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Slow delta wings?

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Norman

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The aoa correction for short span aircraft isn't too complicated:

dCl / dalpha = 2pi *(Ar/(Ar+2))
That formula is only good in the linear range. For really low AR wings (and deltas are just low AR wings with lots of sweep and taper) the linear range gets pretty small. If we stick to the definition used by the guy who coined the name "delta" then the plane being discussed here is definitely not a delta, it's a moderately low aspect ratio trapezoidal wing with a strake. The strake will produce leading edge vortices but the trapezoid doesn't appear to have enough sweep to capture a LEV. The strake vortices will have some affect on lift and stability but with the low power of general aviation it's likely that the sudden drag increase at about 8 degrees AoA will be the most noticeable characteristic. I. E. You may have a good L/D at high speed but your rate of climb will suck and when you reduce power to idle it'll come down like an express elevator.


[video=youtube;rQNPl5DdS5I]http://www.youtube.com/watch?v=rQNPl5DdS5I[/video]
 
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lr27

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I dunno. 8 degrees allows room for a fair amount of lift. If I read the previous equation right, then by an angle of attack of 8 degrees, you've got a Cl of 0.5 or so. Assuming this aircraft has a much lower wing loading than a conventional one, then a Cl of 0.5 may be plenty.

Poking around in that report by Wainfan et al for NASA on a low aspect ration PAV, it appears the Facetmobile climbs best at Cl of maybe 0.3 to 0.35, and the study airplane (still fictional at this point) at Cl of 0.25 to 0.3 or so. These are lower aspect ratios. One assumes that a higher aspect ratio could go to higher lift first, though I don't know about those strakes.

A decent glide probably just requires putting the nose down. L/D of 7.5 or so and 10.5 or so, minus drag from stopped prop. Express elevators, I imagine, would exceed 1000 fpm, which is the approximate descent rate of the FMX-4 when stalled. Should be less in a glide. BTW, those L/D numbers are comparable to conventional aircraft. The max L/D of a Cessna 150 is given as something like 10.3 in the same study. That IS at a considerably lower airspeed, though. Something like 62 knots for the Cessna and 75 for the study airplane, even though the study airplane's minimum speed would be less than the Cessna's.

Of course if you pull the nose up too high and try to climb, forget it.


Norman wrote:
I. E. You may have a good L/D at high speed but your rate of climb will suck and when you reduce power to idle it'll come down like an express elevator.
 

Norman

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I dunno. 8 degrees allows room for a fair amount of lift.
Agreed. Most flying, including best rate of climb, will be done at less than 8 degrees AoA but who operates any machine only within it's optimum limits? Especially when it's potential operating range is 3 times wider than optimum. Low AR 'wings with strakes can slow way down if they have enough power to handle the high induced drag but can also have some really weird post stall behavior that was called "superstall" by the guys who first observed it in the SAAB J-35. The only suggestion I've heard to avoid this is to keep the area of the strake to les than 18% of the total planform area. Also plank 'wings have a pitch isue that is dangerous if ignored.


BTW Thanks for the link to the eclipse video. I remember when Al was working on it and keeping the nurflugel list up on the news. It's great to see it. I've posted it to my facebook wall
 

autoreply

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Well, even worse. 99% of the aircraft need less power/thrust close to stall and thus can climb at any low speed.

Low AR aircraft however might know a coffin corner where - while above stall - you don't have the power to climb and in fact are sinking. That makes for interesting holes in the runway when aborting a landing...
 

lr27

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The low AR guy at least is still alive, at least for a while, while the conventional guy is already dead. If the aircraft is on the light side, it won't make a hole anyway. At least if it's designed for that vortex business. Might smash the l.g., etc. Equivalent to a free fall from 4 or 5 feet for our favorite faceted aircraft, if I've got my math right. With the right kind of seat, the pilot might just walk away, maybe less than 10 g's if the seat travel is, say, 6" or maybe a foot, depending.
Well, even worse. 99% of the aircraft need less power/thrust close to stall and thus can climb at any low speed.

Low AR aircraft however might know a coffin corner where - while above stall - you don't have the power to climb and in fact are sinking. That makes for interesting holes in the runway when aborting a landing...
 

autoreply

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The low AR guy at least is still alive, at least for a while, while the conventional guy is already dead. If the aircraft is on the light side, it won't make a hole anyway. At least if it's designed for that vortex business. Might smash the l.g., etc. Equivalent to a free fall from 4 or 5 feet for our favorite faceted aircraft, if I've got my math right. With the right kind of seat, the pilot might just walk away, maybe less than 10 g's if the seat travel is, say, 6" or maybe a foot, depending.
But in the conventional case you can simply apply power and climb away right?
 

lr27

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But in the conventional case you can simply apply power and climb away right?
I'm just saying if I'm dumb enough to be, say, a couple of knots below the conventional aircraft's stalling speed, at a low altitude, I am far more likely to live if the airplane mushes instead of stalling and diving.

In the scenario I'm talking about, pushing the throttle forward on the conventional airplane will just let in more loam, which makes for a poor oxidizer. Very abrasive as well. Not that the pilot will be capable of pushing the throttle forward, or would want to anymore.

The accident reports are full of this stuff. Plus you can find video of it. Years ago, someone in a Cessna tried to pull up to clear a line of trees near my house and came down very hard. Or that's what I judge happened after seeing the wreck. Looked like he dropped at 30 to 45degree angle, judging by the position relative to the trees. I suspect by the time I saw the wreck, he wasn't very healthy.
 

deskpilot

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Re: Slow delta wings update

Been busy with visitors from the UK so, with others due to arrive over the Easter weekend, I though I'd up-date you with The Eagle-Ray.

I've decided to go with the twin fins and winglets just because I like the look. Arthur, I've restyled the winglets as per your suggestion and they look great. Bear in mind, this is a fun plane, not to be taken overly serious. I've changed the front delta, made it bigger and flared into the main somewhat better. Moved the pilot back to a more 'looks about right' position. Now I've got to get to grips with the maths and figure it all out in reality.

I haven't found out how to model the complex shape of the cowl so it's a bit angular but you get the idea don't you. I'd like to make it longer and more pointed but that would mean having a prop shaft extension and associated torsional vibrations etc. Maybe best to KISS it. The fuselage looks better with the inward taper and will be blended into the wing surfaces to reduce drag. Still looking at using Honeycomb Fiberglass panels but they're not cheap, and that's something I've got to bear in mind at all times. To get the flat panel shapes I've got to revert to my sailing days. I'm hoping that I still have 'Small boat Design' books packed away somewhere. From them I can project the curved surfaces onto what ever material I eventually use.(talking Fuse' only here although it might work for the wings as well)

You'll see a lot of construction lines in the models. Unfortunately, if I turn them off, it's hard to see the various outlines(last image). I have hidden them on the cowl and wing upper surfaces only.

So, what do you think?
 

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BDD

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Neat. Looks nice. The twin vertical stabilizers seem to be a bit close together and could have a bit of interference drag. You could round the canopy and turtledeck. It would be a bit more dificult to build that way but could look more aerodynamic. The tip fins might have to be that large to be effective as Whitcomb winglets and could be a bit destabilizing with the trailing edge swept forward.
 

lr27

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Looks kind of nice. Maybe instead of commercial panels you could use foam faced with fiberglass or ply. A material which is probably viable but which would make me nervous is kraft paper saturated with epoxy. I've made a test model airplane wing with this stuff, and done a bit of testing. It's probably not as strong, but for the weight it seemed to be about as stiff as woven glass and epoxy. I just don't know if I had it well saturated enough to be properly water resistant. Of course ply is probably stiffer for the weight.

Ever seen the plans for the Sky Pup? It uses blue foam for a shear web in many places, including the main spar. Has been around and has a good reputation. You might make foam internal bulkheads, then face the edges with wood, overhanging a bit, then glue on strips diagonally like in a rib, then cut away the foam that doesn't have strips on it. You'd use gussets like a rib too, which is why the edge pieces would have that overhang. That is the overhang would make them even with the diagonal strips.

I need to get a CAD system going here, but I think first I have to get the F!%$#%^ virus out of the computer. It's blocking my use of it, among other things.
 

deskpilot

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Thanks for your comments guys.

BDD, I've put the fins as close to the fuse as possible for structural reasons, plus they're both in the prop wash and should therefore have more control at slow speed and high AOA conditions.

A rounded canopy and turtle back would be more aerodynamic but, as you say, a hell of a lot more complicated to make. I also want to keep some of the body lift with my design(or should that be with John Dyke's design)

Norman, you're right, the winglets are behind the pitch axis, see attached. I need to redo the angle though. They should be the same as the fin. Before anyone asks, the plane sits at a 9 degree angle to assist take-off.

bmcj, so do I, obviously :roll:

lr27, only 'kinda nice' I'm so devastated :dis:....:roll: I appreciate your comments on construction. I think I to would be nervous about kraft paper and epoxy. An interesting thought though. Ply/foam sandwich is a possibility but would it really be lighter than an all wooden structure. I could go with fabric covering as the plan is to have it trailer-able and housed under cover. The original DD had fabric undersides if I recall correctly.
Sorry to hear that you have gremlins in your pc. Why not use GoogleSketchUp instead of CAD. Cheaper(free) and does most things, although I'm still learning how to model even the most basic structures.

A general question to anyone who's interested. What colour(English, and correct, spelling) scheme would you use for this design. I'm useless at colouring in but I have a mean and dirty scheme in mind. Not at all like the real thing, see attached. I've also included the Eagle Ray's 'almost human' face
 

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lr27

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If your using much epoxy, and it's not a special type with high temperature resistance, better paint something close to white. However, I really like that color scheme. Just don't land out in the summer, because no one will ever see you.
 

deskpilot

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Re: Development of a dream

Latest rendition.

Mods are: new, more streamlined cowl, extended strake(also narrowed), winglets reduced in height, and fins reshaped, reduced in height and length.

Now, bearing in mind that this is a dream that just might come to fruition, what building method should I use? Also, I can't weld and neither can most would be builders. I've looked at honeycomb F'glass panels but they're heaps expensive here in Oz. My design is aimed at using panels (or sheets) of some sort. Anyone care to put forward some ideas or potential problems I might run into when using any particular method?
 

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lr27

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You can't weld or you don't know how yet? Not quite the same thing. I would like to learn how.

Very thin plywood over foam would be great if the ply wasn't quite so expensive. I think a panel made like that might be $5/square foot or something if you did it yourself. At least in the USA.

Aluminum would probably give you much better properties than fiberglass, though it's said to be a real pain to bond well. Allegedly, and I have no reason to doubt it, one needs very good process control. I wonder if anyone supplies aluminum with one face treated to be bondable.

Glass is kind of floppy, especially woven glass. Perhaps if you could find light biaxial glass, a panel with that over foam might be good. Or perhaps two perpendicular layers of uni glass on each side of foam.
 

deskpilot

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Whilst I'm still considering building materials, I'm also looking at the wing design. I've searched these forums, and the web at large but can I find any info on delta wing design/construction practices, can I heck. In particular, the placement of wing spas in a light aircraft of this design. Is there, or should that be, are there, any rules that I should be aware of and if so, where can I find them.
 

addaon

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The big question is if you're planning on delta lift as part of your flight envelope. If you're going to be going fast enough (relative to wing loading) and at a low enough angle of attack that delta lift is not a major factor, normal wing design should apply -- although you'll pay the cost of a low aspect ratio. (This is where I am at my design... stall without delta lift is at 19° AoA, and I have no need to go beyond that.) If you need delta lift... dunno.
 

BDD

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Looks like they're aft of the pitch axis. Shouldn't that be ok?
Well, aft but not very far aft. The assumption in my comment is that this shifts the total average rudder side area a bit forward of where it would have otherwise been, even with no surfaces on the wingtips. With the trailing edge kept back farther, as with a straight trailing edge....or even slightly swept back, they would have added more to stability about the vertical axis. When I said "a bit destabilizing", what I really meant, "less stabilizing than it could have been"....maybe even a bit less stabilizing than with rudders alone since the total average area has been shifted forward a bit for less moment arm. This is in the sense that the side area is shifted forward and doesn't really have to be unless there is some other compelling reason like getting a higher aspect ratio, etc.

O.K, yes but could have been a bit more stabilizing which to me sounds like a good thing when you are dealing with a configuration that is short coupled to begin with because of it's geometry.

Don't get me wrong...this looks like an exciting project and there is nothing about it that shouldn't work.
 
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