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Very low aspect ratio planes?

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bwainfan

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Matthew:

A few thoughts for you:

First, we normally take off at a speed significantly above stall speed in order to have positive rate of climb and stall margin for gusts. Typically lift-off speed is about 1.2 x stall speed, so Cl is about 70% of Clmax at that point.

The issue about CLmax for LAR configurations is as much about the fact that they are tailless airplanes as it is about aspect ratio. Without a tail, we can't trim flaps, or highly-cambered airfoils. This limits the max CL.

For reference: The highest CLmax I'm aware of for any tailless airplane was bout 1.4, on a swept-wing design that had large flaps inboard. For most tailless configurations CLmax varies from a low of about 0.8, to a high of 1.1 to 1.2. (The highest I have ever measured either in flight with my own airplane, or in the wind tunnel over my career has been in the 1.2 range)

For preliminary sizing of a LAR configuration I would probaly use 1.0, but do calcs for 0.8 also just to get a sense of how it affects the design.

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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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.
An aside: Those CAFE reports are great, it is unfortunate the work was discontinued. An example of really useful data collection and analysis. They caught some fairly significant misstatements in published acct performance data, too. IMO, the CAFE work/reports are the kind of thing to which EAA should devote at least a tiny slice of their budget.
 

BJC

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I think that there was some EAA support of C.A.F.E. at one point. C.A.F.E. self-distructed, and reemerged as an environmental advocacy organization.


BJC
 

BBerson

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Don't see many LAR or tailless at a Valdez competition. Flaps can be useful for glide path control when the engine quits.
 

poormansairforce

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One of the things that attracts me to this layout is the fact that in an emergency I can use the full CL to get my landing speed as low as possible. Yes I'm going to total the plane but I'm probably going to walk away in most cases.
 

Riggerrob

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Dear Barnaby,
Few flying wings seem to have flown with landing flaps. The only ones that come to mind are an Armstrong-Witworth AW.52 experimental glider of 1945 and the Swift UL semi hang-glider. Both those had fairly high aspect-ratios and significant sweep, that positioned landing flaps near the centre of lift.
AW.52 had aluminum, Fowler flaps that slid aft before hinging down to increase both wing area and camber.
Swift pulls down its flexible, fabric, centre tail for landing.

Have any deltas or low aspect-ratio flying wings flown with flaps ... specifically split flaps under the centre of lift?
 
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Vigilant1

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Would you be willing to pull a Cessna up to 45 degree AOA to try and get 1.85 CL while 20 ft off the ground? At 30 mph there wouldn't be any authority from the tail to go there and even if you could it would probably snap roll.
Let's talk about the C-152 specifically, then. I don't know where 30 MPH comes from, but yes, I would definitely try to max perform the wing (that is, keep it very near its critical AoA) as I approach a touchdown at/near stall speed. And there's enough elevator authority to accomplish that (as evidenced by the fact that it is possible to exceed critical AoA and then stall at very low airspeeds). Obviously, in the C-152 that occurs at an AoA well short of 45 degrees.
Just as obviously, a plane with better glide performance provides more options as to where this touchdown occurs. A C-152 at an 8:1 glide ratio offers about 700% more reachable real estate than a plane with a 3:1 glide ratio. Maybe a meadow instead of the rocks . . .
 
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BJC

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An A152 can be controlled in a stalled descent with full flaps and full nose up elevator. It does take experienced rudder work, though. Hitting the ground in that descent definitely would ruin the airplane, but would be the preferred way to descend into a tree canopy.


BJC
 

Vigilant1

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An A152 can be controlled in a stalled descent with full flaps and full nose up elevator. It does take experienced rudder work, though. Hitting the ground in that descent definitely would ruin the airplane, but would be the preferred way to descend into a tree canopy.
Yes, this "falling leaf" exercise is a good way to get familiar with footwork near the stall. I've done it a bit in the C-152, and IIRC the descent rate was about 650 fpm. So, it would be tough on the airplane to hit at that speed (about like a dead drop from a few feet up), but could be the best alternative in some situations.
 
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poormansairforce

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Never flew a Cessna that did a decent snap roll.


BJC
I should have said "unpredictable stall".
Let's talk about the C-152 specifically, then. I don't know where 30 MPH comes from, but yes, I would definitely try to max perform the wing (that is, keep it very near its critical AoA) as I approach a touchdown at/near stall speed. And there's enough elevator authority to accomplish that (as evidenced by the fact that it is possible to exceed critical AoA and then stall at very low airspeeds). Obviously, in the C-152 that occurs at an AoA well short of 45 degrees.
Just as obviously, a plane with better glide performance provides more options as to where this touchdown occurs. A C-152 at an 8:1 glide ratio offers about 700% more reachable real estate than a plane with a 3:1 glide ratio. Maybe a meadow instead of the rocks . . .
The 30 mph comes from 204 ft of wing area times a CL of 1.85 on the Facetmobile. Basically a big aero brake at the last second. This is the method used by Mr. Wainfan in his engine out event. Not something a 152 could do.
 

rotax618

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A steep descent can be an asset for the class of aircraft that we are discussing, it is not a problem for gyros and being able to land and takeoff from a small patch of land is very usefull. As can be seen from the video of the Arup S2, an undercarriage with a long travel is an asset.
I imagine the technique for power off landing would require maintaining sufficient forward speed to have the energy for a flare at high AOA before touchdown.

 

Vigilant1

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The 30 mph comes from 204 ft of wing area times a CL of 1.85 on the Facetmobile. Basically a big aero brake at the last second. This is the method used by Mr. Wainfan in his engine out event. Not something a 152 could do.
This is just nonsensical now, sorry.
A key takeaway: Use a well maintained 4stroke and reduce the likelihood of any of this in the first place.😁
 

BBerson

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The NASA report shows that cuffs go to 30° angle of attack. I learned to fly in a Cessna 150 after helping the shop owner (I.A.) install cuffs. It could operate with the nose far upward from stock, with the airspeed on zero. I didn't measure the angle of attack, but I would never land at that attitude anyway. With full Cessna flaps the descent rate is excessive even at a nose low attitude and requires power.
 

rotax618

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It is probably more survivable landing at 23mph with a 50ft roll out than 40mph touch down and a 200 ft roll out. Airplanes with uncertified or experimental engines shouldn’t be flown over “tiger country”, I know we all do it but if you think the thrill is worth the risk..........
 

BBerson

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The demonstrated 50 foot roll outs are old Cubs in the Valdez competition. But to win they also must takeoff short.
So just counting engine out emergency landings as a basis for design, how does LAR fly slower with same wing area?
Unpowered emergency slow flight requires a combination of area and span to avoid excessive sink rate impact.
An aspect ratio 4 or 6 can have any stall speed, same as AR 1. With less wing area. The steep angle descent might not even have a measurable stall speed, but isn't much use unless a very low sink speed comparable with a parachute. The BRS people have the sink rate numbers. From memory, the Facetmobile sink rate with full back stick was about 1000 fpm. Is full back stick with high sink best for impact?
So, the only major advantage I see is perhaps ability to crash in a narrow zone.
 
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cluttonfred

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The Arup designs are fascinating but I would love to find some original documentation or contemporary articles that I could trust. Wikipedia lists the weight as 780 lb empty and with that wing area it would need something like a CL of 2.7 to land at 23 mph. And the performance was clearly pretty good from the film footage, so something is off in the numbers.
 
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