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Old Mar 21, 2013, 02:53 PM
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Question
L/D ratio increase methods

Hi guys

I've recently built a 2 meter 'sailplane' with motor from 6mm kinda-depron(cheap in my region, way cheaper than original) foam. Here are the specs
Weight: ~700grams fully equipped
Span: 2 meters
Length: 1.3meters

Motor:EMAX cf2822 with 8060 prop, propsaver mount
ESC: Hobbyking 25-30A
Battery: 2200mAh 20C

For the wings i used kfm-3 airfoil, they are rectangular shaped, i added aluminnium tube spar 50% of wing length. I've flown it and it flies beautiful on power, just point and shoot kind of plane. Top speed around 60 kmh. However when you cut off the power it falls like a brick, at least that was not what i expected of it. From around 3 meters of height ive managed to fly it max 18 meters so max LD ratio would be 6. Is the shape of the wings related to such poor performance? Or maybe its my fuselage or i should say lack of it, everything is mounted on a woodden stick?
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Old Mar 22, 2013, 01:23 AM
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I'm assuming that you are not using a folding prop. It makes all the difference in the world. In fact, I use just enough throttle to kick the blades out as an air brake.
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Old Mar 22, 2013, 04:14 AM
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The wing's the thing!

If you want to improve the glide (L/D ratio) the place to start is the wing.
Lift equals weight (near enough) so you can't change that easily - you need to reduce drag.
Flat plates and KFM type wings fly and handle well, but are draggy.
So first step might be a nice, streamlined top surface to the wing, with blunt leading edge, sharpish trailing edge and high point around 1/3 of the way back.
As Peterlngh notes, a folding prop will also help - you will need to activate the brake on your ESC to make sure it stops and folds - the highest drag available is a windmilling prop (higher drag even than a stopped non-folding prop).
The next step might be a streamlined fairing around the radio gear (we will need pics to see what you have now).
The quest for better sailplane performance is endless, but is basically all about wing span, aspect ratio (long and skinny wings), minimum cross sections on everything, slick streamlined shapes, smooth surfaces and minimal sticky outy bits (wheels, aerials, control horns etc).
Start the journey and have fun!
PeteM
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Old Mar 22, 2013, 06:14 AM
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Hi Pete
Well said
I started the journey 30 plus years ago and am still enjoying the chase
If you want to see what the best L/D looks like take a look at a Shadow or AVA Maxa
And you will see
If you are a home builder type the a pod from www.kristoffair.co.uk may be of use to you
Hope you have as much please trying to improve your L/D as I have over the years
Fly safe
Dandiy
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Old Mar 22, 2013, 07:42 AM
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Weight & airfoil

What you have is a powered airplane that looks like a sailplane. You need to change the airfoil and reduce weight. You can cut the weight of the battery by as much as 1/2. Al is not a good choice for a spar. Find an under-cambered low drag airfoil.
Your goal should be something with a wing loading around 9oz per 1 sq ft.
As stated a folding prop is mandatory!
Similar problem:
http://www.rcgroups.com/forums/showthread.php?t=1856179
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Old Mar 24, 2013, 08:52 AM
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First of all thank you for your responses. I'm planning to mount a folding prop and change the airfoil to a conventional one, probably this would be invention called 'ArminWing' that experimentalairlines use.
I would like to increase the aspect ratio of wings so span would be 3 meters but wing area the same.
I watched a video
KF Airfoil - Wind Tunnel Testing (3 min 36 sec)
that shows tremendous advantage of regular airfoil over kfm one in terms of drag. Also im working on a fuselage that would contain all the electronics and a camera.
I will post result and photos when im done.
As for the plane i already got, im planning to test it with different wing loadings. For now i can say that adding 200 grams of payload not changing the cg does not make a big difference, only maybe in max angle of climb lower about 10-15 degrees.
As you said, working on improvements of your aircraft is very addicting. Once again, thanks for your help
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Old Mar 25, 2013, 12:18 PM
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Eliminate the step in the KFM airfoil. That will make your airplane a flier.
The KF shapes are drag-producers only.
Despite the publicity this thing gets, no one uses it.
It's more expensive to construct, and adds drag.
Not selling points.
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Old Mar 25, 2013, 04:05 PM
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Your wind tunnel tests don't separate the lift and drag forces properly.
Any force on the test specimen is shown on the scale, both lift and drag.
There needs to be a method to mount the specimen to show the effect of lift only.
Here's a first cut at a balance that might do that...
The wing has a smooth tube pushed through at the aerodynamic center. There's a washer on the tube. The specimen can slide freely on this tube.
At any test point, there will be the drag and the lift showing on the scale.
A wheel collar mounted above the sliding tube on the post will let the wing lift be measured as it pushes on the scale due to the drag of the airflow, and the lift.
The specimen pushes on this washer as it lifts. The amount of weight on the scale then drops off from a value it would have with the specimen not free to move.
Probably needs a bit of work.
Keeping the wing on the post when it's lifting, for one...
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Old Mar 25, 2013, 08:12 PM
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Sometime later that same day.... there is no quick and easy way to separate the lift and drag forces... Sob!
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Old Mar 26, 2013, 05:56 AM
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This should separate the lift and drag.
The forces for these small specimens and low airspeed are very low.
The specimen on the post on the scale gives a torque, composed of lift and drag. Which one is doing the most work for any condition is indeterminate.
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Old Apr 05, 2013, 02:51 PM
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Thank you for your advice Paul, i really appreciate it

I've at last had time and a decent weather to fly and flying slower helped A LOT. Decreasing speed to about 15-20kph increased L/D to about 10-12 and this is without changing anything else. However 'draggyness' of KFM at higher speeds is still an issue so the wing still needs rebuilding, not to mention folding prop and a sleek fuselage(any fuselage at least :P )

So wait for progress, in the meantime i will post photos of my plane. Cheers
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Old Dec 27, 2015, 08:21 PM
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Where can I get more graphs on KFm lift/drag ratio? Or more experimental videos :-)

Nice experiments. It would be nice to do the lift-drag separation when measuring.

BTW, I've been trying to simulate KFm wings but the simulations do not converge (XFLR5). I don't know enough about this.
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Old Dec 29, 2015, 03:44 PM
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I'd try playing with the cg location before taking the axe to your plane.
Check out this article. http://www.rcgroups.com/forums/showthread.php?t=884765
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