Winterhawk (oz14013)

About this Plan

Winterhawk. Radio conrtrol sailplane model. Standard Class. Wingspan 99-1/2 in, wing area 882 sq in.

Quote: "Special design considerations produced a tough, simple machine with an outstanding thermal riding ability. Winterhawk, by Bill Evans.

Winterhawk combines the strength of a tank with the flight characteristics of a bird of prey, a combination that delivers superior thermal time and airframe longevity. If it has been your goal to say to your fellow fliers gee it won't come down, the Winterhawk is the answer. The chief complaint from other fliers is you've been up for more than an hour, how about landing so I can use the frequency.

Project Winterhawk was undertaken to deliver a thermal craft with several desirable qualities; they are:

1) Maximum flight time regardless of lift conditions - accomplished by a maximum-lift wing and weight; weight for speed to produce optimum lift.

2) Airframe longevity; since the wing takes the most punishment, a high-impact, almost indestructible, fully-sheeted foam wing provided the answer. With the 3/16 sheet tail surfaces and a glass fuse, the Winterhawk has literally flown through trees, cutting a path, clearing and flying out the other side (testimony Dave Lloyd, Dave's Custom Models, Glendale, CA).

3) Span not to exceed 100 inches - actually 98 inches.

4) Hands-off performance, yet responsive, tested by setting up in a climbing turn, then switching off the transmitter.

Though the majority of Winterhawks built so far have weighed in at just about four and a half pounds, I prefer to fly it just between five and a half and six pounds. The 885 sq in area gives 6.15 sq ft of wing, which works out from 12 to 15 oz/sq ft.

If your reaction is that the wing loading is too heavy for a glider, your skepticism would increase if you could pick up the ship and feel the weight. But all bets are off after it shoots up the line and the first 10 minutes are ticked off on the watch. After the first 60 minutes disbelief changes to amazement. To my knowledge, most flights of Winterhawks have been terminated at the pilot's discretion. Many Winterhawk pilots have developed stiff necks, getting a feeling that they have completely mastered silent flight, after turning in flight after flight of more than one hour.

Design Considerations: Though the nose and tail moments play an important part, they are not super critical. A few minor weight and CG calculations yielded satisfactory ratios. Having flown free flight when it was all there was to fly (more years ago than I care to count), I have a few favorite percentages when it comes to airfoil thickness. So, after considerable plotting and fussing, an airfoil emerged that looked very good. For the wing loading, I took a 180-degree turn from convention, as in free flight, that the ship should be light. I loaded it down, stopping at just over 5-1/2 pounds.

The check-out flight turned in a time of 55 minutes. I landed the Winterhawk because of darkness. Subsequent flights were one hour plus. in each case terminated by my command.

The first Winterhawk was built with 1/16 balsa-sheeted inboard panels and 1/32 outboard panels. All three wing joints were epoxied without the use of glass and without a spar. After several near full-up elevator launches, I managed to fold the right wing panel about four inches out from the root. The right wing panel parted company from the ship at just over 600 feet. With a bit of help from the large elevator and rudder I managed to somewhat soften ground contact just before impact.

Repair to the wing, using epoxy, took about 30 minutes, except for a few splits in the wood fuselage. Subsequent balsa sheeted wings have included a main spar, as shown on the plans. However, I do recommend 1/64 plywood as the ultimate answer. The weight difference between balsa and ply is small - 34 ounces for balsa, and 36 ounces for ply..."

Winterhawk, Model Aviation, May 1978.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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