the answer to the first question myself. Using the formula ##t = \sqrt{2ax}## where a = 32 ft/sec2 and x = 9 ft, the glide time is approx 0.75 seconds. With a horizontal speed of 22 mph (32 ft/sec) the horizontal glide distance is 22 ft giving a glide ratio of 2.666:1 for the brick. The glide ratio of the concrete glider at 4:1 is a bit better than the brick. Interestingly, the brick has a better glide ratio than the Northern Gliding squirrel (1.98:1) and a wingsuit (2.5:1). The concrete glider has a better glide ratio than some (unpowered) powered parachutes (3.6:1). For reference an unpowered 767 airliner has a glide ratio of around 12:1 and a modern sailplane has a ratio somewhere between 40 and 60:1.
     
  5. Apr 2, 2014 #4
    Thanks for that useful information!

    It was! They appeared out of their depth here and had a tight deadline. I think one disadvantage they had was that they were using scaled down models with a wingspans of under 3 ft. If I recall correctly, model aircraft require a lighter wing loading than full size aircraft to achieve the same sink rate.

    The Germans designed a gliding bomb in the last war with concrete wings that a had a glide ratio of about 25:1. http://www.luft46.com/missile/bv246.html

    The point I was trying to make was that launching the concrete glider at 22 mph was to me sort of cheating because it gives a brick an artificial glide ratio of 1.66. If they launched a brick at 800 mph the brick would have a glide ratio of 60:1 using the methodology and criteria used in the MythBusters episode. I guess the key part of the definition of glide ratio is the constant speed part, which does not apply to the brick, nor I suspect, to the MythBusters' concrete glider model.
     
    Last edited: Apr 2, 2014
  6. Apr 2, 2014 #5

    russ_watters

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    A ballistic trajectory would have it dropping 16'.
    It is (to within reasonable limits), but what that doesn't tell you is the required speed to achieve it. No doubt, this "glider", if it wasn't nose-diving (I didn't see it), was stalling and falling, not gliding.
     
  7. Apr 2, 2014 #6
    One approach taken in the program was to buy a propriety model glider and add lead weights to the CoG of the model to see how this affected the glide performance. The glide ratio did appear to significantly deteriorate with increased weight. This might have been due to going outside reasonable limits, or because they did not increase the launch velocity sufficiently. I suspect there is also some factor due to Reynolds number involved with a small models. I found something on a Remote Control Aircraft forum that states while full scale gliders can achieve a L/D ratio of 60:1, a typical model glider only achieves a L/D ratio of around 20:1.

    The relevant part of the Mythbusters episode can be seen at around -24 minutes in this video: http://watchdocumentary.org/watch/mythbusters-s04e23-concrete-glider-video_d00d01cba.html [Broken]
     
    Last edited by a moderator: May 6, 2017
  8. Apr 2, 2014 #7

    rcgldr

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    Full scale gliders use water ballast for weight. For example a competition Nimbus 4T with 26.4 meter == 86.6 foot wing span, with an empty weight of 520 kg?, has a max seat load rating of 110 kg, and max overall rating of 800 kg to 850 kg (TM - not sure what TM means). The wing tanks can hold 300 liters (80 gallons), which would be 303 kg, so there seems to be a conflict in the empty / maximum weight spec versus the maximum ballast weight. The water ballast can be released and drained from the tanks during flight. The Nimbus 4T has a glide ratio of 60:1 at around 60 knots.

    nimbus_4t_htm

    Unlimited thermal duration type competition model gliders have glide ratios of about 15:1 to 20:1, wing spans in the 3 to 4 meter range, and weigh around 2.5 kg, and ballast is used for windy days so the models can fly faster upwind after following a thermal downwind, but it's usually maxxed out at about .5 kg for a total weight around 3.0 kg. The upwind speed is double or more that of speed for best glide ratio, with a reduced angle of attack (and reduced camber accomplished by raising flaps and ailerons a bit), so not a lot of ballast is needed for fast flight with around 7:1 to 10:1 glide ratio. For these models, there's no maximum limit on the weight, since the goal is time duration, not distance covered.

    The highest wing loaded model gliders are the ones used for dynamic soaring. Here the goal is maximum speed obtained from a high energy source, a fast wind over somewhat non-moving (but turbulent) wind with a relatively thin shear boudary on the downwind side of a ridge line on a very windy day. These models have about 2.5 meter wingspans and weigh about 4 kg or so. The most recent record I recall was 498 mph == 801 kph was accomplished March 2012.

    I've seen part of the video. The "modern" version's wing is shaped similar to a flying wing, usually made out of EPP or Z-foam and used for combat since these foam based models will bounce off each other if there's a collision and without any denting (the foam compresses and springs back). These relatively short wing span gliders don't have a great glide ratio even when made from foam. As mentioned already, a brick's glide ratio would be close to zero. Video of combat wings:

    http://www.youtube.com/watch?v=grFuX1SwGu8&hd=1

    update - I finally got to see the test runs. The issue with the lighter glider is that it wasn't trimmed properly. It pitched downwards and slammed into the floor; basically it had "down" elevator input. It needed more "up" elevator, and with sufficient up elevator, it probably would have done somewhat better than the other glider.
     
    Last edited: Apr 2, 2014
  9. Apr 2, 2014 #8

    russ_watters

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    Optimum L/D occurs at a specific point on the lift and drag curves, corresponding to a specific angle of attack (aoa). But since a constant glide requires the lift to equal the weight, a higher weight means either a higher speed (at the same aoa) or a higher aoa to generate that extra lift.

    I've watched the video now -- yes, the glider is not gliding. It starts off in a belly-flop attitude stall and then noses down into a nosedive. It never achieves any sort of "gliding".
    Scaling is tough. If you want a model plane with the same L/D and wing loading as a full-sized plane, then you need your model Cessna to have a takeoff speed of 80 mph! That just isn't done.

    Just as difficult is structure: Since moment of inertia is a square function of beam depth, it becomes very difficult to scale-down a wing while still enabling it to be strong enough to support itself.
     
    Last edited: Apr 2, 2014
  10. Apr 2, 2014 #9

    rcgldr

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    and Reynolds number. Look up the "polars" for an airfoil, and there will be multiple curves, one per evaluated Reynolds number. In the CL versus CD graphs, it's often labeled as CL/CD, but it's not a ratio. The L/D ratio at any point on the curve will be the CL value divided by the CD value. The best L/D ratio occurs at a point where a line drawn from the origin (CL == 0, CD == 0) is tangent to a point on the CL/CD curve. In general, L/D gets better as air speed and/or wing chord increase (up to a point, once past mach 0.3, it probably gets worse): L/D at RE 1,000,000 is much better than L/D at RE 50,000, this is why full scale gliders have much better glide ratios than model gliders.

    Also the polars are usually generated for "2d" air flow, with no losses in lift due to air flowing around the wing tips. In a real world situation the actual L/D ratio will be less.

    Strength isn't an issue when using carbon fiber based hollow molded wings with carbon fiber wing spars for the wing joiners. Those high end dynamic soaring models with 2.5 meter wing spans and 4kg of weight are handling 40 to 50 g's of centripetal acceleration (lift) during flight. The F3J contest gliders with 3 to 4 meter wing spans are launched very hard with a two man pulleyed (doubles the speed) tow and also peak around 40g's or so (they can break 200 lb test fishing line if the launch is too aggressive).
     
    Last edited: Apr 3, 2014
  11. Apr 3, 2014 #10
    Wow! Is that a scale speed?

    Yes, the presenter tacked the fuselage of a normal glider onto a flying wing. For some reason neither presenter stuck to the tried and tested designs. The shopkeeper in the model shop said the flying wing had a better glide ratio than the modern high aspect ration glider, but less stability. It seems the shopkeeper was wrong on that point.

    I agree. It's a shame that the presenters did not seek some input and guidance from expert model flyers. I think that was due to a time constraints. They appeared to only have a couple of days to research the problem and design/build/test their models.

    Agree!
     
  12. Apr 3, 2014 #11
  13. Apr 3, 2014 #12

    rcgldr

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    It's the actual speed measured by a radar gun as the model heads towards the gun coming out of the bottom of the loop. Actual top speed is probably a bit higher, but this is the "official" way to record the speeds. Video of a 468 mph run, hard to see the model, followed by a 405 mph run by the guy with the helmet cam where you can see the model.

    http://www.youtube.com/watch?v=rfoxjNg-eg0&hd=1

    Stability with a conventional wing (flying wing type or high aspect ratio) is acheived with dihedral (the wing halves or wing tips are angled upwards), so that a cross wind component due to rolling results in a corrective roll response.

    I'm wondering if the so called CG adjustment of adding a broom stick to the lighter model was sabotage, since the elevator wasn't adjusted to compensate. The CG should have been left alone and the model trimmed with excessive up elevator, which would have resulted in porpoising, but the model probably would not have been destroyed on the first flight and could be adjusted with additional flights. Then the CG could have been moved forwards a bit at a time and the up elevator reduced a bit at a time until the model was trimmed.
     
    Last edited: Apr 3, 2014
  14. Apr 3, 2014 #13

    A.T.

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    The top airspeed is definitely higher by up to 50mph, which is the true wind that the glider flies into, when its ground speed is measured with the radar gun.
     
  15. Apr 3, 2014 #14
    I think the shop keeper was referring to the lateral stability. The flying wing without the leverage of a tail plane at the end of a fuselage is less able to compensate for any pitch up/down moment.

    I do not think it was intentional sabotage. He might of reasoned that the attachment of a fuselage to the rear of the flying wing needed significant compensating weight at the front. Nevertheless he got his calculations seriously wrong. On the other hand, maybe he just felt sorry for his fellow presenter (Adam) who usually loses all the challenges :P Testing the delicate model from a great height onto a hard surface was obviously not a sensible development plan.

    Anyway, from what I have learned from this thread, it seems that in principle a concrete glider could have the same glide ratio as glider of the with same shape but lightweight construction, but the concrete glider would have a much greater sink rate and would probably be destroyed on landing.
     
    Last edited: Apr 3, 2014
  16. Apr 4, 2014 #15

    A.T.

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    I guess this kind of scaling stops pretty quickly, if your heavy glider requires supersonic speeds to glide.
     
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