Tuesday, October 15, 2013

Comparing Two 2-Meter RC Gliders

To be a good scientist it is important to record accurate data but I still think there is some value in making some estimated observations. Recently I started flying a heavy 2-meter radio control sailplane with a fiberglass fuselage and sheeted foam wings. Weight of this glider is 53 ounces which is a little over 3 pounds.  Besides the heavy structure of the fiberglass glider there are 4 servos in the wing plus the 2 in the fuselage. The fiberglass glider is very sleek and no doubt has less drag than the Gentle Lady Sailplane.  Weight of my Gentle lady was 27 ounces which is just about half the weight of the fiberglass glider. The gentle Lady plans show wing area of 663 square inches, I will estimate the wing area or the heavy glider to be the same.

This would give wing loadings of:
Fiberglass Glider  11.5 ounces per square foot
Gentle Lady balsa  7 ounces per square foot

In a previous flying session with the fiberglass glider I had one very good flight in thermal lift of close to 10 minutes. During my comparison day of flying both gliders I had a much better flight with the lighter Gentle Lady. As I have read there are situations where one glider will perform better than another.

I have been studying some about the Lift over Drag Ratio (L/D) after the new record in the Red Bull Flutag event. L/D or glide angle does not change with weight, only the speed down the glide slope changes with increased weight. To me this has been a little hard to believe it is true but I think comparing these two gliders is proving it to me in a visual way.

Drag Reducing Features of Heavy Fiberglass Glider

For a comparison I gave a hard hand launch to both gliders from the same spot. The heavier fiberglass glider did not climb as high in the launch but glided down the field what appeared to be twice as far as the lighter Gentle Lady glider. Besides reduced drag the heavier glider had more kinetic energy pulling it along but to me it was also obvious the L/D of the fiberglass glider is better.

Drag Sources on Lighter Gentle Lady Glider

Besides the small items on the lighter glider that are sticking out into the airflow such as rubber bands on the wing and control horns, the airfoil of the fiberglass glider appears thinner. The more I experiment and compare model airplanes the more I realize that airfoils do matter. Sometimes by a huge amount.

Bill Kuhl

Additional Blog Posts on Soaring
Thermal Soaring This Week
ALES Sailplane Contest
Competition Altimeter for Models - CAM
e-flite Umx Ask 21 Sailplane
STEPS for Girls Summer Program 2006
Slope Soaring Trips
My Start in Radio Control Glider Flying

GoPro Strapped to Gentle Lady Creates Lots of Drag

1 comment:

  1. I found out that the fiberglass glider was a Shadow 2 meter glider designed by Roger Chastain. It has the SD7037 airfoil which Northeast Sailplanes catalog described as very good for thermal gliders. The wing area is 690 square inches, this will bring my loading down to 11 oz/sq ft but the glider should weigh only 45 grams giving a 9.3 oz/sq ft loading. My glider had been repaired and has heavy servos in the fuselage.