Flying wing design
Flying wings all look similar, but they can fly very differently. This page will collect information about the design process.
If you think of something you'd like explained, add an empty section and hopefully someone will fill it in...
Designing from scratch
Designing for a specific goal
Before you start drawing up your next design, you should decide on some parameters for your wing. What size do you want your wing to be? (will it fit in your car, is it a convenient size, do you want it as small as possible?) How do you intend to fly it? powered to the max and screaming on the way up? duelling on the slope? thermaling in the summer? Target speed? Target stall speed? Want to use a certain power system or fit into your car?
How big is your field and car?
- Cambered wings are good in slow flyers that you want to float around with minimal power or thermal.
- Symetrical or semi-symetrical would be for faster high speed wings that you don't want to have pitch issues from speed.
- The closer you get to symetrical, the better it will work inverted.
- For low aspect ratio wings, the profile used is less important. A simple flat plate or two sheets of foam with a little spar to keep them apart work just as good and are much easier to construct.
A lot of people will recommend the MH45 or similar, but beware of hypetalkers. The ones that swear by lots of carbon fibre are turned on by prestige and price more than anything else.
- Thin aerofoils are faster
- Thick aerofoils are stronger and have a softer stall.
Aerofil plotting software
Useful details here: Airfoils for Tailless Swept Wings
Flying wings seem prone to tip stall (where the wing tip stalls before the rest of the wing, resulting in that wing dropping and the plane starting to spin.)
Tips stall before the wing root if the wing is:
- swept back
- high aspect ratio (long and thin)
For a rectangular wing (like a trainer) the root will stall first because air flowing around the wing tip will delay the tip stall. For a flying wing, all the factors above are working against you, so many designs use Wash out or twist.
Reducing the size of the elevons, and throws also helps to reduce tip-stall because the elevon changes the camber of the wing.
Wing tip fins that extend below the wing also help.
Twist distribution formulas
Mean aerodynamic chord (or MAC) is the width of an equivalent rectangular wing in given conditions. Therefore, not only the measure (size) but also the position of MAC is important.
The CG of a wing should be about 15% of the MAC. This is much further forward than the 25-30% usually quoted for conventional planes.
The full wikipedia MAC article is here.
There's a good (i.e. less mathematical) method of finding the MAC here:
What's the fastest battery/motor/prop for my wing?
Well, the biggest you can fit in. More power = more speed. However you've still got to launch it and land it, so you need to think about your wing loading. If it's to heavy it'll stall before getting up enough airspeed to have any control. You can use a bungy launcher but you still have to land...
There's a useful calculator here Coefficient of Lift Calculator Enter your maximum landing speed and wing area, then play with the weight until you get a lift coefficient of one.
- Subtract the weight of the airframe, plus a bit for the radio gear.
- Allow about 75% of that for the lipos
- Find a battery of that weight, or multiple batteries in parrallel.
- Work out the maximum (continuous and burst) power that battery can provide.
- Buy an ESC that can handle that much power.
- Find a motor that can handle that much power.
Now the tricky bit, picking the 'Kv' of 'gear ratio' of the motor. The higher the kV ratio of the motor, the faster it needs to spin to reach the best efficiency/power output. A faster spinning prop has a higher pitchspeed, but as you need a smaller prop for the higher rpms, this comes at the the expense of thrust. A motor/prop combo with high trust but a low pitchspeed will take off easily, but wil stop accelerating once it comes close to the pitchspeed of the prop. If the airplane where to go faster than this pitchspeed, for instance in a dive, the prop could even begin to act as a brake as the angle of attack over the propblades would be reversed. TPECalc that comes with the free software for the Hyperion E-meter is good for this.
Another program to get a ballpark figure for your setup (though not specific for flying wings) is web-o-calc: http://www.flbeagle.rchomepage.com/index.html (click software->web-o-calc)
You want thrust and pitch speed. There's not much point having more thrust than 1.5 times the AUW, and there's no point having a pitch speed of 200mph if there isn't enough thrust to overcome the drag at 50mph.
To check your thrust line, get some altitude in level flight, hands off and cut power. Glide 50 foot and apply full throttle and see if its attitude changes up or down drastically.
Up thrust on a pusher means point the prop up so it applies down force on the trailing edge forcing the nose to come up
Rules of thumb
- The Leading edge is usually swept back about 25-30%
- Leading edge wing tip is usually an inch ahead of the trailing edge of the root (before any cut-out is made for the prop)
- Elevons need more area at the tips than at the wing root.
How to make a new wing from the scraps of your ‘newly-old’ wing.
Cut out the broken bit in the middle, treat the rest as new wing cores...