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Old Feb 17, 2015, 05:14 PM
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A little help with general design?

I've been messing with DIY foam planes for a while now and at this point I'd basically like some input from someone more experienced than myself on how to move forward.

The setup I'm running on essentially all my planes is a 160W motor and a 2200mah 3s.

What I'm wanting to go for at this point is a long duration (a goal of 25-30 mins of gentle flying) plane using the same components that can eventually be used to lift some FPV stuff as well. I think what I need the most help with is the wing -- airfoil shape, length, wing load, etc. to match my components well. So, any thoughts?

A picture of my current hair-brained project is attached below... I feel it fairly well represents the culmination of my experience to date save for the completely untested airfoil shape. 60" sparless span. The pod containing the motor and battery will stick out about 4" beyond the main fuselage to maintain decent balance.
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Old Feb 17, 2015, 06:43 PM
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That seems a massively heavy battery for the motor and plane to me though may be needed for fpv electrics. I have a 2830 1000kv on a 39" span Mustang that i typically fly not very gently for 10 minutes on average with a 1300mah pack. The few times i was just puttering around flight times were much longer. I'm a minimalist, and tend to use the smallest pack i can. With electrics, you have to balance the additional mah in a larger pack against the amps/watts it takes to carry the pack around. In this sense at least glow is better, as the plane gets lighter as fuel is burnt while with electric, it has to carry a depleted battery. I would suggest getting a couple of pairs of 1500mah packs. You can fly a decent time on them, and if the math works out, parallel two to double the mah.

A closeup of the wing/fuse and sketch of the construction will aid in providing suggestions.
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Old Feb 17, 2015, 08:21 PM
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To get the longest flight time from a given motor and battery, you need a plane that will maintain flight on a very low power setting.
To do this the plane must be light, have a low wing-loading, have an efficient wing, have sleek shape so it has low drag and have a prop that efficiently converts power to thrust.
With your 2200 pack you'll get about 16 min run time if the plane needs 8 amps to fly. If it will fly on 4 amps that gets to your target flight time.
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Old Feb 17, 2015, 08:59 PM
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Interesting.
I just got out a powered glider to feel what thrust it made at 4A, which proved to be at about half throttle.
Just 4 clicks on from that throttle setting, still about half throttle, it was pulling 8A.
At WOT it was about 14A
I'm pretty sure that particular plane would maintain height on 4A though I doubt that it would climb at that setting.
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Old Feb 17, 2015, 09:16 PM
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My thought process was that rather than running a large motor at a very low cruising throttle position, I'd sacrifice some peak thrust for weight and run a smaller motor. Is that reasonable?


Wing and fuselage are pretty dirt simple. All from Dollar Tree foam board, with paper:

The wing is flat bottomed, 60" span, 10" chord. Folded to a triangular cross section with a maximum ~1.6" thk at ~3.3" from the leading edge. Completely untested shape, I was planning on comparing it to a KFM3. It does give great flex resistance. Small winglets on each end run parallel to the upper leading edge -- mostly there for structural stability but should also give a tiny bit of effective dihedral.

Fuselage is ~2.5" square, 12" long with the wing centered on it. Motor/battery pod will extend 4" forward (n.i. motor) and is ~1.875" square. Tail boom is butt joined, 30" long, 3" to 1.5" folded triangular taper with the top surface angled upward towards the rear at about 2 degrees. 12x5" vert. stabilizer (1" elevator) glued to the surface.

When my E-clips come in I can put the motor back together and take some pictures after final assembly. Hopefully tomorrow.


My most successful plane so far had a 48" span, 12" chord with a 3/8" thick KFM2, the rest of the design being similar to this one. It flew about 18 minutes at half throttle but needed enough angle of attack that I know I can do better, and I assume the first step is a better airfoil. I think it was way too thin on this plane, giving me an almost flat sheet wing.

I've done smaller planes that could last longer, but I wouldn't be confident in their ability to handle more weight.
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Old Feb 17, 2015, 10:47 PM
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Whatever.
You have a certain amount of energy in the battery and will need to draw on that energy at a certain rate to maintain flight.
The less efficient the plane, the more energy it requires to fly.
The less drag it has the less power it needs to maintain a certain speed.
A good wing is vital in obtaining sufficient lift with minimal drag.
Do not fall into the trap of thinking a thicker wing is a better wing. It's not.
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Old Feb 17, 2015, 11:35 PM
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Yes, airfoil is the most important and airfoil in the picture you attached is going to be really draggy. For the efficiency you want, you have to go with thin, low drag airfoil. Look up "glider airfoils".
Here is my foam board glider and although I have done over 45mins on a 2200 with this, I spent lot of the time just gliding. BTW, I have same Turnigy motor on this plane, really happy with the motor!
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Old Feb 18, 2015, 01:33 AM
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Thanks elewon. Actually, the first result for 'glider airfoils' in google pulls up something very similar to mine essentially stating that it's functional but draggy and needs more curvature. I'm tempted to go ahead and make up a KFM3 wing for it and ditch the present one, it would also push my weight a bit closer to the nose too which will help with test flights before I drop more electronics on. By the way, you also have the same radio as me!
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Old Feb 18, 2015, 02:11 AM
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All the KFm foils are draggy.
In a great test done by a user, JetPlaneFlier he found a KFm wing gave only 60% of the performance of a 'conventional' wing.
I must say, however, that had the "conventional" wing been made of foam the result would have been closer. This is mainly due to TE shape.
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Old Feb 18, 2015, 02:36 AM
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Besides the good advice from the others: Higher aspect ratio will be more efficient (reduce drag) - which is why gliders tend to have very high aspect ratio.
Of course, this will increase stall speed, so if really slow landings/take-offs are paramount for you, you'll need to compromise.
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Old Feb 18, 2015, 03:27 AM
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Besides the good advice from the others: Higher aspect ratio will be more efficient (reduce drag) - which is why gliders tend to have very high aspect ratio.
Is that a higher aspect ratio per wing area/load, or is that true per span as well? I understand the wingtip vortexes and stuff, I just have a lot more theoretical knowledge than real knowledge at this point.
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Old Feb 18, 2015, 04:37 AM
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Is that a higher aspect ratio per wing area/load, or is that true per span as well? I understand the wingtip vortexes and stuff, I just have a lot more theoretical knowledge than real knowledge at this point.
Aspect ratio (AR) is defined as the square of the wingspan (b) divided by the area (S) of the wing planform: AR = b^2/S
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Old Feb 18, 2015, 07:56 AM
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Or, for rectangular shaped wings, where c=chord, b=span, and S=area:
AR=b^2/S = b^2/ b*c = b/c

It's a lot easier for me to visualize span divided by chord, and is "close enough" for foamies, i would think, even if the wing has some simple tapered tips like my OSG.

Don't get caught up in the math, long skinny wings generate less drag than short fat ones and smoothly contoured airfoils generate less drag than stepped or sharply angled flat surfaced airfoils.

My limited experience has been that a lightly loaded (wing loading) KFm airfoiled wing will thermal as well or better than a conventional airfoil of similar size, shape and wing loading. But if you want long flight independent of any thermal boost, go with a slippery conventional.
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Old Feb 18, 2015, 11:21 AM
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...
...
...
My limited experience has been that a lightly loaded (wing loading) KFm airfoiled wing will thermal as well or better than a conventional airfoil of similar size, shape and wing loading. ...
Springer, how much wing loading are you talking about? My understanding is airfoil characters are less important at really low wing loading but I doubt if that could be achievable for a motor glider.

Badot, sorry that the search didn't come out with expected results but if you are interested in building conventional airfoils, I'd look at UIUC airfoils database. I'm really a newbie in this area but look at Drela/MH/Selig airfoils. Also, I know a guy who did over 45mins with PW51 airfoil. Challenge really is achieving the airfoil shape with DT foam board, unless you can CNC/hot wire the wing. Good luck!
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Old Feb 18, 2015, 12:11 PM
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Elwon: Here are the three motorgliders I have:

OSG is about 7.5oz, and has 300sqin area, for 3.6oz/sqft. She will hang and climb in very weak lift.

Gentle Lady (balsa and monokote) 31oz and 4.8sqft area for 6.45oz/sqft. She flys ... like a gentle lady, decent, but not the best for hanging around in lift.

Elght foot span foamie: 34oz, 6.33sqft area, 5.4oz/sqft. I haven't flown her enough to really understand her yet, but the video on my blog shows her hanging and climbing in steady lift over an orchard, without much effort at all.

I made a wing for the OSG that attempted to approximate a 7035 airfoil, and it is a bit faster than the KFm2 wing I usually fly with, but doesn't seem to perform any better in lift. The wing for the 8 foot glider has the character of an Eppler 377, with the undercamber aft. ( wasn't really trying for that, (and certainly not working off any digitized airfoil sheet) but it's how it came out . That one glides a good long way (especially when trying to land: "will you please come down?????) I doubt that the specific airfoil will be critical, but as Whiskers said, go for a thin (8-10%) wing and use the the foam's tendency to create a gentle curve when bent to make a decent airfoil camber on the top surface. I don't have a good feel for whether a semisymmetrical airfoil would be better than a flat bottom, but suspect it won't help enough to compensate for the increased build difficulty.
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