Getting a grip on gyroscopic precession?

[Kosie]

Seems you guys are looking for an engineer to solve the problem for you. Good news: I am one - bad news: only a "grondkoker / padmaker" with minimal hours behind the stick.

However, some two weeks ago I asked my instructor (no names mentioned) this very same question. It seems the flexibility of the rotors definitely plays a major part in "retarding" or "dissolving" or "minimizing" the gyroscopic precession. Fly the gyro along a straight line (road, rail, etc.) at speed (don't ask me what speed), then turn sharply into, say, a 90 degree turn by forcing the stick left or right. You will definitely notice the effect of gyroscopic precession - the nose will tend to move up or down depending on which way you turn. If you however fly slower and turn more gently, the rotors have a chance to adapt and the effect will be less or mostly not noticeable.

This may not be very scientific, but it is my 1c worth.

[Magnifan]

Wagtail was spot on with his description of what is going on. The mistake that some of you are making is that you assume that the force being applied to the disc is directly due to your control rods pushing and pulling it. This is not so, the force being applied to the rotating disc is a result of the aerodynamic force caused by the change in angle of attack of the blades, a far greater force than you could ever achieve with your puny little arms!

If you are sitting in the gyro with the blade at 90 degrees to your direction of flight and you pull the stick back, you cause the angle of attack of the right hand blade to increase and the angle of attack of the left hand blade to decrease. The right hand blade wants to climb and the left hand blade wants to dive, which exerts a huge aerodynamic forc on the disc to tip down to the left, but because of gyroscopic precession, what actually happens is that the front of the disc tips upwards.

A gyro is certainly not a weight shift machine, all of the huge forces required to move the disc around are caused by our control inputs changing the angle of attack of the blades and it is the airflow over them applies the forces to the disc.

[Wargames]

Eurika... I finally get it. It took a good chat on the phone to Magnifan while sitting in my office chair with my arms spreadeagle shanging the AoA of my fingertips while swiveling in circles! My staff think I'm nuts but I got it. Wagtail & Len - you guys were spot on! All we do when we wiggle the stick is change the AoA... the actually precession input is the rotors flying response off-set by 90'. Pull stick back - AoA changes 90' to the pilot - input is 90" which results in rotor tilting disc back. AAAhh shweet!

Me thinks you staff is correct in their view point.

Me thinks further, that you confusing balance and precession!

You must remember that you are flying nothing else than a weight shift controlled aircraft. Pulling back on the stick, is actually putting your weight behind the center of the rotor. When the undercarriage corrects this balance, your angle of attack increases. Nothing about precession there.

[Wargames]

A gyro is certainly not a weight shift machine, all of the huge forces required to move the disc around are caused by our control inputs changing the angle of attack of the blades and it is the airflow over them applies the forces to the disc.

A gyro has a fixed AoA on the rotor, unlike a helicopter. How can you change the AoA without upsetting the balance. You cannot. The center of weight of a gyro dictates the AoA, and you dictates the center of balance with your stick.

I do understand your stand on the precession powers, and that will only dictate that a small change in stick is necessary to get a bigger effect in position, but it remains a balancing act, but still, this doesn't say there is any procession.

[Magnifan]

Hi Wargames, the angle of attack of the entire disc and the angle of attack of the rotor itself are two quite different things. Effectively you manage the angle of attack of the entire disk by making small changes to the angle of attack of the rotor blades 90 degrees ahead of the desired change to the AOA of the disc.

A gyro really isn't weight-shift at all, all changes to its flight axes are controlled by aerodynamic effects as a result of rotor AOA changes.

[Gyronaut]

We cannot, unfortunately, leave AOA out of the discussion if we wish to understand it fully.

I rest my case.

[Wargames]

Last stance:

Procession is:

a - directly proportional to the force applied, and

b - inversely proportional to rotational speed, and the moment of inertia.

To discuss these alone, I will start with a.
I have never seen any gyro pilot that looks like Arnold Schwarzenegger. (Pic below).

hence the following conclusion. Because your input from the stick is not taking a lot of effort, and because it is taking effect less than 30cm from the center of the rotor, the force applied cannot be big, and thus, on point (a), the procession is small.

To discuss b.
Because the rotor is spinning at more than 200rpm, I think 250 is about average, and it is big and heavy, the rotational speed is high, as well as the moment of inertia. Now, the bigger these 2 are, the smaller procession.

So to conclude, on point (a) we have established a small procession. And on point (b) we have established a small procession. That will leave us with a very small processional properties on a gyro. So, if this force is small, we might start about thinking of leaving it out of the equation. Then the next question must be asked again...If a gyro's flight capabilities is not determined by procession, by what is it determined then??

Weight and balance

[Learjet]

Wargames, so you reckon this is a weight shift???

[Magnifan]

Not at all

Imagine that you are sitting in your gyro with the rotor blade at right angles to your direction of flight:

Now, when you pull the stick back you change the angle of the rotorblade on your right hand side such that it would want to ascend. You also change the agle of your left hand rotor blade so that it wants to descend. Imagine how much force will actually be applied to the disc, given that the rotor blade tip is moving at about 700km/hr and the change in angle of attack is quite big (sit in the gyro and have a look the change when you pull the stick all the way back). So we have a huge aerodynamic force trying to tilt the rotor disk down on the left and up on the right as a result of a small change in the position of the stick in the backwards direction. This huge aerodynamic force tilts the rotor up at the front, not down at the left, because of gyroscopic precession. This is why a gyro climbs when you move the stick back.

The fact that you would have to have arms as big as the oke in your photo to shift a heavy fast moving rotor around using weight shift is excatly the point. All that you do when you move the joystick around is cahnge the angle of attack of the rotor blades themselves and aerodynamic forces as a result of these changes do the rest.

[Wargames]

Wargames, so this is a weight shift???

Certainly looks it. They are shifting a huge weight from point A to point B!!

[Kosie]

To increase your levels of confusion, try this link http://www.unicopter.com/0940.html

[Gyronaut]

Awesome post Kosie!! Well done! Most informative.

This is the function of the blades flying to position. The blade's pitch change precedes the resultant flap or teeter.

As I understand it, this is Magnifan's point. It's the rotor's angle of attack, not weight-shift that changes the disc's flight pattern which the cockpit subsequently follows.

Oh well, whatever.. as I said, it works and it works well.

Sela

[Slabfish]

Not being a gyronaught myself,I have been following this thread with great interest.What makes sense to me at the end of the day is this---
Gentlemen, especially believers in precession. A point to ponder for you.

The Autogyro (yes I've flown those as well). The control of the rotor in simple autogyros is direct movement of the teetering hub; this input is in phase with the disk movement.

Lets look at the forces applied when commanding the disk to tilt forward.

1. An upward force is applied at the BACK of the disk (6 o'clock).

2. A tiny movement of the disk causes the advancing blade to experience an instantaneous reduction in effective pitch angle reducing the angle of attack. The disk begins to move, does it:

A. Tilt up on the side of the advancing blade (3 o'clock [for example]) following the laws of precession?

Or

B. Tilt forward.

Given that we already know that the controls are rigged in phase we know that the effects of precession are not noticeable to the pilot and the disk tilts forward.

[Learjet]

Hi Slabfish, nice explanation! The error in my original thinking was the assumption that the initial precession input control is made by the pilot - this is not strictly correct and in fact it is actually the aerodynamic response of the rotor (as a result of the pilots cyclic change to the AoA) that makes the true precession input. When the advancing blade experiences the reduction in effective pitch - that is when the precession input control is actually being made by aerodynamic responses on the rotor (at 3 o'clock) and which results in the nett effect occurring 90 degrees later when the disc tilts forward (in the 12 o'clock position).

Thus we have not one, but THREE response to the pilot's cyclic control input taking place in the blink of an eye. So to use your example...
Pilots pushes cyclic forward... (1) upward force is upplied at the back of disc (6 o'clcock)....pitch angles reduces AoA (now the true precession input commences!!)... (2) huge aeroedynamic response on the CCW rotor ("downward force" at 3 o'clock position)... and (3) resultant gyroscopic precession manifests 90 degree later in the forward ("downward") tilting of the disc at the 12 o'clock position.

[Magnifan]

Nice post Kosie, I spent last night reading up on aerodynamic precession and discovered that there is still some debate as to whether a gyroplane responds to gyroscopic precession (as described in my post) or aerodynamic precession, which has a similar result on the disc and is also caused by changing the angle of attack of the rotor blade at a position in advance of the point where one requires the disc to react. Fascinating!

What there is till no debate about is that the reaction of the disc is entirely due to changes in the angle of attack of the rotor blades themselves.

The notion that one can push or pull on the disc at all is incorrect. Consider the situation when the rotor blade is fore and aft. When you pull back on the stick, you can have no effect on the rotor because the teeter bearing will simply allow the rotor to swivel about the axis that you re trying to manipulate it on.