Even MORE INFO on Stall Speed, Vortex Generators and Ultralight Aircraft

If you haven't been on this blog in the last few days, there are several new posts.  Don't miss the post on landing in a hayfield!...

The following is a discussion which will intertwine some stall speed testing and also some slight discussion on vortex generators.

A couple of months ago, I mentioned that I had put Vortex Generators on a Belite.  I was not able to provide much substantial information to you, my faithful reader, as to how they performed.  I'm still don't have much to offer, but I have begun to experiment with their placement on the wing.


I also recently discussed the FAR Part 103 mandated stall speed requirement of ultralight aircraft.  My blog post on that topic may be found here.


So here's a little more info.

We've had a few days of smooth air, and smooth air is absolutely intentional when performing stalls at absolute minimum airspeed.  Any disruption of the smooth air causes the plane to stall, screwing up my testing.  This is critically important when we are talking about stall speeds of less than 30mph.


Acting on a hunch related to my impression of the high angle of attack on our Riblett designed airfoil, I moved the Vortex Generators closer to the leading edge of the airfoil, which you can see here:

Vortex Generators on an Ultralight Aircraft, EG:  Belite Superlite Dragon

I then did some test flights. But a new problem was emerging:  my airspeed indication was failing completely at the extremely high angles of attack which I was testing.


In looking at the pitot tube, I suspected that the air intake on the pitot tube was 'micro-stalling', due to the non-parallel flow of air hitting the pitot tube obliquely at the high angles of attack.  In other words, the pitot tube needed to match the flow of air, not be parallel to the bottom of the wing.  I mentioned this to Gene, and he simply grabbed the pitot tube and started to bend it down.


"If you do this slowly, you can do it without breaking the tube," he said.  Hmm.  He was right.  


After he bent it a little, I bent it a little more.  The pitot tube was now bent distinctly downwards.

pitot tube bent downward on ultralight aircraft, EG Belite Superlite Dragon

I then performed another test flight in the plane.  After placing the airplane into a minimum controllable airspeed, I captured the following pic:

A picture showing a high angle of attack on a Belite

The above picture nicely illustrates the very high angle of attack.  Due to the fact that the engine is throttled back, it is safe to assume that the actual angle of attack is higher than what is illustrated:  the airplane is descending!  

As I maintained this very low airspeed, I focused my attention on the instrument panel, and captured another picture of the flight condition:

Cockpit panel view during Vortex Generator Stall Test

The instruments are a little hard to read in the photo above, so I've enlarged that part of the pic:

Closeup of instruments

The altimeter is reading 700 feet; the air speed indicator is reading 30mph, the RPM is showing 4700, and I was able to sustain this low flight mode without stalling the airplane.


The 30mph LED is actually a speed range indication:  the actual indicated speed of the aircraft is somewhere between 28 and 32.5 mph.  This is subject to several different errors, such as pitot installation error (EG, the pitot is not located far enough forward of the leading edge; or it is not parallel to the flow of air....), instrument design / calibration error (I try and make a good instrument, but it's not perfect....), etc.  


And if I pulled the nose back just a smidge more, the airplane stalled.  I was able to get the 28mph LED to flicker on a time or two as the plane progressed through the stall.


The aircraft weight, as tested, was around 522 pounds including airframe, parachute, pilot, and fuel.  This is substantially higher than the FAA mandated testing weight --  I would need to lose considerable weight to get down to the mythical FAA weight of 170 pounds. 


TECHNICAL MATH STUFF STARTS HERE...  

Let's do the math for predicting our stalling speed, and then compare it with what the LED Air Speed Indicator was telling me:


Using the textbook calculations for stall speed, which I first mentioned several months ago:


Vs=SQRT(2*Weight/(Rho*Area*Cl))

Where the following variables apply (using English units of measurement):

Weight = Weight in Pounds of the loaded flying airplane = 522 pounds
Rho = Density of Air  = .00237 slugs / ft3 (Temperature = 59 degrees, at sea level.)
Area = Wing Planform Area in SF = 101 Square Feet for this test
Cl = Coefficient of Lift = estimated at 2.4

So our equation now looks like this:

SQRT (2*(522) / (.00237*101*2.4)) = 42.63 ft/sec = 29.1 mph predicted stalling speed (at a weight of 522 pounds)


and this agrees with my observation -- the stall speed was above 28mph.


Now let's change the weight to a mythical amount of 478 pounds.  (278 pound airplane with chute and goodies, 170 pound pilot, 30 pounds of fuel).


and let's rerun the math with this new hypothetical weight:


SQRT (2*(478) / (.00237*101*2.4)) = 40.8 ft/sec = 27.8 mph predicted stalling speed (at a weight of 522 pounds) (The FAR Part 103 requirement is 28.0 mph.)


END OF TECHNICAL MATH STUFF -- YOU CAN BREATH A SIGH OF RELIEF


Earl Downs told me anecdotally a few weeks ago that the original Kitfox Lite was designed to barely meet the stall speed requirement of FAR Part 103.  Since our wing is the same aerodynamics, we've matched that characteristic.  And we keep finding that the math (and the actual results) point to a stall speed which is right at or under 28 mph.

This all makes sense -- the highest possible stall speed would also correlate to higher cruise speeds with the least amount of horsepower.  In other words, you can touch both ends of the FAR Part 103 ultralight aircraft flight envelope with a Belite - from 28mph stall to 62mph cruise.

Now some thought kickers for you to consider:

1)  These results were tested at fairly high density altitudes.  What would lower altitudes and temperatures do to the results?

2)  Does FAR Part 103 care that the stall speed is higher as the cabin load is increased?  

3)  What effect, if any did the vortex generators due to the stall speed?

4)  Is it fun to fly around with the airspeed indicator showing just 30mph?  [Answer is YES!]

Vortex Generators and Ultralight Aircraft

Vortex Generators.

IF YOU ARE READING THIS ARTICLE, YOU MAY ALSO BE INTERESTED IN THESE OTHER ARTICLES ON VG's and STALL SPEEDS:


http://jameswiebe.blogspot.com/2010/09/even-more-info-on-stall-speed-vortex.html

http://jameswiebe.blogspot.com/2010/06/belite-coefficient-of-lift-and-stalling.html

Vortex generators are mounted on the top side of wings, and are designed to create small vortices as air passes over the wing.

As a result of these small vortices, vortex generators (Let's call them VGs, OK?)  are responsible for doing some pretty amazing things.  Concerning VGs, Wikipedia says the following:


"Vortex generators are likely to be found on the external surfaces of vehicles where flow separation is a potential problem because vortex generators delay flow separation. ^[3] On aircraft they are installed on the front third of a wing in order to maintain steady airflow over the control surfaces at the rear of the wing.^[2]boundary layer, and run in spanwise lines near the thickest part of the wing.^[1] They can be seen on the wings and vertical tails of many airliners. Vortex generators are positioned in such a way that they have an angle of attack with respect to the local airflow.^[1] They are typically rectangular or triangular, about 80% as tall as the boundary layer, and run in spanwise lines near the thickest part of the wing.^[1] They can be seen on the wings and vertical tails of many airliners. Vortex generators are positioned in such a way that they have an angle of attack with respect to the local airflow.^[1]

A vortex generator creates a tip vortex which draws energetic, rapidly-moving air from outside the slow-moving boundary layer into contact with the aircraft skin. The boundary layer normally thickens as it moves along the aircraft surface, reducing the effectiveness of trailing-edge control surfaces; vortex generators can be used to remedy this problem, among others, by "re-energizing the boundary layer".^[1][2]

The use of VGs on FAR Part 103 ultralight aircraft has been debated at least a little; I've decided to get into the debate by actually giving them a try on our Dragon Superlite.  A couple of days ago, I temporarily attached a set of VGs to the Superlite and then flew the plane.  Here's what they look like:

How did they work out?  I don't really know.  The wind was gusty aloft, so it was difficult to check out stall speed, landing characteristics, and I've reached no conclusions so far.  But I've decided to leave them on the Superlite as it heads to Oshkosh.  Come on by our North display area and take a look.