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!]

a Hayfield Landing Highlights Belite's Utility

Long time readers of this blog may be aware that I have had training in mountain flying.  I've been to Idaho many times, and I've enjoyed the skills I've acquired through mountain flying school and through practice into short, odd airfields.  A recent post on this subject concerned several camping trips into the Thomas Creek airstrip.  Another post covered, among other thing, a series wasp bites and and description of the gnarly final approach into the Shearer airstrip, deep in the Selway-Bitterroot wilderness in Idaho.

I enjoy flying into tight, short airfields that curve, hug terrain, pass by trees, and slope uphill.  Ideally, they end in the side of the hill (and offer good camping and fishing nearby).

There is a hayfield next to our primary runway that offers some of these characteristics.  It slopes uphill.  It has a nice collection of trees.  It has bumps.  It has quite a bit of grass and weeds.  It is relatively short.  It is 'unimproved'. 

It is an ideal location for showing the landing capabilities and utility of our airplane.  The rough ground is a great demonstrator for our spring landing gear.

I wanted to demonstrate a nice 3 point landing, uphill, through the hayfield.  I asked my able assistant Gene Stratton to standby and take pictures.  I promised him a series of approaches into landing.  As I 'dialed in' the strip, I was able to establish a final approach of just a few feet over the weeds at the beginning of the field.  (Good mountain flying technique suggests the ability to hit a 50 foot spot... every time.  You can do that with a Belite.)

The photos show the story.

Hayfield Short Final, over the weeds.  I love this picture.

Over the 'numbers', field slopes uphill from here.  Another great pic.

Wheels kiss down on the Hay

Another approach, a little higher

Rollout in the hay, uphill.

Engine shutdown after taxi back

I'm posing by the Superlite

Belite Superlite poses in the grass, by the trees.  Where's the trout stream?

 This flight occured on September 28, about 1:30 in the afternoon.  I tracked down final, adjusting for the crosswind.  (Gene and I talked about my final approach crab angle after we debriefed on the ground, post-flight.)  Winds were out of the north, 12 knots gusting to 19 knots.  Engine power was set at about 35HP, even though the big Hirth develops 50HP when it's fully unleashed.  Landing direction was ENE; about a 50 degree crosswind.  FWIW, Wichita (KICT) Metar weather was:

       KICT 281753Z 01012G19KT 10SM SCT044 
            SCT150 SCT250 24/13 A2998

If you are a potential purchaser, you might compare our Belite with other less capable ultralight aircraft that have stiff gear and no crosswind capabilities and no off-field capabilities and no ChromAloy steel in their structure -- in other words, ultralight aircraft that lack safety, strength, and fun.

A Belite exudes utility and usefulness, and is a blast to fly.

      --  Photo credits: Gene Stratton.

Belite Fuel Gauge Installation

Our Belite Electronics are keeping me running, just catching up with orders.  (And no, we're not caught up with orders.  Not yet, anyway....)

One of the more popular products is the Belite Fuel Gauge.

Types of Belite Fuel Gauges

It's available in a version compatible with standard 2.25 inch instrument cutouts ($74.95):

Fuel Gauge from Belite with 2.25 inch round bezel

 It's available in an ultralight version with dimensions of 1.75 x 1.75 inches (and a weight of about 15 grams) ($69.95):

Fuel Gauge from Belite with 1.75 inch square bezel

The electronic design of each of these products is identical; the only difference is the bezel size and dimension.  They are designed to attach to a "capacitive fuel gauge", which is available from us and from several other vendors in varying lengths.  (No secret here:  we use one manufactured by Falcon Gauge.)  Here's what a "capacitive fuel gauge" looks like ($99.95):

Capacitive Fuel Probe, 12 inch length.

The probe has no moving parts whatsoever.  It senses the fuel level by the electronic effect of increasing or decreasing capacitance.  But, as they say in the movies, that's not important now.  The only thing you need to understand is that the probe outputs a voltage level that exactly correlates with fuel level in the fuel tank.  The voltage varies between 0 volts (empty tank) and 5 volts (full tank.)

Installing the Fuel Gauge and Fuel Probe in your airplane

Hooking these things together is not difficult!  Basically, this is what you need to do:

1)  Install the Fuel Gauge in the panel.  2 screws and a hole in your panel gets this done.

2)  Install the Fuel Probe in your tank.  The one we sell is 12 inches long, it may be put at an angle inside the tank.  Different probe lengths are available from Aircraft Spruce.  Some are bendable.  (You bend them only where they enter the tank, the remainder of the probe goes through the tank at a diagonal.)  Here's a photo of a fuel probe, installed in a tank with a depth of 8".  The capacitive probe was bent and fitted through a hole in the top of the tank, then attached to a small block of wood and bonded to the tank with a fuel proof expoxy.

Capacitive Fuel Probe installed in tank

3)  Attach our Fuel Gauge to a source of power.  A 9 volt battery works great; as does regular +12 volt system power.  (+12 volts is probably a better choice, because the separate fuel probe requires +12 volts).  This power goes to the power connector on the back of our unit.  Attach the ground as well.

4)  Attach the Capactive Fuel Probe to a source of power.    +12 volts works just fine.  This goes to the red wire, attach the black wire to ground.

5)  Attach the Fuel probe sender level wire (yellow color) to our Fuel gauge connector (usually this wire is marked with "FG" or something like that; it's the middle position on the fuel gauge connector).  (Yes, you'll have to run some wire from your fuel tank to your fuel gauge.)

6)  These two units also need a common ground.  This just means that the ground from the Fuel Gauge must be common to the ground at the Fuel Probe.  This is usually done through the metal frame of the airplane, but can also be done by a good ground wire as well.

At this point, if you turn everything on, you'll show the amount of fuel in your tank.  Well, not exactly; because it hasn't been calibrated yet.

Calibrating the Belite Fuel Gauge.

USE CARE AND CAUTION WHEN HANDLING GAS!!!!!

1)  Make sure the end of the fuel probe is about 1/4 or 1/2 inch above the bottom of your fuel tank. 

2)  When the fuel tank is empty, adjust the 'empty' trimmer on the fuel probe so that the only LED illuminated on the fuel gauge is the 0% (EMPTY) indication.  Be careful, it may be a very slight adjustment.

3)  When the fuel tank is full, adjust 'full' trimmer on the fuel probe so that the 100% LED (FULL) is illuminated.  Be careful, try and adjust so that the 100% LED is barely enabled.

4)  Empty the gas tank and verify the LED indications as the tank empties.  Readjust trimmers as necessary.

5)  Use common sense when flying with your new Belite gas gauge.  Assume that your gas gauge is inoperative if it does not agree with your normal pilotage fuel consumption calculations.

Fuel Gauge FAQ's

1)  Will the Belite Fuel Gauge work with anyone's fuel probe?  A:  It will work with any fuel probe that outputs 0 to 5 volts.

2)  What happens when the gas is below the bottom of the fuel probe?  A:  If properly calibrated, the Fuel Gauge will show 0%; your engine will continue to run as long as you have usable fuel in the tank.

3)  How are the markings on the Fuel Gauge designed to work?  A:  When the Fuel Gauge indicates 50% remaining, that means that the tank has at least 50% of the fuel capacity remaining.  When the the 50% LED  goes out, and the 40% LED turns on, this means you have between 40 and 49% of your fuel remaining.

4)  Are the readings accurate?  A:  If properly installed and calibrated in a square tank, they are reasonably accurate.  However, you must test your readings using a real fuel consumption test, with dip level verification of the tank readings.

5)  Are the readings accurate in a round tank (such as Belite's spun aluminum fuel tank, placed on its side)?  A:  They are reasonably accurate at the full, 60%, 50%, and 40%, and at empty.  Due to the circular tank, the fuel reading is not linear.  However, it's still completely useful information, because it can be used to compare fuel burn and gain judgement to fuel exhaustion from flight to flight.

6)  How much power does the Belite Fuel Gauge use?  A:  About 20ma.  It will run off of 8 AA (== +12 volts) batteries for approximately 100 hours!!!!  (Not counting battery current drain of Fuel Probe.)

7)  What is the labeling on the face of the Belite Fuel Gauge made from?  A: It's made from a waterproof membrane.  It doesn't shrink or wrinkle in humidity.  (This is a recent change to our production.)

8)  My Fuel Gauge has four wires going to the connector?!  What's the fourth wire for?  A:  Two wires are power (ground and +12v); one wire goes to the fuel probe, and the fourth wire is a dimmer input to the LEDs.  Leave it UNCONNECTED and you will have full brightness.  Connect to +12V, and the LED display will dim.

Have any more Fuel Gauge questions?  Give us a call or send us an email, info@beliteaircraft.com

-- James

Kitplanes Magazine Comes and Visits...

We were pleased to have Ed Wischmeyer with Kitplanes magazine pay us a visit today!

I had a great time showing Ed our airplane and our shop.  We'll see what he has to say in a few months.

Pictured below is Ed, after he completed a test flight in the Belite Superlite.

Ed Wischmeyer of Kitplanes magazine concludes a test flight and makes some notes.