Hi.. Why does the Cessna 172 SP, align itself after engine start up.. ( the fuselage align it self with prop.?

Question

after engine start up the fueslage align it self with the proppller.. anyone know why ? what does that force called ?

Answer 1

I think what you are talking about is the fact that while holding the brakes at engine start the thrust of the prop will compress the nose strut which gives the sensation, rightly so.. that the entire fuesulage changes in its position of pitch

Answer 2

Me too.  Try your question again, and think about what you're trying to ask. For what it's worth, the propeller doesn't wander off on its own . . .

Or are you by any chance talking about the effect of propwash on the controls?

Answer 3

This is a really seldom noticed movement.  You are very observant to realize it is there.  It is caused by torque and initial slipstream airflow.

It is more noticeable at a high power setting at start.  If you start up at a high thrust setting, the fuselage will wag left (from the torque of the start) then right (from the initial prop thrust hitting the right side of the fuselage), then left again from the initial slipstream hits the left side of the rudder, then straight when the airflow acceleration has stabilized.  But by then most people have come off the throttle.

BTW most flight instructors won't notice this or know why it happens.  

In fact ask your instructor what happens, exactly, in cruise flight when power down.  They'll all say the nose goes down and you descend.  That is not what happens, check it out for yourself sometime:

Trim hands off in cruise and power down without your hands on the yoke.  Observe and join the Stump the Chump Club.

Answer 4

the propeller and the fuselage always have the same alignment.  You need to describe to us what is happening.  Are you flying a real aircraft or a computer simulator?  What exactly does the fuselage do when you say it's moving?

Answer 5

I believe what you are asking is called P-factor.  The effects of torque or P-factor at the initial speeds tend to pull the nose to the left.  The pilot must use whatever rudder pressure is needed to correct for these effects or for existing wind conditions to keep the nose of the airplane headed straight down the runway.  The use of brakes for steering purposes is to be avoided, since they will cause slower acceleration of the airplane's speed, lengthen the takeoff distance, and possibly result in severe swerving.

When takeoff power is applied, torque or P-factor which yaws the airplane to the left.  If you will look straight down the top of the fuselage to the tail you will notice the vertical tail is off set just a little to compensate for P-factor.  On some propeller aircraft this is more noticeable that others.

Also some makes and models the engine mount will tilt the engine propeller an a slight angle down and to the side a different way to manage P-factor.  So when takeoff power is applied, torque or P-factor, which yaws the airplane to the left, may be sufficient to counteract the weathervaning tendency caused by a crosswind from the right.  On the other hand, it may also aggravate the tendency to swerve left when the wind is from the left.  In any case, whatever rudder pressure is required to keep the airplane rolling straight down the runway should be applied.

"P-factor" is present in multiengine airplanes just as it is in single-engine airplanes. Remember, P-factor is caused by the dissimilar thrust of the rotating propeller blades when in certain flight conditions.  It is the result of the downward moving blade having a greater angle of attack than the upward moving blade when the relative wind striking the blades is not aligned with the thrust line (as in a nose-high attitude).

In most U.S. designed light-twins, both engines rotate to the right (clockwise) when viewed from the rear, and both engines develop an equal amount of thrust.  At low airspeed and high power conditions, the downward moving propeller blade of each engine develops more thrust than the upward moving blade.  This asymmetric propeller thrust or "P-factor," results in a center of thrust at the right side of each . The turning (or yawing) force of the right engine is greater than the left engine because the center of thrust is much farther away from the center line (CL) of the fuselage it has a longer level arm.  Thus, when the right engine is operative and the left engine is inoperative, the turning (or yawing) force is greater than in the opposite situation of a "good" left engine and a "bad" right engine.  In other words, directional control may be difficult when the left engine (the critical engine) is suddenly made inoperative.

It should be noted that some light-twin engine airplanes are equipped with engines turning in opposite directions; that is, the left engine and propeller turn clockwise and the right engine and propeller turn counterclock-wise.  With this arrangement, the thrust line of either engine is the same distance from the center line of the fuselage, so there wìll be no difference in yaw effect between loss of left or right engine.

This would be a good subject area to sit down with your flight instructor and have them or the Chief pilot of the school explain it to you in great detail.

Answer 6

I don't can unnerstand too.

Answer 7

The propeller is rigidly mounted to the engine crankshaft, which in turn is rigidly mounted to the airplane's frame.  So the alignment between the propeller and the fuselage does not change.  Sometimes the airplane will seem to shift position when the engine starts because the thrust of the engine--even at idle--causes the nose gear strut to compress somewhat.  But the alignment does not change.

What worries me is the question of why you are asking about this here on Yahoo!Answers, where you will get a mixture of informed responses and nonsense.  Why didn't you just ask your instructor at the moment you noticed this.  That makes much more sense.

Answer 8

It doesn't change alignment vis a vis the propeller shaft after engine startup.

Answer 9

What are you asking.  I been flying Skyhawks for nearly 20 years and I have NO idea what you are asking here,