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Old Apr 12, 2014, 07:40 AM
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I think the graphic below shows that the vortex in the step creates a virtual surface and the air flow around it behaves almost exactly as if the blue area was a solid structure shaped with an under-cambered and drooped trailing edge.
This shape may be adaptive with possible beneficial effects, but my guess is that a solid surface of the same shape would perform in a very similar manner.
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Old Apr 12, 2014, 03:28 PM
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Michael V's Avatar
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Quote:
Originally Posted by Blendedwing View Post
Very interesting report. I have one "scientific" question on the airfoli used for comparison.

By cutting this large step into the lower surface of the NACA 4415 airfoil, the student has effectively increased the airfoil camber, which notoriuosly increases lift.

It would be interesting to discover which part of the performance improvement comes from this additional camber vs. how much comes from the trapped vortex.
Quote:
Originally Posted by Whiskers View Post
I think the graphic below shows that the vortex in the step creates a virtual surface and the air flow around it behaves almost exactly as if the blue area was a solid structure shaped with an under-cambered and drooped trailing edge.
This shape may be adaptive with possible beneficial effects, but my guess is that a solid surface of the same shape would perform in a very similar manner.
Yes and yes, which means the real question is whether the original airfoil used was optimized for the purpose? If it wasn't, then the new airfoil (step or no step) shape might just have been simply better.
A slightly undercambered airfoil might just be better for such application (endurance flying) than a somewhat symmetrical airfoil which will have less lift altogether.
So aside of the "adaptability in shape" that the step provides, it may just be a benefit from a more appropriate shape. Since UAVs are not meant nor really need to fly inverted or perform advanced aerobatic maneuvers, I tend to question the use of a semi symmetrical airfoil to start with.

I mean it is an interesting study, but I think we cannot conclude that the step is the sole source of gain here. This raises a very interesting conundrum, what can we ultimately compare to? Since we are essentially changing the shape of the airfoil with a step, is the gain/loss due to the step itself, or to the change or airfoil shape. And what would be an equivalent shape to measure solely the influence of a step.

For example in this case we see the step being better than NACA 4415, but wouldn't a smooth undercambered airfoil be even better? If that is the case then the step is just a transitional incidental benefit. That doesn't preclude the fact that it may still be better for some compromise (rigidity, easiness to fabricate, adaptability to flight regimes, etc..) than a strict undercambered, but it raises the question of the scope and interpretation of result of comparisons.

So now philosophically I think we are trapped into this "better" concept. Depending on definition of better, and especially depending on scope, it seems that it will always be possible to find better and worse than any airfoil (KF or not). Thus we have to define a set of circumstances, and the KF may be better as certain compromises, and not so good at other, and that is what we've been saying since the beginning. Detractors focus only on the part where it is "worse" (drag mostly), some other only where it is better (easy to build). It is a complete set of a compromise. It will best fit certain sets of circumstances, and not fit others.

Now if we could show that the understepped 4415 had better lift than a 4415 AND an undercambered foil with no step (e.g. cutting the front bottom of the stepped one to have no step) then that would be very interesting indeed, but again how much "different" is valid when we are ultimately comparing different shapes and should we only compare lift?
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Old Apr 12, 2014, 04:21 PM
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We also need to consider another aspect of the extra duration gained.
A UAV required to have a long dwell time would benefit, but one required to carry out a distance task may not if the change of airfoil lowered the cruising speed.
This is a subject where every single change causes many other things to change.
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Old Apr 12, 2014, 04:58 PM
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As I understand it the airfoil selected for the test is the one used on the UAV. If you have a question as to why that one was chosen you might want to ask General Atomics. My guess is that it's over all performance was the best in the regimes in which the vehicle would be flying . UAVs are by nature distance machines. They fly long distances, and then loiter as long as possible. Increasing duration by 20% would be a good thing. No where did I see anything indicating that the aircraft with the KF airfoil would fly slower. The improved performance is under the same conditions the current wing flies, or it isn't an improvement.
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Old Apr 12, 2014, 05:43 PM
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A little background on the KF Stepped Airfoil.

In 1969, when Floyd Fogleman and I decided to file for a patent on a stepped airfoil, we had a problem. The step worked in a number of different ways. We also knew from wind tunnel tests conducted at the LaGuardia School of Aeronautics and again at the University of Notre Dame, the step on top produced higher L/D ratios. So why did we show it on the bottom?

We decided to put it on the bottom thinking that it might be the next step in Whitcomb's supercritical wing. We also knew that when an aircraft goes from subsonic to supersonic, the L/D ratios reverse and we would pick up the higher numbers supersonically.

Whitcomb's wing had a slight cusp at the trailing edge, so that is where we decided the step should be. We did not have the funds to file for a whole bunch of patents.

Fogleman died in 2003, so he never got to see our contribution develop.
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Old Apr 12, 2014, 05:58 PM
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The point being:
If the modified airfoil develops more lift then the aircraft may need to fly slower to maintain level flight.
Note the "if" and "may."
If the AoA was reduced so that the modified wing produced the same lift at the same speed, would the indicated benefit remain?
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Old Apr 12, 2014, 06:32 PM
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As the angle of attack is reduced drag reduces as well as lift. The AoA would only be reduced a degree or two to retain the same lift. The aircraft retains the benefit, without having to slow. However, when loitering speed is not essential. The idea is to remain in the same spot. So reducing speed and retriming for dest L/D will give the best endurance (ie time over the target).
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Old Apr 12, 2014, 07:16 PM
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Overall I agree.
However not all UAV missions are as you describe. As in: Go there. Obtain the data. Come home.
But all this is a side issue, and we lack the information to do other than speculate.
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Old Apr 13, 2014, 04:34 AM
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Just an update on the other thread that Margo is running that I'm assessing the KfM on via CFD. I've actually chosen the NACA 4415 profile to use as the baseline foil with positive camber. The symmetrical will be a 6 series as if you compare a 6 and a 4 that are the same in t/c and PositionMax of t, the 6 has less drag. I'm getting somewhere !
Best
DA
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Old Apr 24, 2014, 10:26 AM
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I came across this very interesting video which illustrates the vortex in our universe. The vortex seems to exist everywhere. From the solar system to a flower to a sea shell.

The helical model - our solar system is a vortex (3 min 21 sec)
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Old Apr 24, 2014, 12:20 PM
Jack
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Man! That is interesting stuff! But who is steering this thing? Do they have a cell phone? Will they text while driving? Are we all at risk here?

I'm scared and it is too early in the day for a drink!

Jack
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Old Apr 25, 2014, 04:23 PM
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Dick, you find the most interestings.

FYI: The video is very impressive when viewed on a wide screen TV.
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Old May 01, 2014, 09:30 PM
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KFm4 Fluid Simulation

Trond created this simulation a while ago of the KFm4 ...

KFm-4 Fluid Simulation (9 min 25 sec)
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