View Full Version : why does the nose of a SEP go to the right when pwr is idle?

green dinosaur
12th Sep 2004, 18:39
I fully know the reasons why the nose goes up and to the left when pwr is added, however why does the nose go to the right when pwr is reduced. assuming amercian engine.

Vee One...Rotate
12th Sep 2004, 18:57
Erm...the same reason as it goes left!?

It's the opposite effect surely. Increase power > increase torque > nose wants to slide left. Decrease power > decrease torque > less tendency to turn left therefore, all other things being equal (inc. control deflections administered by the pilot), nose slides right i.e. to an equilibrium position. The aircraft is designed as inherently stable and will want to return to its most stable position readily in this situation.

That's my take on things. I stand to be corrected though.



P.S. I assume in some aircraft this sliding to the right does indeed happen!

12th Sep 2004, 20:13
For someone who claimed 'not to have flown a twin for ages' on 21 Aug 04, you seem to be asking some very basic questions on various forums, green dinosaur.

What type of licence do you hold, if any?

12th Sep 2004, 20:43
Doesn't when I fly!

green dinosaur
12th Sep 2004, 21:31
well Beagle some of the best questions to ask are the most basic ones. These are the ones that typically get glossed over by textbooks and instructors alike. I'm well qualified I would like to add!!

12th Sep 2004, 22:21
Specifically how 'well' are you qualified?

Or think you are?

Mr Levitator
14th Sep 2004, 23:36
Most of the Piper family of aircraft have the rudder fin set at about 3 degrees to the right of the aircraft center line. This might generate a right yaw/roll when the rudder trim is nuetral and S&L flight is estabilshed in flight.

I have never noticed this in flight-but I always have the slip ball trimmed central! Does this apply to the larger singles such as the Malibu Meridian, TBM700 and the PC12?

Mr L.

15th Sep 2004, 08:22
Here goes... The yaw to the left, as you know comes from the increase slipstream corkscrewing over the fuselage and striking the left side of the vertical stabilizer. This effect is more pronounced at low speed/high power setting.

If you get the chance to look straight down on a light SEP, you'll get the answer. The vertical stabilizer is actually positioned at a slight angle off the centreline. Thus in cruise conditions, the yaw affect of the prop slipstream (trying to yaw you left)will be naturally countered by the built in 'twist' in the vertical stabilizer(trying to yaw you right). Should you suddenly reduce power, you have no slipstream but the vertical stabilizer (with rudder neutral) will yaw you to the right.

This right yaw is not as pronounced as the yaw to the left, and in the good ol'days, I'd apply slight right rudder to make my point. As a prudent pilot, you shouldn't be making these rapid power changes anyway, hence some pilots not noticing it, except during their PPL training days.
I never noticed it on the C208's, but then I wasn't looking for it and perhaps was just cancelling it without giving it too much thought. Obviuosly couldn't have been an issue.

15th Sep 2004, 08:31
Ouch! The torque came out... ;)

(Assuming the initial condition to be trimmed in pitch, roll and yaw and flying straight)

Engine torque acts along the prop shaft. This means it will attempt to roll you around the prop shaft on a power change – and that’s it. If you try to counter this roll tendency with aileron, this will create adverse yaw, the direction of which is left as an exercise for the reader.

Gyroscopic precession will not create yaw until there’s a pitch change.

P-factor will be there, unless the flight path vector is parallel to the prop shaft. The direction of the yaw will depend upon the angle of attack.

Now, the likely culprit: Helical prop wash. The slipstream from the propeller coils its way down the fuselage, striking the fin from one side, creating a side force on the fin. You trim to fly straight, then reduce power... and the strength of the slipstream is reduced. This side force reduces with it and you get... yaw.


15th Sep 2004, 23:40

Are we not forgetting about the assymetric thrust effect of a prop too?! I was always taught this was the main reason for yaw with changes in thrust. It is also responsible for the "ciritcal engine" of a twin that doesn't have counter-rotating prop's!



16th Sep 2004, 09:05
also known as "P-factor"... ;)


Genghis the Engineer
17th Sep 2004, 07:27
By the way, the Jabiru engine at-least swings left. That would cover the Jabiru aircraft, these rather sexy new Spitfire replicas, and homebuilt using one of the Jabiru engines (the 2.2L and 3.3L being most common).


17th Sep 2004, 08:16
left on power reduction?


They're CW-rotating after all.

Now you have me puzzled! :)