June 2006 Issue

Unicom: 06/06

Questions And Answers
In “Cross-Coordinated” (April 2006), David St. George talks about the stall break being more subtle in a turning stall due to the offset of the elevator force.

Why is the elevator force offset? I’ve done some research and can’t find out why this is so. I’d like to add this explanation in my stall instruction but want to make sure I fully understand the aerodynamics behind it.

Thanks for the great article.

Jeff Frye
Pensacola, Fla.

The author responds: “I can refer pilots to some of the literature, but a detailed description is hard to come by. Basically, in the turn the elevator force we are creating is offset increasingly from the vertical. The force we oppose, weight/gravity, still functions straight down rather than at an angle. As the ‘stall break’ occurs the result is a less-forceful pitch down.

As Rich Stowell pointed out to me, the resulting reaction is very dependant on aircraft type, age and rigging. Some ancient, twisted-up trainers can provide quite a ride.”


How To Heat The Carb
“When Engines Fail” (April 2006) incorrectly states that heated exhaust gases are ducted into the carburetor. The correct statement should be that air heated by passing through a heat exchange device of some kind is ducted through the carburetor.

In addition to the information in the article I feel it is very important that pilots understand, when carb ice does form and they apply carburetor heat, the first result is usually a very rough running engine with momentary considerable loss of power.

I have been with pilots who applied carburetor heat and when the result was a worse running engine they immediately put the carb heat back to the off position. Had I not been there to correct them this action would likely have resulted in total loss of power.

Aviaton Safety is a great publication. Keep up the good work.

Franklin Sherman
Via e-mail

You’re correct, Franklin. Somehow the distinction got lost in the editing and production process.

One consideration, as you point out, is that the engine may well run worse during the first few seconds after carb heat is applied. That’s because the a) the ice is melting and water is going through the carb into the engine and, b) heating the air in the carb enrichens the mixture.

The cure for the first problem is to wait until the engine smooths out, which signifies the ice is all gone. For the second problem, pilots need to re-lean the mixture to compensate for the enrichment.


Reversed Downforce?
I’m looking at “Making CG Work For You” (May 2006) by Thomas P. Turner. It appears the diagrams showing down force on the tail are mislabeled.

If the center of lift is ahead of the center of gravity there should be no down force on the tail whatsoever. A CG ahead of the center of lift would require down force on the tail. Apparently, the labels got reversed.

Nyal Williams
Via e-mail


Or Just Dangerous?
As someone who is in the throes of renewing a long-expired CFII certificate I found the article on CG quite interesting. Please, if you are going to fly aircraft with the CG behind the center of lift as shown on page 10, name me as a beneficiary in your life insurance. Obviously this is just an editorial booboo, but a very dangerous one.

Meanwhile, please tell all instructors to do CG calculations on the aircraft they use. For example, a Cherokee 140 is typically forward of CG limits if the instructor and student weigh more than 200 lbs. each and full fuel is aboard.

When I begin flying a new airframe, I always look at CG for the ways I expect to load it and then check the CG for the zero fuel condition to see how the CG moves. If the CG ends up out of limits, I will reconfigure the load until the CG will stay in limits to the zero fuel state. I don’t want to start a three-hour flight with five hours of fuel when the fuel burn will take me out of limits in three hours. Guess when the long hold or other delays will occur?

Erin L. Ireland
Via e-mail

In the graphics at the bottom of page 10 of our May issue, the vertical lines labeled “Center of lift” and “Center of gravity” were inadvertently reversed due to a production error. The center of gravity should be the forward line and center of lift the aft one. Thanks to all the sharp-eyed readers who pointed out this problem.