Sunday, June 20, 2010

Belite Ultralight Aircraft Stalling Speed

I've run into some interesting discussion out on the social networks discussing or questioning the ability of the Belite ultralight aircraft design to stall at 24 knots.

A quick analysis of the FAR Part 103 rules, as it relates to ultralight aircraft, specifies five critical technical conditions for the acceptance of an aircraft as meeting part 103:

a) Weight, not to exceed 254 pounds, although there are several exemptions.  (Under certain conditions, weight may be as high as 338 pounds, and still meet Part 103.)

b) Stalling speed, not to exceed 24 knots Calibrated Airspeed. 

c) Cruising speed, not to exceed 55 knots calibrated airspeed.  This translates to a True Airspeed of as high as 74 mph (conditions:  10,000 feet; 0 degrees C, 55KCAS) or even higher.

d)  Fuel capacity not to exceed 5 gallons.  Part 103 incorrectly indicates that this is 30 pounds of gas -- which is simply not true.  It can represent up to 33 pounds of gas.  See this link for an explanation.

e)  Single seat operations.  Not much of a technical consideration; this is easy to verify.  If you wanted to get two people in our airplane, each would have to have a butt with a width of 8 inches.

For these FAR Part 103 rules, it is easy to verify a, c, d and e, and thus ensure that your aircraft is a legal ultralight.  (There have also been discussions of why Belite would use a 50HP engine, thus potentially allowing cruise > 55KCAS -- I'll get to that in another post in the near future.)

So let's focus on the stalling speed.  It's the hardest to measure (especially in a safe manner) and clearly generates the most controversy.

First of all, let's look at the facts, straight from AC103.7:

   "Powered Vehicles.  A powered ultralight cannot be operated under
   Part 103 when it has an empty weight of 254 pounds or more; has a
   fuel capacity exceeding 5 U.S. gallons; is capable of more than 55 knots
   airspeed at full power in level flight; and has a power-off stall speed 
   which exceeds 24 knots."

Keeping in mind that there are several exemptions for maximum weight, let's consider the following conditions, straight from the Belite aircraft:

1) 253.9 pound aircraft weight.  We achieve this easily, even when using a 50HP engine.  We go over this weight when we add safety devices (a parachute) or floats (but you don't know about that, yet.) but FAR Part 103 provides exemptions for parachutes and floats

2) Wing area - 98.9 square feet.  (We use more wing area on some of our Superlite models.)

3) Wing design - Riblett airfoil with a 'Junkers' style flap.

The original FAA document specified maximum lift wings with a coefficient of lift of 2.0 -- and as a result, our wing is too small based on their paperwork chart.  They did not anticipate the use of high lift wing designs, and our wing is a very high lift design.  How high?

Consider this document, from the internet, written by renowned designer Chris Heintz on the topic of airfoils.

He states:

   "When homebuilders install this kind of flaps it is very important that they
   stick to the designer's geometry because the flap nose position with respect
   to the wing rear end is very critical to obtain maximum profile lift coefficient 
   of up to 2.6 (see NACA Rep. No. 664 - 1939)

   Split flaps, although quite effective, have fallen out of fashion and will 
   not be discussed further [though I have re-introduced them with the ZENITH 
   CH 2000].

   A very interesting flap is the "Junker" type. It is a separate small airfoil
   under the wing trailing edge and hinged in such a way as to always
   create the "funnel effect" to reactivate the upper surface boundary-layer.

   The Junker flap is especially interesting when used as ailerons (the ailerons
   are flap sections on the outboard wing panels, one being deflected down,
   the other up, so that the pilot has "roll control" over the aircraft.) As 
   already mentioned the usual boundary layer is quite thick over the rear 
   part of the airfoil and the ailerons need a certain minimum deflection
   to be effective. This is usually small "ineffective" roll control deflection
   from its neutral position. With the Junker type aileron, this is not the 
   case if full advantage of the possible "funnel effect" is achieved by  
   careful design of the hinge point location and careful construction.

   The drawback of this flap is that at high speed the funnel is always
   consuming some energy so that the drag coefficient is slightly higher
   than for a conventional flap.

   But the 'Junker' flap is a very good compromise when excellent low
   speed in aileron controllability is desired, associated with high lift/low
   drag in climb configuration, and the top speed end is not so important.

What did he say?  He said:  The Junkers flap / wing design has a  "Coefficient of lift of up to 2.6...."

Now that's an amazing number, and it was not anticipated by the FAA when it wrote FAR Part 103 for ultralights.  They only Cl coefficients up to 2.0; probably because they were thinking rag & aluminum construction in their thought process.  They weren't thinking high technology wing designs and they certainly weren't contemplating Belite's revolutionary FAR Part 103 ultralight aircraft, which uses a Junkers flap / wing design.

Let's take a quick look at stall speed calculation.  Given the Stall Speed formula, which is:


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

Weight = Weight in Pounds of the loaded flying airplane
Rho = Density of Air 
Area = Wing Planform Area in SF
Cl = Coefficient of Lift

We will utilize sea level on a standard day for calculating our theoretical stall speeds.  Let's calculate each variable:

Weight = [253.9 Empty Weight + 170 lbs Pilot (per FAA Part 103 Spec) + 30 lbs Fuel (per FAA Part 103 Spec, but almost always an incorrect value)] = 453.9 pounds

RHO =  .00237 slugs / ft3 (Temperature = 59 degrees, at sea level.)

Area = 98.9 Square Feet

So our equation now looks like this:

SQRT (2*(453.9) / (.00237*98.9*2.6)) = 38.6 ft/sec

And 38.6 ft/sec equals a calculated stalling speed of 26.3 MPH which is equal to 22.9 Knots.  That's a pretty good stalling speed!


1)  Our Cl isn't really quite 2.6.  We don't know what it is, but we do know it is a really great airfoil for an ultralight, and clearly is way better than the original FAA experts predicted would be used on an ultralight.  As a result, I do expect the actual stalling speed under the specified conditions to be slightly higher.  How much higher?  See note 2 for some real world observations.

2)  In the early days of our testing, I saw a Belite stall at 27mph -- using a noncalibrated airspeed indicator at a high density altitude.  A lower altitude and lower temperatures would have produced an even better result.  So this is matching the predicted value very nicely.  :-)

3)  We don't have access to sea level / standard conditions to do this test accurately.  We also don't have access to a (reasonably priced) wind tunnel to calculate Coefficient of Lift at stall speed.  However, this may change later this year.

4)  We will submit this data to a Technical Review Committee, should one ever emerge.  In the meantime, I encourage anyone flying our aircraft to keep a copy of this post in their aircraft, along with a weight and balance chart.  However, they are not required to.

5)  All other details of our design are easier to verify.

1 comment:

Grant Smith said...

The Aero Sports Connection will offer a technical review per AC 103-7 requirements.