The Power to Fly

by Dr. Richard Stimson

in Inventing The Airplane

Flight is impossible unless there is enough thrust to maintain the flying speed of an airplane. A key factor in determining whether the 1903 Wright Flyer could sustain flight is to know the thrust required to overcome aerodynamic resistance known as drag. Once drag is known, the horsepower required of the engine can be determined.

What follows is an analysis similar to what the Wrights did to answer the question of how much power was required.

Drag is generated by two different surfaces on an airplane as it moves through the air. One is caused by the lifting effect on the wings and the other by the wind resistance caused by the frontal surface area of the airplane. The first is referred to as induced drag and the latter as frictional drag.


The formula the Wrights used to determine drag is very similar to the formula they used to determine lift. The only difference is that the coefficient of drag (CD) replaces the coefficient of lift (CL) in the formula. The basic formula is as follows:

D = k x S x V² x CD where
D = Drag (pounds)
k = pressure coefficient of air
S = wing area (square feet)
V = relative velocity of air over the wing (mph)
CD = coefficient of drag

For the 1903 Wright Flyer:

k = 0.0033 (Wrights derived from their wind tunnel experiments)
S = 512 (wing area of 1903 Flyer)
V = 30.8 (The wind ranged from 20 mph to gusts of 27 mph at Kitty Hawk on December 17, 1903. I used an average wind of 24 mph on Dec. 17, 1903 plus ground speed of 6.8 mph. Wilbur, running at the right wing tip, had no trouble keeping up with the Flyer as it moved down the starting rail to takeoff.)

The value of the coefficient of drag (CD) in the equation is a little more complicated to determine because the Wrights did not directly measure CD in their wind tunnel tests conducted November 22 through December 7, 1901. Instead, they measured the drag/lift ratio (CD/CL) from which the value of CD can be derived.

The Wrights measured the coefficient of lift (CL) as 0.515 and the drag/lift ratio (CD/CL) as 0.105 in their wind tunnel tests using airfoil #12 and an angle of attack of 5 degrees.

The geometry of airfoil #12 closely resembles the geometry of the wings on the 1903 Flyer. The angle of attack of 5 degrees approximates the angle of attack of the Flyer.

The coefficient of drag is calculated in the following manner:
CD = CL x CD/CL = 0.515 x 0.105 = 0.054

Substituting the appropriate values in the equation for drag:
D = (0.0033) x (512) x (30.8)² x (0.054) = 86.6 pounds

Total Drag

The drag of 86.6 pounds is for the drag attributed to the wings. To determine the total drag of the Flyer, the drag attributed to the wings (D) must be added to the drag generated by the frontal surface area of the airplane (Df).

The Wrights purposely assumed the horizontal position on the wing while piloting their machine to reduce drag. They estimated that the remaining frontal surface area of the Flyer was 20 square feet. Substituting this value in the drag equation:

Df = (0.0033) x (20) x (30.8)² x (0.054) = 3.4 pounds

The total drag (Dt) is therefore:

Dt = D + Df = 86.6 + 3.4 = 90 pounds

On November 23, 1903 from Kitty Hawk, Orville wrote Charles Taylor, their employee who built the engine following the design of the Wrights:

“After a few minutes to get adjustments, and to burn out the surplus oil, the engine speeded the propellers up to 351 rev. per min. with a thrust of 132 pounds. Stock went up like a sky rocket, and is now at the highest figure in its history. We have made some allowance at nearly every point in our calculations, so that with the increase of weight we expect to be a little over 90 pounds, but of course that is coming down to our closest figures.”


Power is force times speed. The power required to overcome drag can now be found by multiplying total drag by velocity:

P = Dt x V = 90 x 30.8 = 2772 pound-miles/hour
Converting this number to horsepower, the power is 7.3

The engine for the 1903 Wright Flyer produced about 12 horsepower. It would reach 16 hp when started, but drop off to 12 hp after a few seconds. While the horsepower of the engine (12) appears to be sufficient to overcome the drag (7.3), there will be additional loss of horsepower attributed to the chain drives that transmit the power from the engine to the propellers. Also, there will be loss of power attributed to the propellers. The propellers had an efficiency of 66%.

The Wrights knew it was going to be a close call. On November 15, Orville wrote home to his father and sister:

“Mr. Chanute says that no one before has ever tried to build a machine on such close margins as we have done to our calculations.” (Octave Chanute was a friend and an aviation historian and experimenter.)

The question as to whether they had sufficient power was answered on that fateful day in December. They made four flights on the 17th, the longest flight going 852 feet.

The following year back in Dayton, they were not so fortunate even though they had more horsepower. The 1904 Flyer had trouble getting off the ground. Dayton didn’t have the wind of Kitty Hawk and the air pressure was less because of the higher elevation.

The 1904 Flyer was little changed from the Kitty Hawk Flyer although they did improve the engine so that it produced 15-16 hp. Wilbur wrote to Chanute on August 8, 1904:

“We have found great difficulty in getting sufficient initial velocity to get real starts. While the new machine lifts at a speed of about 23 miles, it is only after the speed reaches 27 or 28 miles that the resistance falls below the thrust.”

They solved the problem by employing a catapult launch system to give the Flyer a boost on takeoff.

The initial Wright engines were crude, but they did the job. They didn’t need a lot of horsepower because the Wrights had designed an efficient aerodynamic flying machine.

In contrast, Dr. Samuel Langley, Director of the Smithsonian Institution, employed a sophisticated engine that generated a whopping 50 hp, but his Aerodrome was poorly designed. It crashed on takeoff nine days before the Wright’s successful first flight.

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