# The "Beast One": What doesn't kill you makes you stronger

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#### Andy_RR

##### Well-Known Member
Also wing sweep affects where stall occurs first.

#### Lendo

##### Well-Known Member
jedi, sorry BSLD?

#### Aerowerx

##### Well-Known Member
jedi, sorry BSLD?
Bell Shaped Lift Distribution.

You should be able to get a lot of information on it via Google.

#### Lendo

##### Well-Known Member
Andy_RR, Do you have formula for that Wing Sweep effect.
I've calculated effective Wing area loss due to Sweep, it's very small up to 20° at 25% C, after that it rises exponentially.

Aerowerx, looking that up.
George

#### Andy_RR

##### Well-Known Member
No, but I think it's going to be a strong function of the span-wise pressure gradient near the stall AoA. If this generates reverse span-wise flow then effectively you're losing the vorticity at that point that's generating your lift. If you control the pressure distribution near stall with wing sweep and camber/thickness you should be able to control which part of the wing stalls first. Note that Prandtl's theories on lift distribution assume no wing sweep.

#### Aerowerx

##### Well-Known Member
....If you control the pressure distribution near stall with wing sweep and camber/thickness you should be able to control which part of the wing stalls first. Note that Prandtl's theories on lift distribution assume no wing sweep.
A good example of this is the NASA paper on the PRANDTL-D, by Al Bowers. It has been mentioned here on HBA before, and is available on the internet. Although the PRANDTL-D is a flying wing, the paper shows how to control the beginning location of the stall by using wing twist.

#### Scheny

##### Well-Known Member
For academic reasons, I also took this Prandtl paper and calculated a wing based upon it. It has good behavior like slightly improved induced drag and proverse yaw, but it is not very useful in my case.

With the approximately one quarter longer and super thin wingtips, this is nice for a cruiser, but not for an aerobatic aircraft, where roll-rate is also of essence.

In the end, all the paper says is, that the optimal lift distribution is not the elliptic "bell" shape, but instead the lift should run out tangentially to the span. This makes the wingtip vortex go inside and provide negative lift at the tips, which is made up by higher loading on the inside.

Prandtl himself said, that this wing is not the one good solution. Rather, the solution for the perfect wing is defined by its constraints. An elliptic wing is the optimum for a span restricted wing and the "Prandtl" wing is the optimum for a span unrestricted wing.

#### Jay Kempf

##### Curmudgeon in Training (CIT)
This paper was all about proving a flying wing could have pro-verse yaw. Which was a solution for that particular swept and tailless S&C analysis. Not much use outside that config. Not a real statement of efficiency especially outside tailless. The idea of upwash and over twisted tips tipping the lift vector forward when the AOA is grossly negative is a bit out there and I am not sure was really tested. Gross results seemed to stack up. And it was all a good read.

#### Aerowerx

##### Well-Known Member
This paper was all about proving a flying wing could have pro-verse yaw. Which was a solution for that particular swept and tailless S&C analysis. Not much use outside that config. Not a real statement of efficiency especially outside tailless. The idea of upwash and over twisted tips tipping the lift vector forward when the AOA is grossly negative is a bit out there and I am not sure was really tested. Gross results seemed to stack up. And it was all a good read.
Please understand. Per post #40, I gave the PRANDTL-D as an example of putting the lift at the desired point on a swept tapered wing, by using the wing twist. Not as the ultimate solution.

#### jedi

##### Well-Known Member
With regard to a BSLD and the NASA paper on the PRANDTL-D, by Al Bowers the tip does not need to have negative lift. It is all about the design point and speed range. Typical compromises as in all aircraft design work.

I agree this subject is not intended for aerobatic aircraft, per se but wanted to point out that the wing tip design can influence wing stall characteristics, the subject discussed in post #24.

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#### Scheny

##### Well-Known Member
Andy_RR, Do you have formula for that Wing Sweep effect.
I've calculated effective Wing area loss due to Sweep, it's very small up to 20° at 25% C, after that it rises exponentially.

Aerowerx, looking that up.
George
The formula for change in CLmax with sweep angle (correction factor) is:
K=cos(Lambda C/4) according to Raymer
K=cos³(Lambda leading edge) according to Young

Losses are therefore approximately (using Raymer): ~0% @10°, 5% @20°, 15% @30°, 25% @40°, 35% @50°, 50% @60°

#### Aerowerx

##### Well-Known Member
..... the tip does not need to have negative lift.....
In Al Bowers design, the tips do not have negative lift, but they do have negative drag. Because the lift vector is tilted forward slightly.
Losses are therefore approximately (using Raymer): ~0% @10°, 5% @20°, 15% @30°, 25% @40°, 35% @50°, 50% @60°
IIRC, in Nickel's "Tailless Aircraft" book he states that around 20 degrees is "optimum". Of course, he is talking specifically about flying wings but I do not see why that wouldn't apply to a tailed design.

#### Scheny

##### Well-Known Member
... around 20 degrees is "optimum". Of course, he is talking specifically about flying wings but I do not see why that wouldn't apply to a tailed design.
Because the sweep is used to substitute the elevator, by shifting lift between inner and outer portion, therefore between rear and front.

As you have the trade-off between losses due to sweep and benefit of having no tail, there is a sweet spot in the middle with the optimum design point --> which is then at around 20°.

For a tailed aircraft, aft sweep is ALWAYS bad, unless you want to go trans- or supersonic. In this case you want it for a.) keeping the wing inside the mach cone and b.) having a "longer" profile due to the air flowing at an angle over it.

Out jet is flying below Mach ~0.5 so that compression effects are no concern. This is the reason why we have forward sweep rather than backward. The backwards swept wingtips are for optics only.

#### Lendo

##### Well-Known Member
Aeroworx, I found the NASA /TP--2016--219072 paper on BLSD, it's a good read and very interesting, but don't think it's appropriate for our type of Aircraft as it requires accuracy and complexity at a high level and not for the likes of Light Aircraft, IMHO. I would have to be trying to get out of a Spin without a VT and Rudder.
Scheny -Thanks for that info.
Jedi - I will find that PRANDTL-D, Nasa Paper and have a read.
George

#### Aerowerx

##### Well-Known Member
The original Prandtl paper (in German) is here: http://www.ornithopter.de/daten/prandtl.pdf
As a clarification, the PRANDTL-D is a flying wing developed by Al Bowers of NASA, somewhat based on Prandtl's work. Al Bowers wrote a paper describing his design, including the lift distribution. That paper is what is being referred to here.

#### Lendo

##### Well-Known Member
Aerowerx, Yes! I did see that.
Andy R.R, sorry I don'r read German, but I consulted a learned friend on BSLD, he suggested in can only be achieved (on his design anyway) at only one CL, probably meaning one AOA. and then it reverts to Elliptic. He says it's an interesting concept but at present that's about all.
Sceny, Yep! It seems 20° is a golden number alright, I seen it pop up on a number of issues to do with Sweep, with Tapered Wings you can't escape some sweep. I'm a firm believer of the KISS principal, Straight Spar, Tapered Wing, No wash-out, with one Airfoil and use the standard coefficients for the tail feathers. The complicated stuff I leave up to chaps like you.
George