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delta drag

Old 08-21-2002, 06:25 PM
  #26  
banktoturn
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Default If speed is your goal ...

Dusteater,

If speed is your goal, not turning, then you may not need to limit yourself to a delta-wing configuration. The main reasons that high speed aircraft use delta wings are to minimize drag at high mach numbers ( not a problem for you ) and to generate vortex lift at high angles of attack, for maneuvering or landing. If you are able, willing, and allowed by your club to land pretty 'hot', you can probably do better than a delta wing. I have been giving some thought to a really high speed model, and I have been thinking more along the lines of a cruise missile kind of confguration. A minimal fuselage can be used to house most of the junk that Diamond Dust leaves out in the open, and fairly small-area, stubby wings can be placed around the middle of the length, with a fairly conventional vertical and horizontal stabilizer layout. I was thinking about a completely symmetrical 4-fin layout, to complete the cruise missile theme, but that would not be necessary for speed. To reduce drag and weight, the tail surfaces could be mounted on a boom just big enough for strength and to house the control rods. As Ollie pointed out, it is very important to get all the usual crap inside a clean body. Parasite drag, which is the component of drag caused by all the junk, is a huge problem on most models. A small-winged missile with everything inside would be so much cleaner than Diamond Dust that it should not be too hard to beat it for top speed. Landing is a different matter, and might be the reason to go back to a delta wing.

Good luck,

banktoturn
Old 08-21-2002, 11:02 PM
  #27  
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Default delta drag

If you want speed, small stubby wings aren't the way to go. High aspect ratio wings with a taper ratio around .4 would reduce the induced drag. The only problem is wings like I mentioned have bad stall tendencies.

Sweep also reduces the effective coefficient of lift which will decrease the efficiency of the wing at the speeds we fly.

Look at a Q-midget pylon racer wing.
Old 08-22-2002, 01:58 PM
  #28  
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Default delta drag

AQ500,

You're right, everything else being equal, higher aspect ratio gives lower induced drag. In this case, the stated objective is high speed, with no concern about maneuvering. The high-speed, level flight situation is a low-lift scenario. Induced drag is a big problem when high lift is generated, as in the hard turns of pylon racing. For a high-speed pass, there is no concern with lift, as there will be plenty. The cruise missile comparison is not especially valid perhaps, since the Tomahawk acually has a fairly high aspect ratio, probably to maximize range. I think the more important design issue is getting enough lift for take off and landing. If you are willing to launch the way people are launching Diamond Dust, then takeoff is not an issue. I don't know how people land the Diamond Dust, but in principle, the delta wing could generate pretty decent lift at high angle of attack, allowing a reasonable landing speed. You couldn't do that with a typical high aspect ratio wing, but you might be able to with a low aspect ratio wing, possibly using a leading edge extension and some leading edge sweep, like the F-18. If you just don't need to worry about landing, I agree, a highly loaded, high aspect ratio wing would be even lower drag, although a little tougher to build with enough strength.

Banktoturn
Old 08-23-2002, 02:39 AM
  #29  
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Default delta drag

True. It just seems like the theme of this thread is to do everything physically possible to fly fast. The higher aspect ratio wing of the same area would fly a little faster. Doesn't the cruise missile have a fairly high aspect ratio wing?

Here is a plane with little wings. Scale model of the X-15. It is the prototype for an air lauch rocket powered plane. A friend that builds them gave me the first one to play with. It has a length of 53 inches and a wing span of 26 inches. The weight is 27 ounces. It uses elevons to fly. I have been demoing it at fly-ins for about two years now.

Even thought the plane looks sleek, it has a huge amount of drag and a steep glide angle. It wouldn't penetrate into a breeze until I added some weight. It wouldn't make a very fast powered plane.
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Old 08-23-2002, 02:58 PM
  #30  
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Default delta drag

AQ500,

Generally, in the high-speed case, induced drag is not as big a component of the total drag. As I try to think of a good design example, it occurs to me that all the full sized aircraft that are built with an emphasis on speed are designed for supersonic flight nowadays. The examples we have that were designed for subsonic flight, old fighters, were also designed for maneuvering and range, which significantly affects the wing design. The cruise missile, as I mentioned, has range as a major design goal. Dusteater's goal was to fly fast, with no concern for any other aspect of performance. This implies that we want minimum total drag, and that we don't need a high-lift wing. Certainly, this means that the wing should be small, since this will reduce drag, and getting enough lift at high speed is 'easy'. I don't know where the tradeoff is between induced drag and the other components of drag. I don't think the Tomahawk was designed mainly for speed. Maybe the best real-world example we have is swing-wing fighters. When do F-14 pilots decide to sweep the wings, thereby reducing aspect ratio? I don't know, but I'll see if I can find something on the web. In any case, unless Dusteater ends up with the very highly spept delta that he mentioned on one post, it would be hard to acheive a lower aspect ratio than Diamond Dust.

As I look at the X-15 picture, it does indeed look like a fairly low drag configuration, and I think this is a good illustration of the induced drag vs. speed issue. You are flying a high-speed airframe low speed. To get enough lift, it is probably flying at a pretty high angle of attack, even though the weight is low. In this situation, all the other components of drag are fairly small, and the induced drag is pretty big. A curve of induced drag vs. speed shows that induced drag gets much smaller as speed goes up, while the other components of drag go way up as speed increases. The result of this, I think, is that your X-15 IS a high-drag plane the way you are flying it, but if you put a big engine on it, you would find it to be fast, and lower drag than a Q500 plane, for example, for straight, level flight. As I said, I'm not sure what the tradeoff between induced drag and the other drag components would suggest for aspect ratio, but my gut tells me that the high aspect ratio wing does not compare favorably in terms of drag at high speed. I don't know this for a fact, but intuitively it seems that the higher frontal area presented by the high aspect ratio wing would result in a big increase in profile drag. I'll see if I can easily find something about that on the web as well.

Thanks for the replies. When I finally get time to build my 'cruise missile', and learn to fly something that hot, this discussion will be really useful for me.

Banktoturn
Old 08-23-2002, 03:33 PM
  #31  
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Default delta drag

i've made two quick planes bassed on ideas in my head, first was called Dragqueen, it had a thin flat bottom section to give lift at 0 AoA, symertical has to be at a AoA to get lift, therefore creating drag ect ect, also its half the thickness = less drag and weight...
next i went for a all moving tailplane, like full size fast things. scrapped after it didnt work.
what i ended up with was a plane drawn around a MDS38, with heil pipe. it was too small and was a &$$ to launch which earned it the nickname 50/50. once going it was smooth, but landing was a engine cau and keep going affair, no time to turn here. it proved things for me, it showed me better ways of doing things, but a standard MDS38 on a APC 10x6, would keep up with a tuned MVVS 40 8x9 on full pipe.. thats gotta be worth it.
it looked damn good too.
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Old 08-23-2002, 03:44 PM
  #32  
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Default delta drag

the second is DD, not Diamond Dust, but named after the initinals of the guy i built if for. his quest was same as yours.
i had a plan for a Eurofighter type thing, i nicked the wing sections and shape, and built the rest around it.
its a bit thick in the centre, but i built a fuz to hide the rest of the gear, a fuz cant be as much drag as a wing tapering to be thick enough at the centre for the gear ( a 4 part delta wing is the ideal, thin, last bit taper to thick for gear ) the wing had 3 degrees of washout to stop tip stalling, i made the LE real sharp till the last 3" then rounded it, that worked great.
the first fins where too small and it would have a bad stall on landing, the new fins lowered the stall speed to walking pace with the engine off. the pipe and engine where lowered so to be faired inot the wing ( not shown in pic )
the LE it tapered so the elevator is in the centre at the back, where is should be, and ailerons at the tips further forward, where they should be. i've seen Zagi types sip on full up, coz the elevator is at the tips so to speak, this plane had NO tip stall even on viloent turns and climbs.
this plane was 20-30mph quicker than anything else on same engine pipe combos

my conclusions are, lightweight, but work the wing loading out first, and go from there. flat bottom for lift at lowest possible AoA to reduce drag. fair everything, use THICK torque rods for elevons to a fuz to give a thinner wing section ( no servos in there ) lay everything as flat as you can. imagine rear inducton rear exhaust on a Outlaw..... you'd never see the pipe, it would be in rib bay 1.

MOST IMPORTANT!!!!!
make it look fast and fly it close, 6' away and your convinced its the fasted plane ever.

last off, forget the paper plane, it will torque roll into the ground on take off.
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Old 08-23-2002, 03:53 PM
  #33  
AKMac
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Default delta drag

I've gave this a little thought too. With my very limited aerodynamic knowledge, to me something like this would be the most practical design. The major problem is in streamlining the engine to eliminate parasite drag. You would also have to make this very light. Possibly using composites for the entire plane, and aluminum for the spinner.

Old 08-23-2002, 04:25 PM
  #34  
banktoturn
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Default delta drag

AQ500,

I found a great web page:

146.26.194.131/aerodynamics1/Drag/Page9.html

It includes a brief discussion, under the subtitle 'Compromise in Design, Revisited', on the topic of the tradeoff between induced drag and parasite drag, and the implications for aspect ratio.

Phillybaby,

Cool planes. Flat bottom wings do generate lift at zero angle of attack, not because they have flat bottoms, but because they have camber. If one were really optimizing things, the airfoil could be chosen with exactly the right camber for the needed lift at zero angle of attack. I don't recall whether that guarantees minimum drag though.

AKMac,

That plane looks fast to me. I suspect that the aspect ratio ( span ) is too large for maximum straight-line speed, which Dusteater originally was after.

Thanks guys,

banktoturn
Old 08-23-2002, 06:41 PM
  #35  
AKMac
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Default delta drag

banktoturn - I don't think the span is to long, but you may be right since, like I said, my knowledge in serious aircraft design is limited. Plus I just made this one night out of boredom But hey, maybe this one could land at a sane speed.

Here's a different view:
Old 08-23-2002, 08:50 PM
  #36  
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Default delta drag

hmmm, i wonder if you made a bigger diamond dust, like a 90 whiplash or something, and put a turbine on it. that would be fast, no? but then your talking turbines and not prop planes. but, if you are trying to build the fastest r/c plane ever, then i think this would be the way to go.
Old 08-24-2002, 12:05 AM
  #37  
AQ500
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Default delta drag

If you keep the same wing area and percent airfoil thickness based on the chord and go to a higher aspect ratio, the frontal area of the airfoil will remain unchanged. Now if you increase the wing area, the frontal area will increase as well. Higher aspect doesn’t mean a higher lifting wing or a bigger wing, just a greater span, lesser chord, and more efficient one. You will fly faster with a more efficient wing. Again, my point is that a higher aspect ratio wing will give you a “little” more speed. The theme of this thread was doing everything possible for every little increase in speed.

Notice how in the article he is talking about changing the wing loading which means changing the wing area given the same weight airplane. I'm talking about planform configuration. There's a big difference.

The F-14 wings retract automatically controlled by computer. I believe they start to retract around M = 0.6. The F-111’s wings were retracted by the pilot. The wings are retracted to reduce drag at higher mach numbers.

When I fly my fast plane a good deal of time is spent on the turns. If you have gobs of induced drag on the turns, you will lose a lot of speed.

In the case of the X-15 a large amount of lift is generated by the fuselage. It is obvious through the fact that the cg is located beyond the leading edge of the wing. The fuselage does not make a very efficient lifting surface. Even at high speeds the X-15 will still get lift from the fuse and thus make it slower. My point was that small stubby wings on a bigger fuselage won’t make the fastest plan either. Also there is a lot of drag (shear) on the fuselage. A lot of wetted area. I'm not talking about 100 mph difference, maybe 10-15 mph.

Looks awesome AkMac. It would be a fun one to build. How about a brushless direct drive electric motor. You will have the possibility of 150 mph. The same person who built the X-15 built a private jet with twin DF brushless motors. It also had pneumatic retracts. It flew at 110 mph. No streamlining of the motor is needed. The flight time won’t be too impressive, but imagine the speed.

I'm learning every day, that's the reason for these forums.
Old 08-24-2002, 12:08 AM
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Default delta drag

AkMac, Fly-in tomorrow at the S.L. modelport. Just in case you are interested.
Old 08-24-2002, 02:29 AM
  #39  
banktoturn
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Default delta drag

AQ500,

Actually, if you keep area constant and increase aspect ratio by increasing span, frontal area, which is the area projected toward the front ( the silouette seen from the front ) will increase. In particular, the lowest overall drag simply does not occur for the highest possible aspect ratio. I think that relatively stubby wings will indeed give the highest speed, with serious compromises in other areas.

bankttoturn
Old 08-24-2002, 02:45 AM
  #40  
AQ500
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Default delta drag

The area will remain constant if the math is done correctly. A 10 inch chord wing with a 1 inch thickness and a span of 40 inches has 40 square inches of frontal area. Let's now increase the aspect ratio 4 times by keeping the same wing area. An 80 inch wingspan with a 5 inch chord length. The thickness is now half an inch and the frontal area is still 40 square inches. It also works for tapered wings.

If the wing area is decreased and loading increased (as mentioned in the article) then the frontal area will decrease.

If you change airfoils then the frontal area will change given the same area. The article you are referring to is badly written. He could have used the same aspect ratio in his example and the point would have been the same. If you read closely and understand it he is talking more about smaller wing areas than aspect ratios.

I have personally witnessed a high AR glider at over 260 mph. I know it is another bad example. It was out of a dive. It just goes to show that high AR wings can have low drag.
Old 08-24-2002, 08:57 AM
  #41  
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Default delta drag

i woke up this morning and thought "nick the wing from a US Raptor" think it was because of that 45 degree tips thing, + my own TE sweep theorys.
you need this plane to be able to turn without loosing speed, its fast so you need to come back. Deltas have alot of drag in turns so i hear. fly it smmmmoooooooooooooooothhhhhhhhh.
our quickies do big U shape runs, one guy is always faster, main reason is he dosnt yank the plane over at the top and loose speed, he rolls slow, then pulls back so hes facing down same times as everyone else is just yanking over the top, and has the speed advantage right from the top. i've done this with my Dust on a much less fast set up, and i keep up by the time we're over the patch.
AKMacs plane is spot on, anyless span than that, and you'll torque roll too bad.
talking of high AR wings, there are powered gliders here, guess there too loo, brushless motors get them to height damn quick, then they dive. 120mph easy. its seams for small planes electric DF is the way to at the moment, 200 mph is acheivable. i've seen a 40 size DF Hawk with a 90, and it was 208 mph timed, i didnt notice it slowing down on climb either. The airframes top speed had been reached.
DF should be faster than prop, because props are like wings, they cause drag by turning, DF sits in a housing, and that acts like wingletts on the fan, so it can turn faster and give more thrust, thats the idea if you can do it anyway.i wantto try a MVVS 40 GRRT DF in a diamond dust type plane. might take a few of these ideas into the design.
Old 08-25-2002, 01:04 AM
  #42  
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Default delta drag

This thread has been very informative, your knowledge is impressive. I want to build a plane similar to the one AKMAC drew. At these speeds (around 150-200 mph) does the sweep or taper in the wings actually do anything, or would a straight constant chord wing be the same? Any comments about wingtips? Finally, could someone tell me where I can read about hinge systems for composite models? Thanks, Hans
Old 08-25-2002, 02:58 AM
  #43  
Ollie
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Default delta drag

For sweep and taper see:
http://aero.stanford.edu/WingCalc.html
Some of the fastest all composite models are built for Dynamic Soaring and use skin hinges. See:
http://members.tripod.com/douglasturner/id27.htm
These R/C model sailplanes have achieved measured speeds of 195 MPH while doing tight circles which pass through strong wind shear and survived. The limitation on speed so far is the G force in the circles which can easily exceed 50 G's. A typical dynamic soarer might weigh 4 or 5 pounds and experience lift forces of 200 or 250 pounds just before it expoldes into composite confetti. Higher speeds only await stronger structures. See also:
http://www.charlesriverrc.org/articl...rkdrela_ds.htm
http://home.earthlink.net/~jaffee/ds.html

For picture of Kevlar skin hinge in wing layup see:
http://www.rc-soar.com/tech/craig.htm
Also:
http://www.djaerotech.com/dj_askjd/d...ons/hinge.html
http://www.rc-soar.com/tech/silicone.htm

BTW, straight line speed models don't have to deal with such high G's. Dynamic soaring models aren't limited by a prop of a given pitch and RPM.
Old 08-25-2002, 03:14 PM
  #44  
Jeff Leavitt
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Default Fastest RC Prop Planes

Hi,

Interesting discussion. I think that the fastest RC prop Planes are the unlimited class giant scale racers. I saw them run at Jean Nev. a couple of years ago. Rich Verano was flying a Lancair with a long skinny wing. I've heard that he has recently been clocked at around 235 mph.

One thing I haven't seen is comments regarding prop pitch speed required to get to these speeds. The cleanest airplane can't outrun it's pitch speed unless it's in a dive. How much horsepower will be required to drive the prop that will get into these 200+ speeds?

The Formula 1 40 Racers have pretty good designs for high speed. If you like high speed action, catch these races! anyway, one of these racers with a larger engine and the correct prop would be pretty hard to beat and the design work is pretty much all done. Maybe optimize the foils a bit because you wouldn't be concerned with minimum thickness rules for the races, just reasonable structural integrity. Jet aerodynamics are cool looking but most of it only applies at speeds much higher that those our prop powered models run at.

Nice looking design AKMAC, I'd love to see that baby go!!!
Best Regards, Jeff.......
Old 08-25-2002, 05:24 PM
  #45  
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Default delta drag

while our fastest club planes here run 8x9's, i hear that the Plyon guys dont go over 7" in pitch, you get more from RPM than pitch after that. fine if your engine will rev higher than the 18000 max most fast sport engines have.
one reason the big scale planes go faster, there bigger, real planes go even faster than that. most small planes will do 120 without thinking, and they are hardly aerodynamic.
thats just the way physics works. if you wanna get real space age, i also saw ages ago a nose cone with a small rod with a blunt face, i think the idea was to create a shockwave the rest of the plane would fly in, like driving & following a big lorry, it would have less drag. not sure if this worked, another was some sort of plasma fire on the point that did the same thing.

i think those racers are the way to go, what other type of pilot would de-tune a plane from 200+ to 190mph just to make it better in the turns!!!!
Old 08-27-2002, 02:15 PM
  #46  
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Default taper & sweep

Flybike,

As I understand it, the main reason to taper a wing is to tailor the distribution of lift along the span of the wing, to reduce induced drag. At high speed, induced drag is not important, so the spanwise lift distibution is not a big deal from that perspective. The other consequence of taper, if it is excessive, is to make tip stall more likely. For a plane that must turn hard, as in pylon racing, induced drag is a bit more important, although it remains much less important than parasite drag.

Most of the reasons to sweep a wing have to do with airspeeds which approach Mach 1. For a purely subsonic aircraft, the only reason I can think of, besides looks, to sweep a wing would be to generate vortex lift at high angles of attack. This could be to enhance maneuvering or to allow lower take-off and landing speeds for a plane with a small wing. Apart from that, sweep generally makes a subsonic wing less efficient.

Apart from truly well-designed winglets, which tend to be effective only for very specific flight regimes, there does not appear to be any significant drag benefit to any of the various wing tip designs. Aesthetics and ease of construction may be the best reasons to choose one wing tip over another.

banktoturn
Old 08-27-2002, 02:27 PM
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Ollie
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Default delta drag

The main reasons to taper a wing for aerobatic aircraft are to increase the roll rate, ease entry to maneuvers requiring tip stall and to decrease the bending moment load near the center of the wing.
Old 08-27-2002, 02:34 PM
  #48  
banktoturn
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Default aspect ratio & high speed drag

AQ500,

Certainly, on paper, we can keep frontal area constant if we allow thickness to vary as well as chord and span. It is impractical to decrease thickness, however, as span increases, for structural reasons. Moreover, the increase in profile drag with wing span is not due only to the increase in frontal area.

The article may have been poorly written, but it was pretty clear about the relative importance of induced drag and 'parasite' drag at high speed.

You will get no argument from me on the point that a high aspect ratio wing can have low drag. It is still the case that the lowest drag for maximum speed straight flight will not be obtained with the highest aspect ratio wing. To put it in design terms, pursuing reductions of induced drag for the case of high speed flight is pursuing diminishing returns. Your effort would be better spent reducing weight or other sources of drag.

High aspect ratio sailplane wings are beautiful marvels of aerodynamics, uniquely well suited for their purpose. Short, ugly, stubby wings just happen to be better for brute force high speed straight flight, because they make less drag in that flight regime.

banktoturn
Old 08-27-2002, 02:37 PM
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Default taper for aerobatics

Ollie,

Makes sense. Thanks,

banktoturn
Old 08-27-2002, 03:20 PM
  #50  
Ollie
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Default delta drag

Banktoturn,

Your point is well taken about the advantage of low aspect ratio wings for straight line speed.

I was looking at the drag polars of laminar airfoils as reported in Theory of Wing Sections by Abbott and Doenhoff. At a reynolds number of 3,000,000 the profile drag coefficient of an NACA63-OO6 was only about 0.00425. This compares very favorably to an NACA 0006 which has a drag coefficient of 0.0055 under the same conditions. That's a profile drag reduction of about 23%.

The benefits of laminar airfoils can be had at speeds of 250 MPH with chords of 16 inches (corresponding to a reynolds number of about three million) and perhaps at somewhat smaller sizes and speeds as well.

To realize this significant benefit, the wing would have to be of moderate taper and without a lot of sweepback. A pusher to eliminate the propwash turbulence over the wing might also be necessary. A lot of sweepback produces turbulence near the center of the wing due to "middle effect." Too much taper reduces the reynolds number of the tips so that the benefits of laminar airfoils are no longer obtainable there.

Also, the airfoil contour has to be so ripple free and smooth that a fly attempting to walk on it will end up on crutches.

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