Comments on: Gyroscopic Precession

By: JPH

JPH — Tue, 19 Jun 2012 04:37:57 +0000

Comparison of the fundamental equations describing the behaviour of a
gyroscope and that of a helicopter rotor (specifically the blades)
clearly indicate they are not same physical phenomenon.

The control advance angle of the Robinson R-22 helicopter is 78 degrees,
that of the Air & Space 18A gyroplane 52 degrees, and the Lockheed L-286 helicopter about 45 degrees. This is not gyroscopic precession which is always 90 degrees.

The unstabilized rotor also does not hold its position in space but left
to its own devices wanders all over the place. Try flying an R-22 hands-off.

A rotor is more accurately described as a system of whirling pendulums
subject to – and readily explained by – the laws of vibration.

Unfortunately, at some time early in the helicopter era the analogy of a
rotor as a gyroscope took hold. This analogy is not only incorrect but
results in confusion as to what is really going on.

By: Philip Casey

Philip Casey — Thu, 19 Apr 2012 15:00:14 +0000

Good read! Thanks for the info. I’m very interested in how helicopters operate and I hope to learn more by reading your blog.

By: Dale Long

Dale Long — Tue, 28 Feb 2012 13:31:55 +0000

Enjoyed reading all the comments. Thanks for creating a platform to share HELI info.

By: Alan Barnes

Alan Barnes — Tue, 07 Feb 2012 05:16:07 +0000

That’s not gyroscopic precession but, rather, another important property of gyros called rigidity in space. Essentially the gyro instrument wants to stay in the same location while the aircraft rotates around it. Having to reset the DG occasionally is more a function of drag inside the instrument. It can be due to age (it’s just worn out ), a manufacturing defect, or even a vacuum failure.

By: Bob Kuriger

Bob Kuriger — Tue, 07 Feb 2012 03:27:57 +0000

With regards to resetting your DG every so often, be advised the DG wants to stay aligned with some point in space, while the world rotates 15 degrees per hour. That is not exactly gyroscopic precession, but close.

By: Bob Kuriger

Bob Kuriger — Tue, 07 Feb 2012 03:22:50 +0000

Gyroscopic precession is not as mysterious as most college physics professors and their texts would have you believe. Helicopter rotor blades, for instance, climb and dive under the influence of increased or decreased cyclic pitch. Since maximum climb rate occurs at a point 180 degrees from maximum dive rate, it is only natural that halfway between, i.e 90 degrees in the direction of rotation, the blade will be neutral and at its highest point of the revolution. Also good to know, gyroscopic precession only occurs when the forcing function is relatively mild compared to the inertia of the rotating disk.

By: Rich

Rich — Tue, 07 Feb 2012 03:14:28 +0000

Tail dragger pilots also become familiar with gyroscopic precession. When raising the tail during take off at high rpm the effect is noticeable by creating a sudden left turning tendancy as the propeller rotates forward and down.

And that need to reset the DG once in a while is caused by gyroscopic precession.
Holy moly Batman it is all around us..

By: Gary

Gary — Mon, 06 Feb 2012 15:26:59 +0000

Well, I thought I had learned something. Then, from the comments above I’ve reverted to my prior understanding. I knew it couldn’t be that simple so yes, helicopter flight IS a fig-newton of our imagination, collection of parts, etc. I even tried it. Once. Rub stomach and pat head, or was it rub head and pat stomach? Repeat.
My hat is off to all that do it!

By: Paul Goldasich

Paul Goldasich — Fri, 03 Feb 2012 15:41:17 +0000

I enjoy learning about technical things especially having to do with helicopters. I often watch the mechanics and ask questions to learn more about the machines I love to fly. During training, there are so many things to learn, and the principles of flight is one of the biggest. Hitting the highlights is about all you get and if you want to learn more, it’s on you. To demonstrate the precession principle, my school had a bicycle wheel (with the tire mounted) and a handle bolted to the axle. You would hold the handle and get the wheel spinning. You could then feel the gyroscopic force applied by the spinning wheel. Great learniing tool to explain this concept. I cannot do a better job explaining it than Christian did, and I agree with Jim, that as pilots, the explanation is just fine.

By: Alan Barnes

Alan Barnes — Wed, 01 Feb 2012 18:50:44 +0000

To further what Christian said… a rotor system is *like* a gyro and has some gyro-like properties. But it’s not a fixed system like a gyro. The blades are allowed to flap or flex and, in a fully articulated system, lead and lag. This moves where the reaction takes place from 90 degrees to a spot based off that specific rotor system – it’s different for every helicopter.

One of the best explanations I’ve found for it is in the Navy’s Introduction to Helicopter Aerodynamics workbook (P-401) available here https://www.cnatra.navy.mil/pubs/folder5/TH57/P-401.pdf It starts on page 103 of the pdf, section 405 and 406. This is also a valuable resource for a lot of other rotor aero subjects.