Prop Theory


Good article on props (“Propeller Theory 101,” November). Many years ago, I had an old airplane that when you applied power rapidly on takeoff, you would get a surge in rpm that I think was caused by the prop first being stalled, and then “unstalled.”

The airplane was a North American P-51 with a 1490-hp Merlin engine driving a four-blade Hamilton-Standard prop, which was geared down to where it was only turning 1400 rpm or so. (I could afford it back then; I paid $12,000 for it and it came with a new spare engine in a crate!)

On takeoff, smooth application of power would stabilize at 3000 rpm. About halfway through the takeoff roll, engine rpm would drop quickly to about 2700, and then surge back up to 3000. (This occurred on several Mustangs I flew, not just mine.) 

I talked to many people about this at the time, and the best “guess” was that the prop was partially stalled at the first part of the takeoff roll, and when it unstalled itself, the airfoil’s lift/drag increase was what pulled the rpm down until the governor got it back to 3000. At the time of this surge, you could really feel the airplane’s rate of acceleration jump up.

It would be nice to hear from someone who knew what they were talking about on this. Good publication, keep it up!

Jeff Cannon – Via email

We’ll pass the word. Maybe someone else has the details on how that engine/propeller combination works.


Jim Wolper’s article covered P-factor and its characteristics fairly well, however slipstream and gyroscopic effects came up short. The spiraling slipstream is real. The propeller constantly provides a rotation to the air, which continues to rotate as it is pushed rearward by more rotating air, hence the corkscrew. One may observe this helix at times during another plane’s runup or takeoff in dusty or snowy conditions. Consequently, anytime the engine is producing power, spiraling slipstream exists and it is the dominant airflow over the empennage in the initial takeoff phase until the airplane achieves enough airspeed to minimize its effect.

Gyroscopic effect happens when a force is applied perpendicular to one edge of a rotating disc (gyro) and results in that force reacting perpendicular at 90 degrees in the direction of rotation. Consider the powered propeller as the rotating disc. Raising the tail of a taildragger is equivalent to applying a force (push) to the top of that rotating disc which reacts 90 degrees clockwise as a force (push) on the right side of that disc, swinging the nose to the left. More right rudder!

Wolper’s example in his gyro discussion is really an example of torque, which also is propeller-related and deserves some understanding as well.

Frank L Miller – Ninilchik, Alaska


Much of Canada has limited VOR coverage, and the most reliable navaid for long-distance VFR navigation was and still is an ADF tuned to the destination’s commercial broadcast radio station. This is especially helpful for winter flights over the featureless prairies when frozen, snow-covered lakes are indistinguishable from the snow-covered fields under overcast skies. The strong, 50,000-watt stations could be clearly tuned in from hundreds of miles away…I could pick up a Saskatoon station while on the ground in Calgary, 325 miles away, with the ADF needle precisely fixed even before takeoff.

Canada’s VFR supplement lists the frequency, power and coordinates of all broadcast stations across the country. I wouldn’t cross the middle or northern tier of our country without the ADF. Not everyone can either afford and/or have room to install a GPS. I suspect GPS navigation is the norm in the U.S., but certainly less so in Canada. That which is old is not necessarily obsolete.

Morton Doran – Fairmont, British Columbia


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