Outrunners and Gyroscopic Precession

[Iso Octane]

I was watching an excellent documentary today on National Geographic called "Warplanes", which talked about how the discovery of powered flight lead inevitably to guns on airplanes.

They talked about rotary engines as an alternative to heavy water cooled engines. They go on to explain that rotary powered aircraft suffered from odd handling characteristics because of the massive rotating mass of the engine.

Outrunner motors are easily the electric flight equivalent of the rotary. :p So I wonder, when mounting the rotary such that the whole thing rotates, do we get the same odd handling characteristics as those early fighters did?

[JetPlaneFlyer]

The procession effect must be there, as for any aircraft with a spinning propeller (or impeller). The degree it manifests itself is down to the diameter, weight and rotational speed of the spinning mass... An outrunner is relatively small and light so the procession effect would be much less than a rotary engine WWI fighter, but it must still be there to some degree. The prop itself, not the motor is probably the biggest contributor.

The effect is a 'couple' between yaw/turn and pitch.. For a conventionally rotating prop; left turn will cause nose up pitch and right turn will cause nose down. Also pulling back on the stick will give a right turn reaction and pushing forward causes a left turn reaction. If the rotation direction of the prop is reversed then the effect is opposite. Twin engine aircraft with contra rotating props, or contra rotating props on the same shaft cancel out the effect.

You can check the effect out yourself with a spinning cycle wheel, a mountain bike wheel with a big heavy tyre works best.

Steve

[downunder]

Precession from the prop is a minor problem in CL stunt where a hard turn yaws the nose out from the circle with up elevator and vice versa with down elevator. This of course will happen with RC too. The ultra fussy compensate for this by using a Rabe rudder (named after Al Rabe who came up with the idea many years ago) where the elevator is connected to a moveable rudder in such a way that up elevator moves the rudder inboard a little to counteract the outward yaw and down elevator does the opposite. Using a wood or CF prop reduces the affect.

[ghoti]

The rotary engine spins the block. The radial engine spins the crankshaft. The mass difference is what. . . 30 to one at least, so the Sopwith Camel, for example, has amazing flight charactoristics from huge precession forces.

Our outrunner motors spin a light aluminum can and a few tiny light magnets so are not much impacted by procession. And the heavest part, the copper wires, are stationary.
bill

[MarkusN]

The large effect of precession on WW I planes came from the high proportion of mass they concentrated in the engine and the relatively low rotary inertia of the lightweight, short-span planes. This relation is skewed much more in the direction of stable flight / little influence of gyroscopic effects on a typical model plane employing an outrunner.

On a lightweight shockflyer relations may approach WW I fighter orders of magnitude again.


Gyroscopic effects are present even in full-size aircraft with standard engines. This is used in certain aerobatic maneuvers, such as the Lomkovac. (And produces some jaw-dropping results.)

[Sparky Paul]

The awful ground handling problems of the WWI rotary-engined models comes from the very short nose lengths the massive engine demanded.
Getting a Dr-1 to takeoff at all is a feat, and to NOT end up resting on the top wing a miracle! :)

[Neil Stainton]

Our outrunner motors spin a light aluminum can and a few tiny light magnets so are not much impacted by procession. And the heavest part, the copper wires, are stationary.
bill

No, every outrunner has a mild steel flux ring to contain the magnetic field. The flux ring and magnets are a substantial part of the weight of an outrunner, and as they are on the periphery they will contribute most of the moment of inertia (distance^2 * weight) of the spinning motor. However I agree the prop will probably contribute more to any flywheel effect.

Neil.

[Iso Octane]

It seems like, and probably is, such a clumsy idea to have the whole block spin instead of just what's needed. I had the same reaction to outrunners. Actually I wasn't aware of the distinction between 'radial' and 'rotary' until I watched this program. Rotary engines just seem so non-intuitive.

[JetPlaneFlyer]

The rotating cylinders of the rotary were needed at the time to give adequate cooling. More modern materials and oils helped to make the rotary engine obsolete.

Steve

[vintage1]

I am not sure that cooling was the issue. A prop makes a pretty fair cooling fan.

I suspect that in some way they are just simpler to make. Valve gear possibly?

[raptor22]

No, they gave a better thrust-to-weight ratio.

The reason was that, back then, the engines weren't very strong so they were operated at very low RPM's....which necessitated a very heavy flywheel to keep things running smoothly. The block's spin allowed the engine to operate smoothly without the flywheel, which reduced the weight of the powerplant.

--Alex

[JetPlaneFlyer]

Raptor,
Good point but I think we are maybe both correct to some extent...
From Wikipidea:

The first effective rotaries were built by Stephen Balzer, who was interested in the design for two main reasons:

In order to generate 100 hp (75 kW) at the low rpm at which the engines of the day ran, the pulsation resulting from each combustion stroke was quite large. In order to damp out these pulses, engines needed to mount a large flywheel, which added weight. In the rotary design the engine itself doubled as its flywheel, thus rotaries were lighter than similarly sized engines of regular design.
The cylinders had good airflow over them even when sitting still, which was an important concern given the alloys they had to work with at the time. Balzer's early engines did not even use cooling-fins, a feature of every other air-cooled design, and one that is complex and expensive to manufacture.


I'm sure however that as you point out the lighter weight was in fact the prime reason their continued popularity in early aircraft.

Steve

[yoyoML]

Please don't forget that, while an in-runner may have much lower moment of inertia, to give the same output as an outrunner will require high rpm and reduction gears. So in the end, the gyro forces from an inrunner and an outrunner may be very close, provided they drive the same props.

I'd hate to fly in a Camel or Dr 1. I remember they didn't even have (useful) throttles. There was perhaps a very short operable rpm range, and a blip switch to cut ignition at times...

Oh dear, is that the WW1 version of today's PWM?!!

[ghoti]

Radial engine has super simple spark distributer, too!

[Salto]

Radial engine has super simple spark distributer, too!So how did the ignition system work on a rotary engine?

Graham.

[harveyweewax]

Again, from Wikipedia:

"....The Gnôme (and its copies) had a number of features that made it unique, even among the rotaries. Notably, the fuel was mixed and sprayed into the center of the engine through a hollow crankshaft, and then into the cylinders through the piston itself, a single valve on the top of the piston let the mixture in when opened. The valves were counter balanced so that only a small force was needed to open them, and releasing the force closed the valve without any springs. The center of the engine is normally where the oil would be, and the fuel would wash it away. To fix this, the oil was mixed in liberal quantities with the fuel, and the engine spewed smoke due to burning oil. Castor oil was the lubricant of choice, its gum-forming tendency being irrelevant in a total-loss lubrication system. An unfortunate side-effect was that World War I pilots inhaled and swallowed a considerable amount of the oil during flight, leading to persistent diarrhoea. Finally, the Gnôme had no throttle or carburetor. Since the fuel was being sprayed into the spinning engine, the motion alone was enough to mix the fuel fairly well. Of course with no throttle, the engine was either on or off, so something as simple as reducing power for landing required the pilot to cut the ignition. "Blipping" the engine on and off gave the characteristic sputtering sound as though the engine was nearly stalling, though it did not stall as quickly as conventional engines due to its great rotational inertia....."

The ignition system was probably just a simple magneto and condenser like you would find on an old lawn mower engine.

Regards,

Sven

[ghoti]

So how did the ignition system work on a rotary engine?

Graham.

Why not have the spark plugs spinning on the "top" of the block and one by one brush across a high voltage metal stud. . . at least that is how I would accomplish it. Rotate the stud to adjust timing. What could be simpler?

[vintage1]

Why not have the spark plugs spinning on the "top" of the block and one by one brush across a high voltage metal stud. . . at least that is how I would accomplish it. Rotate the stud to adjust timing. What could be simpler?

A magneto.

[harveyweewax]

Why not have the spark plugs spinning on the "top" of the block and one by one brush across a high voltage metal stud. . . at least that is how I would accomplish it. Rotate the stud to adjust timing. What could be simpler?

you might want to check out the pics on this link:http://modelrotaryflyer.tripod.com/

If you look closely at the photos you will see how the spark plugs are connected to commutator on the main shaft which is then connected to the magneto. This guy does some really nice work!

Regards,

Sven

[rebell]

I have a little 30" stick with a fairly short nose, a 2208 outrunner and an 8" prop on it. The prop, prop adaptor and motor weigh about 17.5% of the total flying weight. Whenever I level out from a turn, especially from a left hand turn, you can see its tail wagging like a dog for a while. That is because of the gyroscopic precession still in action and dying out. I see it some times at the club with glow planes as well. You will see it mostly when the engine / motor has no down or side thrust, like my little stick.

[JetPlaneFlyer]

Whenever I level out from a turn, especially from a left hand turn, you can see its tail wagging like a dog for a while.

This sounds like a little yaw 'left over' from the turn damping itself out. It does not sound like anything to do with gyroscopic effects. the Procession reaction to a turn is a nose up or down pitch, not tail wagging ;)

Steve

[rebell]

This sounds like a little yaw 'left over' from the turn damping itself out. It does not sound like anything to do with gyroscopic effects. the Procession reaction to a turn is a nose up or down pitch, not tail wagging ;)

Steve

At first I also thought it to be a dutch roll. I then level out with the power off, and there was no wagging. With a dutch roll the effect will continue, with power or not and is visible with gliders (no motor / engine) as well. I repeated the test both ways a few times and the results were the same with each test.

As I understand the word, precession means it is a repeating process. Here is a description I scanned from MODEL AIRCRAFT AERODYNAMICS by MARTIN SIMONS. Some other books describe it as well, but it is quite lengthy.

[JetPlaneFlyer]

Good point... I guess I stand corrected and procession could be playing a part though would not also have a nose up and down element to the motion if this was the case?... Perhaps this part of the motion would be less evident from the ground?
I've seen just this sort of behaviour in gliders where procession is obviously not an issue, In gliders I’ve noticed it most in rudder only model that naturally tend to yaw around the turns. High aspect ratio and resultant high moment of inertia in yaw makes it more pronounced.

[rebell]

From the ground Dutch roll almost looks the same. The best way to see if it Dutch roll or gyroscopic precession is power on / power off test. You will notice the Dutch roll mostly in models with large dihedral and a small fin and it is aerodynamic forces that are the cause.

[rebell]

Here is something more to read:
http://en.wikipedia.org/wiki/Gyroscopic_precession#Torque-induced_precession
http://en.wikipedia.org/wiki/Dutch_roll

[JetPlaneFlyer]

Yeah... I was not thinking so much of Dutch Roll but more of the phenomenon whereby an induced yaw takes a few cycles to dampen itself out, it's related to Dynamic Directional Stability...

It’s described here under ‘Dynamic Directional Stability’: http://selair.selkirk.bc.ca/aerodynamics1/Stability/Page4.html

This could well lead to Dutch Roll in a model that has strong Dihedral Effect.

Having said all of this, if the model only does it when the power is on then it could well be related to procession.

Steve

[rebell]

There are many phenomena that are so close in appearance that one must do a test to see what it actually is. It is more of a science than anything else. That is what makes this hobby so interesting.

One way to test for directional stability is to move the rudder vigorously left and right to yaw it that way. Then leave the stick so to centre the rudder and observe what is happening. If the yaw motion stops immediately, it is a stable plane. A directional unstable plane will continue "shaking" its tail. Another way is to do a stall turn and see what is happening. I did the test, this little stick of mine is very stable.

[vintage1]

From the ground Dutch roll almost looks the same. The best way to see if it Dutch roll or gyroscopic precession is power on / power off test. You will notice the Dutch roll mostly in models with large dihedral and a small fin and it is aerodynamic forces that are the cause.

You may have solved a problem I had on a plane but never really isolated. It started out with a speed 400 motor, and ended up by strange upgrades with permax 450T on 3s LIPO.. it was a massive motor in a tiny plane, and I noticed that if it got hit by a gust..the tail would wag for a while before settling down. Zero dihedral.... I couldn't reproduce it in calm air (no rudder you see, no way to yaw it)

[rebell]

From the last discussion I tested a few different prop sizes and weights on the 30” stick. With the smaller diameter and lighter props there is definitely less gyroscopic precession visible. I also put on a folding prop and even though it is the heaviest prop of all tested, with it I don’t notice any tail movement like with the others.

[rebell]

Recently I bought a small Hacker A10-9L motor with a propsaver on it. It is the first time I used a propsaver. I noticed that in tight turns, there is a loud vibration sound. At first I thought there is something wrong, but thought about this discussion. I tested it a few times and accepted it was from the prop gyroscopic precession and the o-ring forcing it back into place. NO tail wagging visible here.

[vintage1]

From the last discussion I tested a few different prop sizes and weights on the 30” stick. With the smaller diameter and lighter props there is definitely less gyroscopic precession visible. I also put on a folding prop and even though it is the heaviest prop of all tested, with it I don’t notice any tail movement like with the others.


Thats because the blades will flex instead of the tail wagging. The mass is somewhat decoupled from the airframe by the folding bit..

[rebell]

Back to the question in post #1. To sum up, I think that any noticeable gyroscopic precession, P-factor, momentum or other unwanted forces is not so much from the outrunner motor, but mostly from the prop blades. The motor itself should therefore not contribute much to the "odd handling characteristics as those early fighters".

[ghoti]

Rebell, I beleive that is possible although I hae never experienced such. Dollars to donuts it won't happen with a 3 blade prop but much easier to prevent with tighter rubber banding. bill

[flypaper 2]

When I put the 46 size electrics into a flatspin, the noise from the prop/motor sounds like a scalded cat howl. Very loud and proportional to how much throttle is on. I imagine a lot of the noise is reverberating through the airframe too. When I let someone handle the transmitter and hold the nose up at full throttle there is quite a resistance when you wag the tail. On the WW1 rotaries, on some engines they could cut out just a number of Cy's. to get part power. The reason they used castor oil is that it wouldn't mix with the gasoline, one being mineral base and the other vegetable base. And your right, the pilots were well cleaned out :D

[HELModels]

Yeah, they went straight to the ....house when they landed. Cold with distressed bowels, so much for the romance portrayed.

As for the observations of rebell, I have seen some of what he describes. I had a model with lots of polyhedral, an outrunner and big prop. In a stall turn, I attributed the waggles to the V-tail, but that was probably wrong. It was very brief and dissapeared as soon as airspeed built. I seem to remember differences between left and right turns too, but I attributed those to control horn placement, but that was probably wrong too. So, whoever said tests to isolate some behavior make the hobby interesting was close. Using instinct which tests might verify is what makes it interesting to me.

On the model which seemed to react to power oddly at certain speeds, I increased span, reduced dihedral, increased AR and the character changed, but I also increased oz./sq ft. Which change worked? I never bothered to test and isolate it, but I liked the result.