THE INFLUENCE OF WEIGHT ON MAXIMUM CRUISE SPEED By Tom Jewett (EAA 61986) 472 E. Lancaster Blvd. Lancaster, Calif. 93534

AIRCRAFT'S CRUISE speed has always been of prime importance to a prospective owner and/or builder. There are many factors that influence this parameter. Factors having a significant effect upon aircraft cruise speed include horsepower, size and shape, and the Public Relations Department. Conversely, cruise prop efficiency, sweptback vertical tails, new paint schemes, and weight have little or no effect on cruise performance. I would like to zero in on the last factor-weight, because it is one that is often misunderstood. Basically, for the following class of aircraft WEIGHT HAS A NEGLIGIBLE EFFECT ON CRUISE SPEED: 1. Wing loading less than 15 lb./ft.2. 2. Cruise speed over 150 mph. 3. Aspect Ratio of wing greater than 5. Why is this so? Because weight affects only the induced drag component of the total aircraft drag. Induced drag is a measure of how hard the wing is working to support the aircraft. At the high speed, Ig condition, the fast flowing air allows the wing to loaf. Therefore, it follows that increasing the weight some would still not make the wing work very hard. As a result, induced drag is low at high speeds and is not appreciably influenced by weight. Figure 1 illustrates a more technical approach to the same argument. A power required curve is shown for a ficticious aircraft at two different weights. The aircraft was sized and shaped to represent a VariEze or a KR-2 although the results would apply to a Thorp T-18, Smyth Sidewinder, BD-5, etc. As can be seen from the curves, for a 160 mph cruise speed, adding 125 lbs. of weight (equivalent to increasing the empty weight by about 30%!) reduces the cruise speed

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by only 2% mph which is less than 2%. Note also that above 200 mph the difference becomes nearly unreadable. If you would like to prove this point for yourself, try

the following elementary flight test task. Some sunny morning, go out to the airport early, take off at a light weight, and climb until you find smooth air, probably around 5000 ft. AGL. Set up the power for maximum cruise speed and hold altitude and power setting constant for at least 3 minutes after the airspeed has stabilized. Note the IAS. Back on the ground, determine your TAS for the point. Next, load your wife or girl friend into the right seat and onload fuel and baggage until the aircraft is at maximum gross weight. Then repeat the test at the same conditions: i.e. same altitude and power setting, smooth air, and airspeed stabilized for at least 3 minutes. Again note the IAS and convert it to a TAS once you're on the ground. If you have done the test accurately, you will not notice much difference in the two airspeeds, certainly not over 3 mph. Although weight may not affect cruise speed much, it does adversely affect most other performance parameters; stall speed and takeoff and landing distances increase; range, rate-of-climb, and service ceiling decrease; and generally there is a deterioration in structural integrity, handling qualities, and stability margins. So, there are many excellent reasons to keep any aircraft light, but cruise speed is not one of them. Now you can put the aspirin back in the bottle and discontinue that crash diet; your 550 lb. KR-2 will still go as fast as a 425 lb. version. ABOUT THE AUTHOR

Tom Jewett is a graduate of Ohio State University with a degree in Aeronautical and Astronautical Engineering. In addition, he holds a CFII certificate and averages about 5 hours of flight instruction a week. Presently he is a Flight Test Engineer for Rockwell, assigned to Edwards Air Force Base and working on the B-l bomber program. He was formerly the Project Engineer on the BD-5J air show team aircraft and was with the company through the construction of the first three jets.