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Very low aspect ratio planes?

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cluttonfred

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Actually, it’s landing not takeoff that is the critical scenario when considering ultralight, microlight, or light sport categories with maximum stall or landing speeds. Since the simple spreadsheets I mess around with don’t make allowances for any special behavior at low aspect ratios, I am wondering if there is any way to generalize about LAR designs at low speeds and high angles for attack to help me size the wing area.
 

Vigilant1

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Actually, it’s landing not takeoff that is the critical scenario when considering ultralight, microlight, or light sport categories with maximum stall or landing speeds. Since the simple spreadsheets I mess around with don’t make allowances for any special behavior at low aspect ratios, I am wondering if there is any way to generalize about LAR designs at low speeds and high angles for attack to help me size the wing area.
I don't know if this helps, but Dr Wainfan's Facetmobile Home Page says that plane weighs 740 lbs max, has 207 sq ft of wing area, and that it doesn't stall, but:
Accordingly, the true minimum aispeed could not be measured because the fixed pitot tube stalled at high AOA, leading to an airspeed reading of zero on the airspeed indicator before buffet onset. Wind tunnel data predicts that the steady-state airspeed at the angle of attack for maximum lift is approximately 33 knots.
A 740 lb airplane with 207 sq ft of wing area requires a Cl of 1.0 for level flight at 33 kts, standard day.
 
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BBerson

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The NASA Cuffed leading edge can go to 30° also with zero on the airspeed. AR6 with cuff is more efficient and virtually stall proof. Parachute landing with flaps.
 

cluttonfred

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OK, but are the Facetmobile types a special case or does a LAR aircraft using a conventional airfoil also behave that way? Something like a Clark Y can generally get to about 1.5 depending on conditions in more conventional wings.

Another problem I have is that they sources I used often don't have the airfoil data for high Reynolds numbers in any case. A 12' chord moving 40 mph gives a Reynolds number of about 4.6 million.
 

Vigilant1

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OK, but are the Facetmobile types a special case or does a LAR aircraft using a conventional airfoil also behave that way? Something like a Clark Y can generally get to about 1.5 depending on conditions in more conventional wings.
My caveman technique would be to take a few of these planes with numbers you trust (Zimmerman, Hoffman, etc), plug the minimum level flight speed, wing area, and aircraft weight into the lift equation to determine the Cl under those conditions. The prop thrust will bias things a bit, but probably not a huge amount. And if you use acft close to the same size you're interested in, the real numbers will include any Reynolds numbers effects.
 

Sockmonkey

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Now I am confused. I thought one of the appeals of LAR was high lift at high AOA due to vortex effects. A CL of 1.0 is not terribly impressive.
In some cases yes, but remember you also get a light cantilever wing with plenty of internal volume to put stuff in to reduce parasite drag.
 

pictsidhe

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Now I am confused. I thought one of the appeals of LAR was high lift at high AOA due to vortex effects. A CL of 1.0 is not terribly impressive.
Vortex lift does help. The problem with LAR is that they are capable of a very high AOA. If you are measuring speed with a regular ASI, it will under-read horribly. If you can find NACA or similar quality wind tunnel data, then that is well worth trusting. But lone experimenters getting fantastic numbers that the experts don't, needs a bucket of skepticism.
I am pretty sure Barnaby Wainfan did quality measurements. He's also a very clever guy. If he can't get anywhere near the very high CL claims for LAR, what chance do us amateurs have?
 

Riggerrob

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Dear pictsidhe,
When test pilots want accurate airspeed measurements, they either trail a cone on a long hose (outside the airplane's wake) or attach a gimballed pitot tube to stick well out in front.
 

rotax618

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There is an old saying that you should believe only half of what you see and none of what you hear, this is true but it is hard to ignore the un-doctored video footage of LAR ( Little bird, UFO, FMX4 Arup S2 etc) aircraft flying, watch the video of Hatfield’s Little Bird’s takeoff and climb on 28HP, Barnaby’s Facetmobile and Rowe’s UFO on less than 50HP, if the L/D is so bad how do they perform? The cruise performance of FMX4 and UFO seems to be as good or better than most other 503Rotax powered “ultralight” airplanes.
 

Speedboat100

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I agree....you cannot get anywhere if it is slow. There is a lot of room between fast and crazy fast.
 

berridos

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The facetmobile is revolutionary in two aspects. The planform and the weight (the facets are consequence of a very optimised weight). Its very hard to draw conclusions without mixing both features. The facetmobile with the weight of its peers would be disapointing in slow speed and in cruise vs its peers. That thesis at least, is what i am going to verify with emprical experiments on deltas without facets.
 

Vigilant1

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The facetmobile with the weight of its peers would be disapointing in slow speed and in cruise vs its peers.
Why would an increase in the Facetmobile's weight have an appreciable impact on its cruise performance? Increased weight has zero impact on parasite drag. At cruise speeds, drag due to lift is minimal, so even a substantial % increase due to weight still yields only a small increase in the total drag.
 

berridos

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Personally i dont believe the stated cruise perfomance, but really dont want to get into forum trolling or "naysayer" discussions on this.
 

Arfang

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Norman and all, I don't think I ever did get an answer regarding a rule of thumb or rough approximation of the max CL to expect with a LAR design, say an aspect ratio of 1.0-2.0 which would cover circular, square and diamond (rhomboidal) designs?
From 'Charachteristics of clark y airfoils of small aspect ratios', by C.H. Zimmerman (see attached file):
  • AR 1, rectangular tips: Cl 1.3 @ 32 degrees
  • AR 1.15, faired tips: Cl 1.0 @ 30 degrees
  • AR 1.27, rounded tips (circular planform): Cl 1.85 @ 45 degrees
Re number=860000

Now I am confused. I thought one of the appeals of LAR was high lift at high AOA due to vortex effects. A CL of 1.0 is not terribly impressive.
As I understand it, with such low wing loadings, there's probably no need to fly at an ''extreme'' AOA. I think nestofdragons post sums it up.
 

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Vigilant1

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From 'Charachteristics of clark y airfoils of small aspect ratios', by C.H. Zimmerman
Thanks for the attachment, great stuff.
  • AR 1, rectangular tips: Cl 1.3 @ 32 degrees
    [*]AR 1.15, faired tips: Cl 1.0 @ 30 degrees
    [*]AR 1.27, rounded tips (circular planform): Cl 1.85 @ 45 degrees
    [*]

At those extreme AoAs, drag becomes very high. That's why we see L/D of 2 at these Cl (Figure 5).
If we are at 30 deg AoA (and achieving those Cl) we're either carrying a lot of power or we are coming down at a steep angle.
 
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BJC

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Dear pictsidhe,
When test pilots want accurate airspeed measurements, they either trail a cone on a long hose (outside the airplane's wake) or attach a gimballed pitot tube to stick well out in front.
One can get good (but, perhaps, not perfect) data using a GPS and flying multiple directions to cancel the wind speed.

A pet peeve is that pilot reports on aircraft, with the exception of the now-abandoned C.A.F.E. reports, make no attempt to validate performance claims.


BJC
 

cluttonfred

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If the Zimmerman data from NACA TR-431 is still accurate at full scale, then the simple square wing of AR 1.0 reaches a max CL of 1.32 at about 32 degrees before experiencing a sharp drop in lift, not really a stall, though it keeps flying to at least 40 degrees. So if I take CL 1.25 at 30 degrees as my practical maximum, I get something like this with a very light aircraft using a basic 55 hp VW conversion:
  • 12' span x 12' chord = 144 sq ft area (3.66 m x 3.66 m = 13.38 sq m)
  • 550 lb gross weight (249.5 kg)
  • Minimum speed 35 mph (56 kph)
  • Climb rate 1144 fpm (5.8 mps)
  • Max speed 97 mph (156 kph)
  • 75% cruise speed 88 mph (142 kph)
  • Power off sink rate 1331 fps (6.8 mps)
Obviously, the one very concerning number here is the dramatic power off sink rate. I don't know if my pedestrian LAA performance estimate spreadsheet is accurate under these conditions, but assuming a best glide speed of about about 45 mph than that 3:1 glide angle means landing right now, pretty much right where you are in the case of a lost engine.
 
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