Gyroscopic Effect on a prop aircraft.

[Mohit_C]

Hello everyone,

I was trying to look up information on why a prop aircraft behaves the way it behaves with regards to the "gyroscopic effect" of its propeller especially when pitching and yawing. I've looked through some notes but I really can't get my head around it.
For instance it stated that when the aircraft pitches up, there's a tendency to yaw right, and when it pitches down there's a tendency to yaw left. Likewise, when the aircraft yaws left, there's a tendency to pitch up and vice versa.

Is there some graphical representation of this to help me understand this better?

Thanks.

[ft]

I'll try it without pictures first if you don't mind?

Look at the bit of the propeller which is on top of the propeller disc. It's going to the right, assuming RH spinning propeller.

Now, pitch down. You have to make the bit going to the right go to the right and forward to make the propeller dish pitch down. This means exerting some torque on the propeller disc.

Then, the same bit of the propeller comes through 90 degrees of rotation and is now to the right. Here, it has to go straight down again and not forward or aft.

A further 90 degrees of rotation and said propeller bit is pointing straight down, going to the left... and due to the pitching also moving aft.

In those 180 degrees of rotation, the bit of propeller has gone from moving forward to moving aft, on top of the rotational movement of the prop. This means it has to have been accelerated to the rear. Equal and opposite reaction dictates that it must have exerted a force on the rest of the aircraft to the front during the half-circle on the RH side of the disc.

The torque you used to accelerate the bit of propeller forward comes back 90 degrees of rotation later as a result of all the bits of the propeller having to go from going forward to going aft.

[Capot]

And there was me thinking it's precession wot does it. ie push a spinning gyro one way and it turns in another at 90 degrees.

Or something like that; never did fully grasp it.

[ft]

Capot,
we're taking the non-precession approach here... ;)

Joking aside, that is the 'why' behind precession.

[sapperkenno]

Scroll down to page "4-27" here: http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2004.pdf

Also, your comments on airplane behavior are only true of airplanes using (most) western built engines. If you flew a Yak for instance, you would expect different reactions, due to the prop turning the other way.

In really old machines circa WW1, (such as a Sopwith Camel) with rotary engines, the effect was a lot more pronounced due to the cylinders turning with the prop giving a bigger/heavier gyroscope in a lightly weighted airplane. I've heard rumors of the gyroscopic reaction on these airplanes actually overcoming the desired effect wanted from the control surfaces. So you might try and pitch up abruptly, but instead you get a small pitch up and massive amounts of yaw.

Hope the pictures help, as you seem to have an idea of the theory!

[HiFli]

Wiping away the cobwebs, having been a pupil and an instructor on Harvards (AT 6 Texan if you must) I recall the following. Bear in mind this is a tailwheel aircraft. There were four reasons why the aircraf yawed to the left when power was applied for take-off.

1. Torque effect ie the engine wants to turn in the opposite diection to the propeller.

2. Asymmetric thrust caused by the lift produced by the down-going blade.

3. Precession caused by the tail coming up, the force being applied at the top of the propeller and taking effect 90 degrees later (clocwise rotating propeller when viewed from the cockpit).

4. Slipstream effect caused by the slipstream of the propeller flowing around the fuselage and striking the vertical fin on the left (similar to a corkscrew). It is for this reason that the fin is offset by two degrees when viewed from the rear.

In S&L flight the nose also pitched up when power was applied.

The converse applied when the power was reduced ie nose pitch down and yaw to the right.

[Wizofoz]

Gyroscopic precession is also used to great effect by aerobatic pilots to fly lomcevaks and other tumbling type maneuvers. Big prop powered by big engine in light airframe makes for a GREAT demonstration of the phenomena.

[Mohit_C]

Thanks for those explanations and that diagram in the link. I was getting confused with some notes I had which mentioned "force "emerging" at 90º of rotation". Didn't make much sense to me, but it's clear now.:ok:

[Nubboy]

I believe that the reason we fly left hand circuits as a default, unless told otherwise, due to the old rotary engines of the first world war, and before, era.
As already said the propellor was bolted directly to the crankcase and there was a huge spinning mass on the front of the aeroplane. Due to the direction of rotation, a left turn caused the nose to pitch up (good thing) and right turn caused it to pitch down (bad thing).

Incidentally I believe this is why the captain sits it the left hand seat, as he would have had the airfield on his side for a convential circuit.