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New Aproach in Flying Wing

#1
Sorry for bad English

Hi `m designing a wing whit kinda NACA 60212 and elliptical leading edge to make a flying wing, it will have a APM board and almost 1.80m wing span ( 90cm each side central cord within 20cm).

I would like to make a wing for FPV somewhat slow flight , I´m not sure if 15 sweepback is the best for this type of flight.

I hope some one cam help.
desenho asa.JPG
 

Craftydan

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#2
Pualo,

I'm not sure I understand what question you're asking, but I think you're asking about the back-sweep angle on the wing.

Flying wings, as you know are generally built flat, but 0 dihedral gives you no roll stability. Sweeping the wings back in the wing will give you a dihedrial-like effect at a rate of 1 degree of dihedral for every 2-3 degrees for sweep back, so a sweep of 15 degrees equates to 5-7 degrees of dihedral -- a fairly good dihedral for any wing.

It also gives you added pitch control, since your elevons are moved further back from the CG -- the elevator-like behavior on the elevons has a MUCH longer moment arm on the pitch axis, the longer the sweep angle.

Can you live without it? Sure, but you'll end up with a pitchy wing that likes to drift in roll. Will the APM take care of that instability? maybe, but why make the APM work harder, instead of making the plane fly better with-or-without it?

It's easy to make an airframe less stable, but unless you have a VERY good reason for it, don't do it.
 
#3
Very good explanation Crfty Dan! You got the point.

When you mean 15 degrees you are measuring it from the central cord so it will be 60 between both leading edge?

More one point by sweeping back it will make the STOL velocity higher in any reason degree x CL?

Regards.
 

Craftydan

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#4
Everywhere I've seen sweep defined is "how far back from straight", so 0 degrees would have the wing root and wing tips centered with each other, with any taper between root and tip equal, forward and back. 10 degrees back would be measured on the yaw axis, back along the wings center pitch axis -- 10 degrees back would have the wingtip's center at "(length * SIN(10))" behind the wing root center.

Sweep and stall speed is a complicated relationship. Effectively, the airflow across the wing at slower speeds bounces/drags across the leading edge causing it to flow *both* across the wing -- chordwise --, and along the wing -- spanwise. On a straight wing or a fast swept wing, it flows mainly across and most goes toward producing lift. the spanwise flow that comes at the slower speeds will reduce the lift, raising your stall speed. How much per degree . . . there are equations, but I've never had reason to fool with them, so I don't really know how complicated it really is.

There's also the issue with stalling near the tip -- there's more spanwise flow out at the tips -- adding on an unfriendly stall characteristic to the already higher stall.

You *could* mitigate this by installing wing fences to add drag on the spanwise flow, or washout to ensure the AoA of the tips prevent a stall before the root. Wing fences in general increase drag and can look cool or ugly, depending on taste, and washout only forces the stall speed to occur at the root, not decrease the stall speed. These mostly improve the handling, not as dramatically reduce the stall speed.

The best thing you can do to reduce the stall speed -- reduce the wing loading. keeping your wing loading low will allow you to maintain that 1:1 lift-to-weight with less and less airflow over the wing. Adding weight means you'll have to raise the AoA to maintain altitude at the same speed . . . and eventually your airflow will detach from the wing and into the stall you go.
 

Craftydan

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#8
5cm seems short, but it might be enough.

Just making sure you haven't forgotten about yaw . . . elevons REALLY won't help you there.