gyroscopic precession

[Alex Becerril]

Hi All

which of these two factors affect the Gyroscopic Precession (yaw to the inside of the circle) more?:
a- Prop number of blades
b- diameter
c- prop weight

[RandySmith]

Hi All

which of these two factors affect the Gyroscopic Precession (yaw to the inside of the circle) more?:
a- Prop number of blades
b- diameter
c- prop weight

All the above affect GP

If your having problems with GP the lighter the prop the better for less GP (although it may not perform as well) If you can thin out the outer parts of your blades..without... them getting too flexible that will help.

Also the smaller diameter of the same will give you less  GP

3 blade seem to be better with GP since the force manifest itself 90 degrees ahead of the blades

Randy

[Larry Cunningham]

All above and the distribution of the prop's weight along its diameter..

(3-blade vs 2-blade may have something to do with moment of inertia as well.)

L.

"What is now proved was once only imagined." -William Blake

[Douglas Ames]

<snip>
3 blade seem to be better with GP since the force manifest itself 90 degrees ahead of the blades

Randy

?? - Are you sure it might be because your 3-blades are light-weight carbon vs. a heavier (larger dia.) 2-blade?

[RandySmith]

?? - Are you sure it might be because your 3-blades are light-weight carbon vs. a heavier (larger dia.) 2-blade?

No the 3 blades are better on GP than the same CF 2 blades that weigh less
But one advantage of the 3 blades is that you can, and do, normally run a little less diameter

Randy

[Trostle]

As explained, all of the factors listed affect gyroscopic precession.  Gyroscopic presession is a funtion of moment of inertia.  Moment of inertia is a function of a mass and the square of the distance from a center of gravity of the rotating mass.  Props with lighter tips would be better to reduce groscopic precession than props with heavier tips of the same diameter.  Lighter props with smaller diameters are better than larger heavier props to reduce gyroscopic precession.  A multi-blade prop with a smaller diameter will be better to reduce groscopic redcution than a hevier 2-balde prop of a greater diameter if its moment of inertia is less than the two-blade prop. This response is in regard to the question of what most affects gyroscopic precession, not necessarily what is the best prop to use for a given model/engine combination.

Also, gyroscopic presession causes a model to yaw to the right on inside turns and the model will yaw to the left on outside turns with a counter-rotating engine/prop (viewed from the front).

[Dick Pacini]

Hi All

which of these two factors affect the Gyroscopic Precession (yaw to the inside of the circle) more?:
a- Prop number of blades
b- diameter
c- prop weight

Um, I count 3 choices.  Is this a trick test?

[Bob Reeves]

Run reverse rotation then it won't be as much of an issue. I'm converting all my larger Saitos and most of the electrics are running reverse.

[Howard Rush]

Has anybody ever calculated the amount of rudder it takes to counteract the yawing moment from the spinning prop in an hourglass corner? For the amount of elevator it takes to counteract the pitching moment from flying level in a circle?  I ask the latter because today I'm rigging my elevator for a backwards-running motor.  I see the same qualitative stuff over and over, but nobody seems to own a pencil. 

Folks look at a prop like it's a uniform disk.  It's not.  What does the yawing moment from a two-blade prop do during a turn relative to a three-blade prop, and how does this affect my score? 

[phil c]

.......
Folks look at a prop like it's a uniform disk.  It's not.  What does the yawing moment from a two-blade prop do during a turn relative to a three-blade prop, and how does this affect my score?  

Three blades eliminate the vibration from one blade flying at a higher angle of attack in a turn.  Any number of blades over 2 does this according to NACA reports. 

The left handed prop probably does better for you.  It puts the precession to work for you in the third turn of the hourglass and the upper left corner of outside square corners.

[Tim Wescott]

Has anybody ever calculated the amount of rudder it takes to counteract the yawing moment from the spinning prop in an hourglass corner? For the amount of elevator it takes to counteract the pitching moment from flying level in a circle?  I ask the latter because today I'm rigging my elevator for a backwards-running motor.  I see the same qualitative stuff over and over, but nobody seems to own a pencil. 

Well, get cracking!

http://en.wikipedia.org/wiki/Gyroscope

Folks look at a prop like it's a uniform disk.  It's not.  What does the yawing moment from a two-blade prop do during a turn relative to a three-blade prop, and how does this affect my score? 

AFAIK (and I may just be bloviating here; I ain't gonna do the math), the major difference between a spinning stick and a spinning disk with the same radial distribution of mass is that the spinning stick will tend to deliver it's torque in thumps (well, more likely as a raised cosine w.r.t prop position).  The whole behavior of a gyroscope is really the ensemble average of the behavior of a collection of point masses; in the case of a stick vs. disk the difference is that the instantaneous behavior is different than the time-average, so a two-blader would induce more vibration.

[Douglas Ames]

And a 4-blader, a 5-blader or a contra-rotating 3 or 4-blader???
The later must have huge amounts of precession, just look at the empennage design of a Westland Wyvern. Dihedral in the stab, finlets and a huge vert. fin/rudder.

[PJ Rowland]

Question for you Howard must be :

Will 3B be optimal for Electric Counter rotation ?

Id be very interested to see electric setups with a 12 " 3B Undercamber thin carbon prop - might yeld excellent speed control.

I noticed little to NO GP Prec in Bob Hunts electric model in flight, and from what I can recall there was very little in the way of rudder offset.

[Chris McMillin]

And a 4-blader, a 5-blader or a contra-rotating 3 or 4-blader???
The later must have huge amounts of precession, just look at the empennage design of a Westland Wyvern. Dihedral in the stab, finlets and a huge vert. fin/rudder.

Hi Doug,

The Wyvern has a big rudder and fin because of the propellers mass, and the fact that it and most propeller driven airplanes of that era are too short in tail moment to be very stable. It's the same reason the Red Baron RB-51 racer had to have it's fin enlarged even though it ran a CR propeller.

The CR propeller blade work to cancel gyroscopic precession.

Chris...

[RandySmith]

And a 4-blader, a 5-blader or a contra-rotating 3 or 4-blader???
The later must have huge amounts of precession, just look at the empennage design of a Westland Wyvern. Dihedral in the stab, finlets and a huge vert. fin/rudder.

A 4 blade Contra rotating prop has almost zero GP

Randy

[Brett Buck]

And a 4-blader, a 5-blader or a contra-rotating 3 or 4-blader???
The later must have huge amounts of precession, just look at the empennage design of a Westland Wyvern. Dihedral in the stab, finlets and a huge vert. fin/rudder.

  Assuming the props are properly designed and the mass properties are right, the precession is ZERO for a contra-rotating props.

    Brett

[Doug Moon]

A 4 blade Contra rotating prop has almost zero GP

Randy

And the Badass factor would be off the scale!

[Eric Viglione]

And the Badass factor would be off the scale!

Yeah, and so would the noise I think! 

[Brett Buck]

Yeah, and so would the noise I think! 

  Better have fast fingers to flip it through compression, however.

    Brett

[RandySmith]

  Better have fast fingers to flip it through compression, however.

    Brett

Spinner  flip  works !

[Matt Colan]

A 4 blade Contra rotating prop has almost zero GP

Randy

In one of Windy's videos, there's a flight of Randy flying a Magnum with Contra-rotating props back in the early 90s.  The sound and coolness factor was up there,even on video!!

[PJ Rowland]

If its electric just press the Go Baby Go button.

[Howard Rush]

AFAIK (and I may just be bloviating here; I ain't gonna do the math), the major difference between a spinning stick and a spinning disk with the same radial distribution of mass is that the spinning stick will tend to deliver it's torque in thumps (well, more likely as a raised cosine w.r.t prop position).  The whole behavior of a gyroscope is really the ensemble average of the behavior of a collection of point masses; in the case of a stick vs. disk the difference is that the instantaneous behavior is different than the time-average, so a two-blader would induce more vibration.

Yup.  The general equation for moment from a spinning prop in a corner has a bunch of jive in it.  I was hoping you would tackle it.  Maybe Tom will.  I'd like to be able to plot yaw moment vs. time for different props, RPM, and such.  Brett probably does this stuff every day at work.

[RandySmith]

In one of Windy's videos, there's a flight of Randy flying a Magnum with Contra-rotating props back in the early 90s.  The sound and coolness factor was up there,even on video!!

I flew the Magnum at the NATs and other contest with the contra rotating props.
The Magnum was flown in many many configurations, It has a skinny fuse and when I ran big motors (64) and 13 inch props it had very bad GP, you could watch the wheel pants moving fore and aft.. When  I set it up with a contra prop, that all went away completely, It was so clean in the corners and rounds that it really was transformed into a far superior Plane from before.
I still have it, and it maybe used again as a test bed for another system.

Randy

[Brett Buck]

Yup.  The general equation for moment from a spinning prop in a corner has a bunch of jive in it.  I was hoping you would tackle it.  Maybe Tom will.  I'd like to be able to plot yaw moment vs. time for different props, RPM, and such.  Brett probably does this stuff every day at work.

     Oh, no, I am arguing over whether you should define the requirements for a part before buying it, or after. Surprisingly, I am not  winning that one right now.

    Brett

[Tim Wescott]

     Oh, no, I am arguing over whether you should define the requirements for a part before buying it, or after. Surprisingly, I am not  winning that one right now.

I'll bet that you're on the side of boring predictability.

You know, trying to negotiate the requirements for a part after it's bought and paid for creates a lot more jobs.  For lawyers.  So it's not all bad.  It keeps them from splashing mud over the tail lights of their Escalades and M3s and going around braking hard at intersections.

[Howard Rush]

     Oh, no, I am arguing over whether you should define the requirements for a part before buying it, or after. Surprisingly, I am not  winning that one right now.

I was assigned to write the requirements for the last-generation 737 flight controls.  I set a personal goal of getting the requirements written before the first flight, and I almost made it. 

[john e. holliday]

All I know is the more blades, the more cuts you  get before ralizing you are being cut.

[Will Hinton]

And just how do you know that, Doc?

[Dennis Adamisin]

And just how do you know that, Doc?

I think he learned...."first hand".   

[Bill Little]

I always start my engines standing up.  I flip the plane over if it has an inverted engine.  I always tell my "helper" what we will do beforehand.  Once started, I get behind the engine and turn the plane over (if needed) while the helper gets a new hold on the model.  THEN I remove the battery.  The helper holds the plane standing up until I give the release signal when they then put the model on the ground and release in one motion.  Since this is usually Aaron, we have it really well worked out and the only "bite" has been when an engine kicked back and the Bolly prop cut my uncovered finger.  No stitches, though.  I will not reach over, or around, a prop under any circumstances.

One thing I noticed about the only electric system we have is that you do not ever get near the prop and the model never needs to be turned over! LOL!!

BIG Bear
RNMM/AMM

[Douglas Ames]

These contra-rotating motors would be killer for a Pro Stunt Ship, but they're above my skill level and paygrade. 
The total motor pkg. might be a little heavy though?

http://www.maxxprod.com/mpi/mpi-266.html

[Tim Wescott]

One thing I noticed about the only electric system we have is that you do not ever get near the prop and the model never needs to be turned over! LOL!!

Yet I have heard stories (and seen icky pictures, and the scars on my RC club president's arm) that attest to the fact that electrics will bite, too.

You gotta treat those spinning steak-knife thingies with respect, no matter what's doing (or about to do) the spinning.

[Bill Little]

Yet I have heard stories (and seen icky pictures, and the scars on my RC club president's arm) that attest to the fact that electrics will bite, too.

You gotta treat those spinning steak-knife thingies with respect, no matter what's doing (or about to do) the spinning.

I understand that part, Tim.  I have a whole lot of respect for things that can cut off body parts!

BIG Bear
RNMM/AMM

[john e. holliday]

And just how do you know that, Doc?

Will, you want to see the scars?    Dennis is partly right.

[Doug Moon]

I get it that GP yaws the model in and out during the turn and the mass of the prop is one of the larger reasons for this.

What is it that causes larger diameter props to open up the corners?

And why is it that I can run a 12.5 3 blade and have pretty stout stick force but a larger 13.25 2 blade will have less stick force in the corners even though the diameter is larger?  I can guess at it but I would rather just know.

[bob branch]

Just a note on the noise factor, they are not noisy. Counter rotating props have been used with considerable success in world championship RC Pattern competition with electric power and they have very strict noise level limits.

bob branch

[Will Hinton]

Doc, I'd rather not - I don't even like to look at my own!  (Ask Jeff Traxler about his red trimmed finish on his old profile.)

[kbf1kbf1]

In one of Windy's videos, there's a flight of Randy flying a Magnum with Contra-rotating props back in the early 90s.  The sound and coolness factor was up there,even on video!!

 

Anyone know if this video or other videos of C/L models with contra-rotating props is on YouTube?

[Matt Colan]

 

Anyone know if this video or other videos of C/L models with contra-rotating props is on YouTube?

I could put it on Youtube, but the video is at my Grandparents, so it will be a couple days before I could do it.

[dale gleason]

I'm having trouble seeing the apparent advantages to be had with a reverse rotation prop in stunt applications. A number of people who don't fly have pointed out how it is extremely positive in overhead tricks.

The GP is in an unfavorable direction when cornering on outside tricks....is this correct? If it is, since there are more inside tricks than outside tricks in the pattern, standard rotation would seem more better, to me.

Speaking redundantly, it looks like reverse rotation lends itself to improved outside cornering, but there are more inside corners in the pattern......

I hope someone can shed some light on this anomaly before I get my new electric ship designed which will be flown clockwise.

dg

[Bob Reeves]

Hi Dale,

As one that recently made the transition with my 4 stroke I may be able to help. What you are saying is correct however the two most demanding corners in the pattern are the two at the top of the hourglass, both of which are outsides. I really didn't notice any difference on insides but sure noticed the difference at the top of the hour glass and the vert eight.

[Brett Buck]

Hi Dale,

As one that recently made the transition with my 4 stroke I may be able to help. What you are saying is correct however the two most demanding corners in the pattern are the two at the top of the hourglass, both of which are outsides. I really didn't notice any difference on insides but sure noticed the difference at the top of the hour glass and the vert eight.

  The effects of yawing are asymmetrical. If it noses out, the line tension is (at least temporarily) increased, increasing the yaw restoring torque, limiting the motion. If it noses in, the line tension goes down, reducing the restoring force, and allowing more motion. So you can tolerate A LOT of nose-out than nose-in.

    Brett

[dale gleason]

I toiled late into the night last night trying to get this figured out (assymetric recovering forces)....I didn't have much luck. I'm wondering about  trying a ROJett 67 with a reverse crank (was a Carrier possibility a few years back), but the lack of "cutting edge" props is problematic.

dg

[Brett Buck]

I toiled late into the night last night trying to get this figured out (assymetric recovering forces)....I didn't have much luck.

   If the line tension was constant, the restoring torque from the leadouts is

     line tension*distance from CG to leadout guide*sin(yaw angle)

   and just define a value K as the line tension*distance so you have a "coefficient of restoring force" , so for small angles this becomes:

      K*yaw angle.

  In unperturbed level flight for my airplane, say, the value of K  is about 10 lbs*28" or about 280 in-lb/radian. But the line tension is a function of the yaw angle, too, and it goes down as the yaw angle goes negative (nose-in) and up as the yaw angle goes positive (nose-out). So, the K is not really a constant, but changes as the yaw angle changes- so the value of K goes down for nose-in yaw and up for nose-out yaw. For example say 9 lbs at some nose-in yaw angle, and 11 lb at some nose-out yaw angle. For the nose-in case K goes down to 252 in-lb/rad and the nose-out case it's 308. Asymmetrical.

    Of course, if you knew how much the line tension changes as you change the yaw angle, you could put that function in where the line tension goes, have some function of the yaw angle and then simplify and get rid of the need for the "constant" K that isn't really a constant. In reality that isn't a known function - not because it's magical but because we just haven't worked it out. There are other restoring torques that aren't asymmetrical, like the fin/rudder and dihedral effects, and others that are not simple functions of the yaw angle (like, some of them have to to with the yaw rate and yaw angle past history)  so to build up the entire equations of motion is a very difficult task.

   Brett

[Howard Rush]

I'm fixing to measure line tension in flight, but I have a contest coming up, so it will be awhile.  I guess the range of pitch rates we fly is too big to assume linearity of delta line tension vs. pitch rate.  Hence Keith Trostle's extremely clever cam-programmed rudder on his Bearcat. 

[Douglas Ames]

Is there a need for such a thing as a reverse Rabe rudder for a reverse running prop.?
...or would it just be a trimming nightmare?

[Brett Buck]

I'm fixing to measure line tension in flight, but I have a contest coming up, so it will be awhile.  I guess the range of pitch rates we fly is too big to assume linearity of delta line tension vs. pitch rate.  Hence Keith Trostle's extremely clever cam-programmed rudder on his Bearcat. 

   The best feature of Keith's mechanism is that the degree of adjustment is more or less infinite, meaning the movement can be made relatively small. That's not possible with the original, and most people wind up with WAY too much travel.

   Brett

[PJ Rowland]

Howard : Why do you want to measure line tension ? To be of what benifit ?

I have a pretty good idea of force.. this is based on a more simplistic measurement device known as a scale.
I wanted to know how much line tension I was feeling in the process of flying the bomber. DIRECTLY after one flight I had someone inside the circle with such a scale, whereby I said " pull till I tell you when its the same "..

I was staggered at how much it was..

To be honest I havent done a comparision of a modern ship, but I could give a fairly accurate level for what I feel.

 

[Larry Cunningham]

The "counter-rotating" prop I saw in a video had the second prop just free wheeling.. It certainly looked cool, and seemed practical, but I don't remember it being overly noisy.

However, how about the efficiency of such props, free wheeling or driven? I know we tend to not be terribly concerned with propellor efficiency in CL Stunt (exception for electrics, right). But it seems like a lot of energy would be lost just correcting the airstream.

Of course, with a driven version, you have the gears associated with the reverse drive, which means mechanical loss of efficiency not even considering the aerodynamics of the system.

Interesting thread.

L.

"Sometimes you get shown the light in the strangest of places if you look at it right." -Jerry Garcia