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Old Oct 31, 2014, 06:48 PM
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ForeverFlying's Avatar
Christiansburg, VA
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Incidence Angle for Slow Fly Motor Glider

Hi Group!
I am designing and building a 66" span, 430sq in motor glider using an AG35 airfoil. It is very light, the final should be ~2.7 oz/sq ft. I am fascinated with slow, floating flight, so this will be a slow flier for calm conditions. I am thinking that I will need considerable incidence at those speeds, so am thinking of ~4 degrees incidence. Does this sound right?
Thanks!
Mike
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Old Nov 01, 2014, 02:17 PM
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Illini_Rob's Avatar
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Yeah should be fine. Your target loiter CL is roughly 0.7? Foil Cl/Cd should be quite high there.

Wing incidence isn't critical in this application though. The only thing dependent on your incidence setting is fuselage drag .. which will be quite small during loiter.
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Old Nov 01, 2014, 07:07 PM
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If you plan to fly at about 15 ft/sec, then the CL needed would be about 0.63. In order to get a CL of 0.63 with your planform and an AG35 section, you'd need a wing AOA of just under 4 degrees.

The chord-based Reynolds Number at 15 ft/sec would be about 47,500. You should be OK with an AG35 at a CL of about .63.

The slowest you could fly would probably be about 10-11 ft/sec at about 8 degrees wing AOA.
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Old Nov 01, 2014, 07:12 PM
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The slowest you could fly would probably be about 10-11 ft/sec at about 8 degrees wing AOA. That is pretty optimistic. As I recall we figure about 1/4" rise in one foot for one degree
so --------------------
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Old Nov 01, 2014, 07:18 PM
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Sure enough, the "rise over run" for one degree is indeed just under a quarter inch per foot. Not sure how that is in any way related to the stall speed of this design.
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Old Nov 01, 2014, 08:39 PM
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Forever, if your goal is to fly this as slowly as possible what about using a more highly cambered airfoil such as those used on a lot of free flight models? Such airfoils will provide for an even higher Cl value and thus you can slow down even more for that wing loading.

What it would give up to some extent is the ability to penetrate back upwind when desired. But I'm thinking that this light and large a model won't be flow in anything other than calm to mild conditions anyway.
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Old Nov 01, 2014, 09:00 PM
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Good idea Bruce!
I've got that on the agenda once I see how this one flies.
I will probably have to get a CNC hot wire cutter working in order to do it, though. I am using pink dow light foam with a carbon rod spar, top and bottom, and covered with coverite microlite covering. It seems quite good at this point, but would be challenge with a thin undercambered airfoil!
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Old Nov 01, 2014, 09:56 PM
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Yes it would if you don't include some stiffening of some sort that simulates the shape control you'd get with ribs. Mind you you're also going to find this issue with the thin trailing edge area of the AG airfoil. What are you using to ensure that the wing stays true and doesn't wobble and curl now? I know that the pink is pretty good. But it fails when the material becomes as thin as it will at the trailing edge. Or are you using balsa caps glued on and shaped down for a stiffer trailing edge?
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Old Nov 01, 2014, 10:17 PM
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Just the stiffness imparted by the covering. I also have a 1.5mm carbon rod inset top and bottom for a spar. I overlapped the TE an inch or so top and bottom to thicken it up, and it is pretty stiff. Overall the wing has OK torsional stiffness.
Maybe a thin carbon T shape would work for a spar on the free flight airfoil?
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Old Nov 01, 2014, 10:19 PM
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Quote:
Originally Posted by ShoeDLG View Post
Sure enough, the "rise over run" for one degree is indeed just under a quarter inch per foot. Not sure how that is in any way related to the stall speed of this design.
I was visualizing a stick fuselage a foot long with prop end 2" higher than the tail (an example only)
Believe it or not I do have ultra slow flyers built up tissue covered and that AOA It is tough to hold that angle without power vector flying
Stall speed power on /power off? what is your interpretation of a stall? I am no good at math models but for actual flying models - I ain't all that bad
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Old Nov 02, 2014, 12:40 AM
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If you use rigging much like an Eindecker or other similar style aircraft or functional wing struts you can easily get away with a "T". Otherwise I'd suggest top and bottom caps with something a little firmer than foam in between. The thinner wing sections used in free flight work well at the lower Reynolds numbers found in super slow speed flying. But it makes the job of the thinner top to bottom wing spars that much tougher. So the filler between the upper and lower caps needs to be more firm to avoid crushing. Or you need to use external bracing like struts or full upper and lower rigging. The extra strength from any of these isn't needed for straight around flying. It's for when you pull back on the stick at some point and the G loads build up. And yeah, this still happens even at super slow speeds if you make it pitch up or loop tightly enough.

I know that the flat foamie models get away with just a carbon blade spar. But they are small in wingspan. I'm guessing that your motor glider is using a longer and higher aspect ratio wing. And the extra loading that provides makes me suggest the proper spar or external support options. Certainly you CAN make a spar that will take the load. But it would likely be heavier than using functional wing struts which can be very light.
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Old Nov 02, 2014, 08:55 AM
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For those really into the idea of a very light slow powered model - a few points to consider -- In power off mode -the AOA for flying has a pretty small window.
increasing it above best slow glide speed , will result in instant speed loss and rapid sink. the model shown is 250 grams all up It was made a number of years back by a Czech model model maker.
Note the airfoil -thin and highly undercambered. I added a modern motor (EFlite 300 and 8x3.8 prop on 3, 450 ma pack)
IF the model were to be ballasted - then it would glide much faster and usable AOA envelope would INCREASE.
The stack of various models was included to show different types which have--much different AOA tolerances .
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Old Nov 02, 2014, 09:12 AM
An itch?. Scratch build.
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Go bigger ?

Spectacular HUGE RC plane. 16 square meter wing SLOW MFK FALKEN (11 min 23 sec)



There is a very good view of the wing section at 10:00, clip below, but very small picture, and at 10:45 after it has landed.
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Old Nov 02, 2014, 07:02 PM
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The airfoil on that big wing is fine because the size of the model provides for a higher Reynolds number despite the flying speed. But for smaller models at very low speeds the Free Flight community has proven that thin and sharp is in over many years of trial and error.
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Old Nov 02, 2014, 07:20 PM
hul
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Switzerland
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I put this into FLZ Vortex, just out of curiosity. Had to assume the geometry and the size of tail.

Angle of attack is about 5 degrees while incidence is 0 for wing and tail.
NOTE: wing incidence is measured against the flat bottom because airfoil coordinates reference to that.

Hans
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