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.

Hope it's helpful.

https://youtu.be/VGQJtiGEU8U

Views: 2011

Comment by Mark on September 24, 2015 at 5:43pm

Doug,

Very nice review of the options. Did you by any chance calculate a sum of the lift generated by all sets of wings for each one of them?

It would be very interesting to see which one is actually the greatest in terms of combined lift produced.

Birds on the other hand are not completely a flying wing type of aircraft strictly speaking, and they achieve stability and steering by slightly different types of control surfaces and geometry.

So it would not be fair to compare flying wing with the birds as apple to apple. Birds can change their airfoil in the air and control airflow on different parts of the wing, go really high alphas without spoiling the airflow.. list can go on and on. Fact is, birds are very efficient flyers!

Thanks once again for great material!

Mark

Comment by Muscate on September 24, 2015 at 7:27pm

the issue is known but the reality of engineering is that:

- the elliptical wing is much harder and expensive to build

- the elliptical wing is much weaker and need a lot of material (weight)

- the elliptical wing's elevons will not be properly position for maximum effect (bird wings can move in all kind of positions/shape an don't have this issue)

That is why we end up with the more traditional "arrow shape" - albeit having a straight center section can help a little bit - in particular if the front center is an arrow/triangle and the rear is flat (best trade-off with current engineering capabilities).

Of course, there's a well known wing like this - the B2 - for such reasons! (except for the radar-avoiding-shapes of course for the engine exhausts, which can be simplified - and the wingbody optimizations)

Don't get me wrong though, it was cool to watch and see your interpretation of it, we need more of this.

Comment by Doug Hunsaker on September 24, 2015 at 10:13pm

@Mark: Thanks. It was fun to put together.

I didn't quite get your question about calculating the sum of the lift. Each case had the same wing area, and each case was set to the same lift coefficient. So each wing I studied produced the same amount of lift (although they were at slightly different angles of attack). Hope that clears up your question, but let me know if not.

@Muscate: You're right - there are lots of good reasons not to use an elliptic wing. I could have demonstrated the identical principle with any wing shape (rectangular, tapered, etc.), but I chose elliptic since it is seen as the "ideal". But you can repeat the process for any planform and see the same phenomenon. Pretty interesting.

Comment by Mark on September 24, 2015 at 10:37pm

OK Doug,

I got the idea now. So basically graph shows how lift spreads and sum of if identical for every angle.

And you're right, if wing is produced by modern glass technology - elliptical wing is just as easy to produce as square and it is not heavier than any other shape while aerodynamically it is much more efficient.

CNC does not care much of a shape nowdays. It can produce any plug or mold with phenomenal accuracy.

Comment by Cliff Dearden on September 24, 2015 at 11:38pm

Nice work Doug.

Comment by Andrew Rabbitt on September 25, 2015 at 12:10am

Interesting video Doug.  Of course you can recover much of the ideal lift distribution by washout profiling.

Comment by Martin on September 25, 2015 at 2:13am

Hi Doug, that's a great video showing the effect of sweep on the lift distribution.

I think that the comparison with birds is fair. Birds a very versatile and have great efficiency in their entire flight envelope, but model airplanes can be more efficient if we consider just a single operating point. One reason is that planes can have much higher aspect ratio than birds.

Birds can partially fold and sweep their wings to decrease the wing area which gives better efficiency at high speed flight. Model airplanes can't match that.

Could you also plot the (sectional) cl distribution for the wing with varying sweep? I would like to see if it's prone to tipstall.


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Comment by Gary Mortimer on September 25, 2015 at 5:20am

Excellent most excellent I wish we had a place to put great posts like this and keep them top of mind.

Comment by Mostyn Gale on September 25, 2015 at 6:16am

Great website and software there Doug. I always dreamed of having access to such things.

I agree with Mark's first comment. You can only take comparisons between birds and planes so far. Birds have different constraints on their design than aircraft. Birds are constrained by evolution, we humans can create all kinds of designs. Birds have to use muscles and bones and feathers, we have to use props, fans, metals, wood, plastics and composites.

For example, when people first started to try to make planes, they looked to birds for inspiration. They noted that bird wings had very thin airfoils, and warped their wings for control. So they built early aircraft with thin airfoils and warping wings. To achieve enough structural strength, they had to make trusses using multiple wings and drag creating bracing wires. 

But it turned out that the most efficient way for humans to build a wing is to make a thick airfoil, trading off some aerodynamic efficiency for greater structural strength and to make it stiff and use control surfaces.

The reason birds don't have swept wings is because birds don't work like that.

Comment by Doug Hunsaker on September 25, 2015 at 7:57am

@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!

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