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Old Jan 04, 2013, 02:11 PM
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Yak 52's Avatar
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Well I'm starting to firm up my ideas on what I want the design to include:

- R/E woodie style 'floater' with reasonable penetration.
- Small enough to launch with the same bungee as the FF'ers in my local club.
- Small enough to aerotow with the UMX Carbon Cub.
- Hand launching would be nice but it's less of a priority than the Hi-start/aero-tow.
- Big enough to take some radio bits I have hanging around here. A 10g rx, two 6g servos and 250mAH Nicads totaling 70g. (I could get a lipo that would be a lot lighter if necessary)

I'm looking for something around 1.0 to 1.25 metres but my problem is that with limited larger builds under my belt I'm a bit hazy on estimating wing and airframe weights. I'd guessed around 160-180g AUW (with the 70g radio) for a 1.0m built up woodie. Any input on this would be great...

I've done a few initial drawings. Settling on:
b 40"
S 216in^2
A 7.3
This gives me Re numbers of approx 40-50k at thermalling speeds. (6" root chord)

I did have a look at the span squared loading. Most of what I looked at came out around 100-150g/m^2 (not sure what units others use) But some of the the lower aspect models (e.g. GP Spirit) were higher - up to 200g/m^2. Interestingly the UMX ASK 21 was one of the lowest for b^2 loading but obviously with a high wing loading.


A side point here, but wouldn't the span squared loading be a more accurate comparison if it were Weight divided by 'pi * (0.5*span) ^2' ? In other words the area of a circle with the same diameter as the wing span. Wouldn't this relate better to mass flow? Or am I missing the point?


Jon
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Old Jan 04, 2013, 04:01 PM
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You're just adding a bunch of complexity to the equation, in the form of additional constant factors. The only thing that matters in that is the span itself, and the weight, so the other stuff just complicates without adding any value to the result.
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Old Jan 04, 2013, 04:20 PM
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Take a look at the "der falke"
I like mine for small fields. Thermalling is no problem.
http://www.rcgroups.com/forums/showt...light=der+falk.
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Old Jan 04, 2013, 05:58 PM
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love the looks of the Der Falke, wish i could just get a plan for it. i am building a Bug now but that falke looks much better
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Old Jan 05, 2013, 03:35 AM
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Quote:
Originally Posted by Don Stackhouse View Post
You're just adding a bunch of complexity to the equation, in the form of additional constant factors. The only thing that matters in that is the span itself, and the weight, so the other stuff just complicates without adding any value to the result.
Ok I see that, thanks. So squaring the span (linear measure) makes it comparable at different sizes then.

At 1.0m span my W/b^2 is quite high although the low aspect ratio gives a modest wing loading.

I'm wondering If I ought to go a bit larger on span, but a bit higher on aspect ratio. Keep the same 6" chord but about 1.5m. That would give me a bit more head room weight wise. Plus give me the option to ballast it up for penetration. ie low span loading, moderate wing loading, and an airfoil with a decent power factor at low Reynolds numbers....
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Old Jan 05, 2013, 06:33 AM
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It's a compromise, and it really depends on how good you are with designing low-Re airfoils. The highest performance 1.5 meter HLG we ever made was the Spectre VR, with an aspect ratio of over 15, but I have a lot of experience with low Re airfoils.

More wing area increases overall whetted area, which hurts high speed. High span-squared loading (not enough span for the weight) hurts induced drag and low speed performance. However, those two requirements together push you into high aspect ratios, which hurts Re. Figuring out how to deal with all of those together determines where the balance comes out.

The other approach is to go with low aspect ratio, "racing" airfoils and low wing loading to get the low speed performance, like we did with the Chrysalis series. This does not have as much potential for raw performance, but it gives you a plane that is easier and more forgiving to build, and more tolerant of weight, making it more versatile.It's also going to have higher chord for a given span, making it easier to see. In your case, that could be a major issue. Visibility is mostly a function of chord.
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Old Jan 05, 2013, 08:27 AM
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Originally Posted by Don Stackhouse View Post
Figuring out how to deal with all of those together determines where the balance comes out.
Am I right in saying that for a high aspect ratio you need a reasonably high Cl_max?

For the low Re/high aspect type I was looking at some of the F1C airfoils (FF power duration) which were aimed at a good compromise between penetration and glide at low Reynolds numbers. The MA409 was one I looked at but it seems the AG's can do better.

The Verbitsky BE50 looks interesting. I've used it for FF rubber at low very low Re. Compared to the thinner AG airfoils there is more drag at Cl_zero but still less than say S3021. And the Cl_max is high, as is the section power factor.

My thermalling Re will be around 50k which is above it's critcal Re. But X-foil suggests it needs a turbulator at lower Re than that. The drag at Re100k and Cl0.2 is still close to that of the AG's.

I'm thinking for a bungee launched floater I can sacrifice a little of the zero_lift performance for the sake of section power factor? The other option would be AG35 but the Cl_max is a bit less. It's a thicker airfoil though so easier on the structural side of things. This is not intended to be a seriously optimized design, rather a straight forward model so perhaps the AG35 and modest aspect ratio would be better.
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Old Jan 05, 2013, 04:12 PM
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At lower Re's thicker tends to be bad. Too much thickness and/or camber can actually reduce your max usable lift. Note, the max usable lift is NOT the same as Clmax. It's the max lift coefficient you can get before the drag starts to go up too quickly, a sort of "point of diminishing returns".

For high aspect ratio you need high lift, but you still need low drag at zero lift. Getting both in one airfoil can be quite challenging, especially at low Re's.

The airfoils (note the plural!) for the wing are one of the last things you determine. Start with a survey of sections at the approximate Re's where you expect to operate, just to get a feel for what's reasonable. Then, do your wing planform design, including the range of operating conditions and the effects of turning flight. Note that for models in this category in a tight thermal turn, the airspeed and Re at the tip on the inside of the turn could be half that of the outside wing tip, and therefore the inside tip might need more than four times the lift coefficient needed at the outside tip. This could actually be a higher lift coefficient than at the root, but with a substantially lower Re!

The result should give you a map of lift coefficients along the wing at various flight conditions. Only when you have this information are you ready to design/select the airfoils along the span. Because the conditions at the tip are different than at the root or at the mid-span locations, it's unlikely that one airfoil will be optimum along the entire span. The Chrysalis has several different baseline airfoils along its span, with non-linear blending in between, and that's before I put in the alterations at several places on each individual rib to correct for covering sag.
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Old Jan 07, 2013, 01:39 PM
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Don, thanks for taking the time to explain this.

Quote:
Originally Posted by Don Stackhouse View Post
At lower Re's thicker tends to be bad. Too much thickness and/or camber can actually reduce your max usable lift.
My last design was flying at about Re11-15k so with a 1.0m model I'm moving up in size But I have a lot to learn about the specifics of glider design... Having come from FF I'm not used the need for tight turns so I've been able to ignore the effect of curved flight path on the lift distribution. I would really like to get into exactly how you work out local speeds in curved flight and how you know what turning radius to expect and design for?

I appreciate it's rather involved so if anyone has links or book recommendations that would be great too.


Quote:
Originally Posted by Don Stackhouse View Post
Note, the max usable lift is NOT the same as Clmax. It's the max lift coefficient you can get before the drag starts to go up too quickly, a sort of "point of diminishing returns".
So are you talking about the best Cl^1.5/Cd here or something else?


Quote:
Originally Posted by Don Stackhouse View Post
The airfoils (note the plural!) for the wing are one of the last things you determine. Start with a survey of sections at the approximate Re's where you expect to operate, just to get a feel for what's reasonable. Then, do your wing planform design, including the range of operating conditions and the effects of turning flight. .....

....The result should give you a map of lift coefficients along the wing at various flight conditions. Only when you have this information are you ready to design/select the airfoils along the span.
I'm aware that's how you should do it but I don't have access to an airfoil design program at moment (been looking at Profili?) I do work out local Reynolds but for past designs I've stuck with a single airfoil and relied on planform and washout rather than aerodynamic twist. But then I've been designing what are essentially one-speed aircraft so that has worked ok. I would like to get into this level of detail though so I appreciate the input.


Thanks
Jon
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