### Why sweep hurts endurance and what to do about it

Hey guys,

I just wrapped up a project that taught me a lot about the effects of sweep on flying wings. Turns out sweep can significantly increase drag and hurt endurance. Sweep isn't all bad, though. There are ways to make a flying wing stable if you do it right.

I put together this film to explain the physics of what's going on and how to get stability without sacrificing endurance.

https://youtu.be/VGQJtiGEU8U

We are trying to make the software available outside the US and will let you know when it is.

With regards to your airfoil thickness question, this is a matter of sweep definition. Sweep is not actually a rotation about the normal axis. Instead it is a skewing of the wing. In this way, the airfoil always maintains its same orientation to the freestream, and also maintains its thickness and geometry.

In contrast, dihedral is a rotation of the wing about the axial axis. Dihedral is the actual rotation of the wing, not a skewing motion.

So in short, the alleged change in thickness of an airfoil due to sweep is a misnomer. The sweep should not change the thickness of the airfoil because the wing is skewed, not rotated.

Does this make sense?

• Doug,

Very interesting, and especially well presented, thank you!

Please can I +1 the request for MachUp to be available in other countries (South Africa). I have used XFLR5 extensively for swept flying wing design, esp. foregoing any extra possible drag items such as winglets, and going for a basic blended wing-body, but I have hit the limits there to some some degreee, and would love getting a fresh data perspective on the problem.

Does MachUp consider the change in airfoil thickness (reduction) percentage and profile as a result of sweeping the planform?

• Great ... Many thanks for great video and information and effort.

• Please allow for Israeli users to use the soft :-)

• I really like how you went into detail with one particular design facet! Where it starts to get complicated is with the multiple degrees of freedom available to the aircraft designer (washout, airfoil, trim condition, cg placement, sweep angle, structure weight, etc) and how each interacts with the others. Sometimes you have no choice but to add sweep, sometimes you find that sweep is pretty detrimental. This seems like a good primer on some of the effects of sweep and hopefully people find it useful!

• Achieving good efficiency is a lot easier with larger span wings. You can even use cambered airfoils if you have enough sweep and span.

• Martin, I agree. Commercial UAVs probably designed to sustain flight at minimum computer "power" in case of any sort of damage to it. Negative stability won't help.

We see some nice examples on Youtube of RC planes with negative stability. I did try flying high performance glider with Vector controller. Did not like the idea of not be able to fly it manually but with autopilot it flies very nice. Still on a learning curve.

At the same time building a 110" flying wing. Small wings I flew did not fly very well.

• @Mark, it's impossible to design swept wings that have the performance of non-swept wings. At least at the model airplane scale. If you want good stall characteristics you'll have to accept higher induced drag. No way around it. Some compromise has to be made. There's a reason why you don't see tailless airplanes winning competitions.

When it comes to UAVs aerodynamic efficiency is usually not the driving factor.

I've also wondered why there aren't any commercial flying wing UAVs with negative stability.

• As I recall swept wings are very hard to design properly. Tip stall is very common design flaw.

Twist along with proper airfoil help a lot in that department but not fool-proof. Even in classic configuration swept wing is much easier to stall than a straight wing.

As an example of very expensive flying wing tip stall  - we all saw B-2 crash on a take-off where a billion dollar computer system did not help.

Swept flying wing is not that horrible but flight envelope is much narrower than on a straight wing classic configuration.

In addition since we flying UAVs with flight controllers it is very feasible to move CG a bit back or far back in classic config where we could easily bring stabilizer down effect to up-effect, in-turn bringing a flying wing design benefits to zero or close to zero.

• @Martin: I just looked at the sectional Cl distribution along the wing, and as you suspected, it does show that it will tip stall. I plan to release a video soon on the effects of twist/washout, which could mitigate this issue for tapered wings.

@Gary: Thanks!

@Mostyn: I agree we can't assume too much about bird flight from this simple example. The intention of this post was to show the effect of a single parameter (sweep) on the efficiency of wings. Whether it is a bird's wing or a human-made wing, the physical principles are the same. You're right - birds have many constraints I'm not accounting for here, and the fact that they don't have much sweep is affected by more parameters than I mentioned here. This is just one principle that is important to understand for anyone wishing to create aerodynamic-efficient airframes.

As @Mark pointed out, birds are very efficient flyers - and we still have a lot to learn about the how's and why's of what they do. For example, the US government has poured millions of dollars into studying bird flight and trying to develop an airframe that works like a bird. They've had many of the best thinkers working on the problem (Drela from MIT, professors from Brown University, and guys at Aerovironment to name a few. And I believe anyone who worked on those projects would have to admit that birds are still evolutionary years ahead of us!