Stall Speed / CLmax /magic

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Foundationer

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Here in the UK we have SSDR ultralights, the rules are: MTOW under 300kg (660lbs), Stall speed 35 knots CAS (18m/s).

Having a look at the numbers for a few of these the stall speed numbers seem... unlikely. Taking published numbers for area they seem to have CLmax in the 2.1 to 2.4 range with plain flaps. Ones I was looking at were B612 (CLmax 2.43), SD1 (2.1) and MC30 Luciole (2.2).

With a decent airfoil and plain flaps I can come up with something in the region of wing CLmax 1.65.

So what's going on here?

Links to the various aircraft:
http://www.a4aviation.co.uk/b612-aircraft-2/
http://sdplanes.co.uk/sd-1/design/
https://en.wikipedia.org/wiki/Colomban_MC-30_Luciole
 

Topaz

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Without commenting directly on any particular model, especially the ones you've listed here, it's truly amazing how much you can "change the performance" of an airplane by moving the pitot-static system around. Throw the pitot tube in a high-pressure area, suddenly the airplane "flies a lot faster". Put it in a low-pressure area, suddenly the airplane "flies a lot slower." Flip that for the static port. A GPS would say there hasn't been any change, but the ASI would indicate a big change. This used to be an old game in the kit industry, especially on higher-end "faster" airplanes where there was a top-speed "spec war" going on. It's why CAFE developed their own completely independent wing-mounted pitot-static system for their independent flight tests - they didn't trust the manufacturer's pitot-static system.

Never assume that, because the ASI says something, it's true.
 

Marc W

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Pixie dust! I have one of those "magic" airplanes. The designer claims it stalls at 40 mph. It actually stalls at 55+mph. Another one is empty weight. All prototypes are sprinkled with pixie dust to make them lighter on the scales. The planes built from plans always weigh 100 lbs. more than the designer claims.
 

BBerson

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The rule states 35kts calibrated airspeed.
The owners may be using "indicated" airspeed.
The aircraft in the first link has an unusual wing loading. (213 lb/ft :) )
 

Dan Thomas

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Getting accurate pitot and static pressures is one of the most difficult jobs when building an airplane. Finding the right spots for the ports can take time. Copying the locations found on production airplanes is no solution, either. I tried several sorts of static pressure pickups; one was the static feature on the pitot head. It made the airspeed read low, since it was under the leading edge where the air slows and its pressure increases. The pitot's pressure was also reduced, but the effect was much smaller than the increase in static pressure. I tried taking the static from the cabin, which made the airspeed overread considerably. Most airplanes have a few leaks, and the air rushing past them lowers the cabin pressure. I machined a static port similar to a Cessna's and installed it in the left cabin wall well ahead of the wing; it too made the ASI overread. I milled a notch across it, like some seen on the Cessna 206, and that made it worse. I rotated it to orient the notch in various places. No good. At high AOA it really made the ASI underread; apparently air spilling from under the belly and cowl was turbulent enough to raise the static pressure at the port.

I finally cut the original static line inside the wing and got accurate static pressure. The still air inside the wing was the closest result. That's a fabric-covered wing with very few holes to the outside, mostly drain holes and aileron cable fairleads. Some metal wings would be a bad place for static pickup. Too many lightening holes in aft spars or false spars, and leaks all along the roots.

I wish I could find some magic way to make the airplane lighter. It, too, is at least 100 pounds heavier than the plans say. Some of that is the result of using birch instead of mahogany, spring leaf gear legs instead of the oleos, and a heavy finish, but much of it is just the weight of the pixie dust the plans had on them.
 

pictsidhe

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At some point, the CAA will smell a rat, and start GPS speed measurements for certification. All that would need is a smart phone with an app going along for a test flight.
 

Aesquire

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Hang glider speeds are verified using trailing wire "bomb" instruments. The rotation and slowing of the flow field is magnified at such low speeds. Only the manufacturer ever bothers and only during testing and certification.

I found up to a 20% error range with a airspeed meter on a tube ahead of the pilot over a one foot position change.

You just make corrections. .. often just using the wrong readings and adjusting the desired speeds to the gauge. I'd just tweak the speed ring on the vario a touch based on experience. And made sure the airspeed instrument was in the same location. ( it pivoted out of the way for takeoff and landing )

Most pilot tubes get non linear at high angles.
 

Foundationer

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It seems a bit like these airplanes are not entirely playing by the rules and getting away with it.

Anyone got any suggestions on how to design single-slotted flaps? It looks to me like you either get quite hefty CFD or follow the broad brush guidelines in 'Theory of Wing Sections' and hope for the best.
 

TFF

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Remember too that the job of a production test pilot is to get the numbers. There was an article years ago about the Cessna 172 test pilot and through his training and practice, he could get 2-3 or more kts difference from some dude yanking on the yoke. If the numbers did not come fast, he had all day and mechanics to tweak the wings.
 

karoliina.t.salminen

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Certified aircraft, at least the newer ones, tend to have IAS and TAS very accurately agreeing with the ground speed (G1000 on Diamond, or Avidyne or Garmin on Cirrus). Published numbers in handbook also agree with actual ones quite accurately. The manual of Cirrus even has tables for speeds for different altitudes and they are accurate enough for planning. Stall speed also agrees, when measured with GPS. I think to pass certification nowadays, there is no room for pixie dust, but it needs to be exact numbers given or someone gets sued in no time. I have calculated that these planes achieve typically achieve around 2.0 Clmax, rarely better, based on published stall speed and wing loading. Diamond does that with plain flaps unlike Cirrus, but then the Diamond uses the Wortmann FX63-137 high lift profile, although modded from trailing edge for lesser lower surface curvature in horizontal flight.

I think experimentals should be designed like these in the sense that all data should be exact and not something into that direction and falsification should be big no-no.
 

Topaz

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... I think experimentals should be designed like these in the sense that all data should be exact and not something into that direction and falsification should be big no-no.
It already is, ethically, but short of doing a type certificate for every model airplane ever flown, how do you enforce it? Sure, I can be honest, but if the next guy fudges his numbers and gets a slight sales advantage, who's going to call him on it? His customers? In many cases, they're so loyal to the make and model they chose to build that they'll perpetuate the fantasy. That's what was happening in the old days. Am I going to call him on it? That's bad-mouthing a competitor, about the worst kind of PR you can generate.

CAFE was the industry (meaning the flyers, not the manufacturers') attempt to create an honest testing regime, but manufacturers ended up hating it and so...
 
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BJC

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It already is, ethically, but short of doing a type certificate for every model airplane ever flown, how do you enforce it? Sure, I can be honest, but if the next guy fudges his numbers and gets a slight sales advantage, who's going to call him on it? His customers? In many cases, they're so loyal to the make and model they chose to build that they'll perpetuate the fantasy. That's what was happening in the old days.
That has been a problem with homebuilt airplanes, fishing boats, motorcycles and how good I was at sports in school. Probably always will be.
CAFE was the industry (meaning the flyers, not the manufacturers') attempt to create an honest testing regime, but manufacturers ended up hating it and so...
Do you know the inside story of why CAFE ceased the APR’s? Would love to learn the true story.


BJC
 

Topaz

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I suspect that, whatever the real reason, they don't care to tell it. The organization seems to have "moved on."
 

Norman

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Having a look at the numbers for a few of these the stall speed numbers seem... unlikely. Taking published numbers for area they seem to have CLmax in the 2.1 to 2.4 range with plain flaps. Ones I was looking at were B612 (CLmax 2.43), SD1 (2.1) and MC30 Luciole (2.2).

With a decent airfoil and plain flaps I can come up with something in the region of wing CLmax 1.65.

So what's going on here?
Aside from any possible pixie dust there are also a couple of real variables involved in an airplane's CLmax besides section cl max and airspeed. These are propeller and fuselage effects. In slow flight the slipstream is quite a bit faster than the airplane's airspeed so any part of the wing behind the prop sees a much higher airspeed and therefor produces more lift at a given CL. The prop itself also produces some lift when the plane is pitched up; the amount of this prop-lift can be calculated quite easily simply by drawing a triangle of vectors with the hypotenuse equal to the thrust. The fuselage also produces some lift. Fuselage lift is very inefficient but it can be substantial at low speed especially if the top is flat in the front view and convex in the side view.
 

pictsidhe

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It already is, ethically, but short of doing a type certificate for every model airplane ever flown, how do you enforce it? Sure, I can be honest, but if the next guy fudges his numbers and gets a slight sales advantage, who's going to call him on it? His customers? In many cases, they're so loyal to the make and model they chose to build that they'll perpetuate the fantasy. That's what was happening in the old days. Am I going to call him on it? That's bad-mouthing a competitor, about the worst kind of PR you can generate.

CAFE was the industry (meaning the flyers, not the manufacturers') attempt to create an honest testing regime, but manufacturers ended up hating it and so...
If I built an aircraft with and advertised 40mph stall speed, and it actually stalled at 55mph, I wouldn't be very happy. That is not a small difference. I certainly wouldn't be recommending the design to anyone.
 
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