Autogyro specific technical information.

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Autogyro specific technical information

Rotor Mechanics and The Rotor

Every autogyro has a rotor, and that put simply is what enables them to fly. As far as autogyros in general are concerned the rotor consists of two blades, which are attached at the axis with a hinge system. Without these hinges the Autogyro of early years tended to tip sideways, the inertia made for a dangerous and unpredictable craft. The invention of the actual hinge is what makes gyros safe.

Present day autogyros and helicopters are fitted with far less hinges than those described below. The advent of modern materials has allowed flexing not just at the root but also predictably throughout their length. This has the same effect as hinges used to achieve with all of their complicated hinge mechanisms: Hinge-less rotors have a flexible blade root, which acts as a combination of all hinges. They are also referred to as an elastomeric hinge.

The (always two-bladed) teetering rotor with direct axis control is a special case. It uses the fact opposing blades have opposing movements. It has therefore one central flapping hinge for the beam that forms the two blades. It uses the rotor shaft as a central lead-lag hinge. Feathering hinges are usually omitted on light and ultra-light autogyros.

Bearing in mind what has been said about elastomeric hinges let us consider what they replace. Flapping hinges allowed the rotor blades to move up and down. In static air on a runway, both blades if balanced properly will track more or less in the same identical rotational line. But as the autogyro gains forward speed, the advancing blade gains a relative higher airspeed compared to the retreating blade; the advancing blade then will produce more lift. Due to the gyroscopic effect and to the relative high and low air pressure areas around the rotor, the advancing blade will rise higher than the retreating blade. By having a "Flapping Hinge" this extra pressure is absorbed into useful energy, rather that trying to tip the autogyro/helicopter over to one side.

Lead-lag Hinges

The lead-lag hinge is designed to allow a blade to "run in" or "get behind" with respect to other blade. This is mainly to compensate for the extra rotational speed that comes with "blade flapping". The same hinge also compensates for differences in blade drag in various moments of one cycle. It is common for the rotating blades to react to forces upon them a quarter of a cycle later, and this is seen in the response time that an aircraft takes to respond to pilot induced changes made, to in flight controls.

Let's take this a little further with respect to blade flapping, a perfectly natural and expected phenomena. If you were to place a set of blades on a flat surface and measure from tip to tip, let us say that your measurement is 29 feet. If you take those same blades and rest them in their place at the top of the mast, unsupported they will droop, if you measured the distance tip to tip, that is point to point in a straight line the measurement would be somewhat less than 29ft. If now you powered up your rotors and could take a measurement of a fully upward coned set of 29ft blades, and if you were able to measure tip to tip they would now be significantly less than the 29ft. Why then is blade flapping of concern? In this particular scenario it is predictable and necessary. In the example of a ballet dancer making a pirouette with her arms stretched outwards. If she moves her arms closer to her body (that is, her axis of rotation), her rotational speed increases. This is the same with autogyro and helicopter blades. As they "flap" up or down, they come closer to the axis of rotation and there is a significant increase in their speed. So, when one blade wishes to advance a tad by speeding up, or lag on the retreat cycle the "lead/lag hinge" has its moment.

 

 Feathering Hinge

The feathering hinge is not at all common today on autogyros; it has been tried though and proved to work. By making it possible to change the pitch setting of a rotor blade at the root, it is possible to do a " jump start". The general concept is that the blades are set to minimum drag and powered up to at least one and a half times the normal rotor flight speed that is needed by that autogyro to maintain flight. When sufficient rotor speed is gained sufficient energy has been temporarily stored in their rotation. But in order to use that energy the power to the head must cease.

Now that the power is taken away the rotor speed will decay; slowly at first as there is no (little) drag. By increasing the pitch by as little as 3 degrees the inertia present in the "over speed" rotors is sufficient to get the autogyro airborne, but because the drag has dramatically increased, the rotor speed will decay rapidly. But as said, there is sufficient "power" stored in the spinning rotors for the autogyro to "take-off". Of course this "flight" is short lived as the power to the rotors has ceased. Forward speed now MUST be gained and gained quickly during this "short life span" to obtain long-term flight. What has to happen is, we have to swap the energy stored in the ground-powered rotors for genuine autorotation. That's where the propeller comes in!

In addition, by changing the pitch setting of a rotor blade, you change the lift characteristics on that blade and therefore influence the flapping angle of that blade. If you change the flapping angle periodically, so that the front blade moves down and the rear blade moves up, you tilt the rotor forward without tilting the rotor axis. These rotors, which tilt by changing the blade pitch, are called "Articulated Blades". Today every helicopter has articulated rotors.

Rotors that tilt by simply tilting the rotor axis are said to have "direct axis control". The first autogiros by Juan de la Cierva all had direct axis control. Direct axis control is still very common with light autogyros; they are relatively very simple mechanisms and consequently trouble free. What's more they work!

Rotor Trim and Axis

Autogyro stability was a real design problem at the beginning. Cierva put his theories to work but stability was not easily found, gyros kept tipping over to the side. For gyros to work stability had to be maintained. Experience was the teacher employed back then, rather than scientific research; my thanks to those who have gone before! They all contributed in a school of hard knocks and have left us a legacy to build upon. This is where the "teeter" hinge came in and with it the physical placement of the axis of the rotor. As far as stability is concerned, it is vastly improved if the rotor's axis was NOT just placed on top of the mast as a matter of fact. Adjustment was built in that allowed movement of the axis fore and aft of the central mast position. This helped rotor stability.

The position of the head to the mast is located with the "Hang Test". Next, the rotor or more correctly the DISC trim ATTITUDE is tensioned up to give neutral stick at cruise speed. This can be a trim adjustment that one can tune during flight to suite the in-flight load situation.

A. Balance Test = Position of head. Located with hang test from teeter bolt.

B. If using a Benson type regular head (like most do) then some sort of spring tension may be necessary to give neutral feel at the chosen speed. With this system one would give forward pressure on the stick when flying very slow, and need some aft pressure on the stick when flying faster than the trimmed speed. OR re-trim the control feel to your new selected speed.

Placement of the Teeter bolt in relation to the mast.

By adjusting the Balance of the gyro in the correct place, we gain a flight envelope that is safe to operate within. Then if, when rotor load was increased by an action of the pilot in pulling back on the stick, or by an increase of power to the motor and therefore thrust, or by a sudden wind gust then the rotor angle of attack is decreased by those actions rather than increased and thus safety is increased.

For, if the rotor had been in extreme load before the action of the pilot by pulling even farther back on the stick, increasing the thrust or even an increase of load via a sudden gust of wind then the rotor could if not balanced correctly be overloaded to the degree that breakage could occur. Again, join with me in thanks to those who have gone before and experimented with their lives!

The rotor and therefore flight can be trimmed by the careful addition of springs (usually). The result is much the same as is applied in a fixed wing aircraft's trimming system: Aircraft can and should be trimmed for climb, descent or level flight.

ROTOR VIBRATION, IS IT INEVITABLE?

To get to the bottom of this question it is necessary to understand some of the peculiarities of autogyro blade function; as this has a direct baring on the answer. (Again, I will not attempt a totally scientific answer to this, as I am just not qualified to do that.)

Remember: Rotors you purchase, will need to be installed on your gyro (again whatever manufacturer) and tuned correctly.

Unbalance of rotors is just one rotating component of your gyro that may cause the entire machine to vibrate. This induced vibration in turn may cause excessive wear in bearings, bushings, shafts, gears, cabling, hoses, cowlings and exhaust systems substantially reducing their service life. Vibrations set up highly undesirable alternating stresses in structures, which may eventually lead to structural failure. Not only is your gyros performance decreased because of the absorption of energy by the supporting structure. But, the enjoyment factor is driven low too.

Generally the smaller diameter Rotor will give a higher top speed, and the longer they are the more maneuverability you will have at slower speeds. With this information the Rotors rated by the Manufacturer to do this will be recommended. Within the Hub-Bar there are provisions made to adjust and tune the blades when they have been installed on your machine. HOWEVER: Please check with us with your recommended ROTOR size and chord etc., BEFORE your purchase.

ROTOR IMBALANCE!

All rotors exert variable forces on to the Rotor-Head and Mast and then to the frame of the gyro resulting in vibrations through inertia. Though manufacturers do their best to provide "perfect blades" in reality there is no such blade, errors in geometrical dimensions and slight differences within the material used and some irregularities in the mass distribution are always present.
The result when they are spun up - vibrations. To remove these vibrations and establish safe and quiet operation, tracking and balancing becomes necessary. Vibrations due to irregular mass distribution occur at a frequency that is related to the rotating machine's speed of operation and therefore measuring such a vibration requires that the balancer utilize a filter that isolates the vibration that occurs at the machine's speed of operation. This imbalance is made worse because of another force acting when the unbalanced rotors are spun - centrifugal force increases all these vibrations by amplifying the vibrations differently.

IMBALANCE AND CENTRIFUGAL FORCE

Centrifugal force varies with speed. When rotation begins, the unbalance will exert centrifugal force tending to vibrate the rotor and its supporting structure. The higher the speed, the greater the centrifugal force exerted by the unbalance and the more violent the vibration. Centrifugal force increases proportionally to the square of the increase in speed. If the speed doubles, the centrifugal force is quadrupled, etc. Fortunately, Manufacturers are totally aware of, and understand in far more depth than I, what is required of them. They have the machinery, not only to balance your pair of blades statically but dynamically. In so doing they can supply a set of blades that are as "clean" as possible. But, sorry to say, the story of unbalance does not end there. Even when the gyro has its Rotors perfectly balanced by the manufacturer, and the Rotor Head and all other equipment too are perfect. There will still be vibrations!

These vibrations are a natural consequence of the dynamic associated with a Rotary-Wing. If the ROTOR (Rotary) wing turned while the Gyro was in perfectly still air, and every other piece of engineering was perfectly balanced also, then it may be possible to produce perfect harmony between the two blades, and thus no vibration. But as soon as you introduce a variable as simple as moving air, perfect conditions cease. Add to this the complexity of a flying gyro bolted on underneath and all sorts of other criteria present themselves. I will restrict this article to just one of those variables; "The different forces at work and competing when one wing advances in flight compared to the wing that is retreating in flight". But before we get into that please be aware that when your rotor and teeter bar set-up is installed on your machine it MUST be checked BEFORE YOU GO FLYING!!!!

Generally this takes the form of two procedures, or checks already mentioned:
1. Tracking.
2. Balancing. (Chord Wise - Lengthwise)

ADVANCING / RETREATING ROTOR BLADES AND THE CONNECTION TO VIBRATION

When we consider what takes place to the rotors while an autogyro is flying it is immediately plain why there will be some vibration. Let us make some assumptions upon which to give us a few numbers . That way, it will be more easily understood. Lets keep things simple first of all. For our purposes:
A Rotor Diameter (disc) is 29ft.
A Rotor speed is 300 rpm.
A flight speed (TAS) is 100 mph and our gyro is flying quite happily.
At that speed the Rotor is turning at 96ft (circumference) times 300 rpm times 60 per hour = 1728000 divide by 5240 ft per mile = 327 miles per hour. DON'T GET IN ITS WAY!!!!!!!

327 MILES PER HOUR?

But this is not the true airspeed of the Rotor's tips! At cruise speed we have to add 100 mph to the advancing side and subtract 100 mph from the retreating blade. Now we have a forward airspeed of 427 mph on the one advancing tip and 227 mph on the retreating tip, and each time either blade has to swap its direction relative to the moving gyro, the advancing blade, then becomes the trailing blade then becomes the advancing blade ------- etc. etc. etc.

This now translates into reduced lift (and stress) on one side and increased lift (and stress) on the other side of the craft, and each time the rotor swaps its direction the stresses it encounters are absorbed by the blade first, the mast second, and the fuselage finally. The craft because it has a teeter hinge can cope with this and continue to fly, but as the craft increases its forward motion more and more, the problems become greater and greater. To the point where one side of your craft wants to roll up and the other down. This is why in layman's terms there is a limiting factor to gyro speed, and for our discussion vibrations (knock) WILL always occur.

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