rcacro

Here's another way IMHO tothink about coupled gyros. The gyroscopic force operates in the plane of the rotating mass. Take a flat horizontal plate and mount a gyro mass (spining wheel) with its plane parallel to the horizontal plate (on a short axle). Mount a second gyro on the opposite side of the plate ( spining in the opposite direction). If you attempt to tilt the plate, it will resist the angular change. Moving the plate up or down will not cause such a resistiveforce to be generated if the plate is maintained in a plane parallel to it's original position. It can also be moved right or left without generating a resistive force( think of the off balance washing machine drum).
The yaw and pitch changes in our model are are causing the plane of the prop disc(s)to be tilted resulting in a resistive force that will oppose the change.

John W.
Scottsdale, Az

serious power

Hi Jim/Malcolm,
Jim your two gyros stacked(fixed together) is the same as Malcolm's experiment.
It has been done in a lot of school/college labs.
They either add or cancel depending on if they spin the same or opposite.
It is difficult to stop thinking about this stuff once you start.
In the link below there is a chat about this.
There is lots of stuff out there, but I choose this one as in it there are a group of guys some of who have weird and wonderful ideas. A bit like us.

See Syd post 10 and follow him down. - I have no idea who he is.

http://www.physicsforums.com/showthread.php?t=173215

We should leave this thread to Brenners drive.
Sometimes less is more.

Brian

Dsnow

Hi Jim/Brian, I’m going to keep this short since I am not up on all the math but here’s a video with two gyros in a rigid frame showing the effects when spinning in same direction and then in opposite directions. http://www.youtube.com/watch?v=bzbVwiIeM0M

What do you think?

This was the best example I could find on counter, not contra, rotating gyros and shows the forces canceling out. Does the gyro alignment make any difference? At this time I do not think so since it's in a closed system, it would be great if someone could find another video showing different results. Question: if having two contra rotating gyros cancel out the precession but maintains or adds gyroscopic stability why aren't some mechanical gyros built this way?

One gyroscope that has been left out of the talk for the contra system is the motor itself. Is the motor gyroscopic effects small or large, I do not know but maybe someone can do the math and then compare these forces to the gyroscopic forces from a single prop. I would hope that the motor forces are small and the prop is large because then with the use of the contra gyroscopic forces would be reduced.

Assuming the gyroscopic force of the props is relatively large, compared to the motor, and are canceled this loss of gyroscopic stability could explain why the elevator and rudder are more sensitive. This should be good news, now this is one more force/load out of the system that the plane does not have to be designed to fight but at the potential loss of some stability. Any thoughts?


Dave

serious power

Hi Dave,
You are correct about the motor.
I have run up in my hand. When I tilted it up or down I could feel the torque to one side that resulted.
It was a small force, and I tilted it more quickly than any model pulling or pushing except for a high power very aggressive snap.
The spinning parts in an inrunner are very close to the center of axis so the moment is very small.
It is there though. This was some time ago when setting up an ESC.
On my Midrex I went to great lengths to install the drive with 0 right/left thrust.
After flying I had to shim in a tiny amount of right.
Never worked it out ; perhaps 0.1 degrees or less ????

I think it is very small relative to a YS or normal motor/prop/spinner combo.
I'm sure this could e also balanced out, but by adding weight to the contra component.

Brian

serious power

Hi Dave,
Your last paragraph - you nailed it.

Re the vid, side by side is the only way to keep the moments the same, stacked is not as good.We used to mount lab ones on a U shaped Y. If the structure weights were kept low the whole thing would orbit the base and the gyros each other - nice to see. A mini solar system.
It the contra,excluding the motor , the masses are close and some on the same plane. The props have to be separated and this is a compromise I'm sure Brenner had to deal with for prop efficiency reasons.

Brian

OhD

I suspect that people will go for Brenner's drive in any case once they see how well they work. I flew one for just a few minutes and was convinced it was a winner. After thinking about it, it made sense that all those forces due to torque, P-Factor, gyro precession and spiral slipstream are making our planes a lot tougher to fly. Right thrust can't be helping when you are trying to do axial rolls. I also felt the airplane seemed more "stable" meaning it would tend to stay where you pointed it. Perhaps it was due to that plane being setup better than mine. Anyway I'm convinced the precession is cancelled out but I've been back and forth on whether there is still a stabilizing effect. If you carefully read Syd"s post he clearly proved the precession is cancelled as the spin axis moved in the direction the force was applied. What he didn't say is what happened after the force was removed. Did the spin axis remain tilted or did it return to horizontal? I'd love to know.

What have you observed? Does your plane have a more solid feel?

Jim O

serious power

Hi Jim,
I just love Brenner's CR drive.
I really came to hate the trim compromises immediately before it became available.As the models evolved they became more obvious.
With all this talk of 'instability' your question re 'solid fell' is apt,, .
It took me a while to dial down the elevator and rudder movements (for normal flight ).
I had flown my Ventura for a season with the Pletty prior to that.
I copied the model in the TX and went flying.
I put the rudder flair on straight away. The bipe is extra stable in pitch anyway.
I ended up moving the CG 10 to 15 mm forward (I already had a 'forward' CG ) from where it was.
I then dialed down both the rudder and elevator - to approx 2/3rd's to 1/2 what I had.
I also removed some of the rudder - 45mm at the bottom 0 at the top to get a more square TE. It was pitching the model going vertical up and at 45 degrees up under power.
I now have a model with a very solid feel - or perhaps more to the point it does what I tell it to do and ,often in my case, it doesn't do what I fail to tell it to do. In other words it is now very very honest.
I have gone from playing pool on a table with two rolls to playing pool on a very level table.
Now a better player player than me will take greater advantage of that - however I know it's me not the table !!
So yes my Model is flying better now than ever before.
Pitch and yaw control in rolls is incredible - it takes so little, you can dial out to so little movement that big mistakes are a thing of the past.

Brian

Brenner

Brian and Jim have some good points here in that, all this theory is well and good, but the real proof is in the flying.

I remember when we were developing the original Contra Drive mechanical design, that we theorized and speculated a lot about what the various differences, trade-offs, and benefits would be, but when I actually had the chance to fly a Contra Drive for the first time, what struck me most was all the things I didn't predict or imagine I would see. For instance:

1/.. Rolling maneuvers of all types became much easier to do because the nose of the plane didn't drop nearly as fast when I initiated a roll, so I had something like two to three times more time to apply rudder corrections as the plane rotated about its axis. This gave me a big advantage in AMA masters, because the current masters pattern has a lot of rolling maneuvers. (three rolls opposite, four of eight from inverted followed by a reversed slow roll, four of eight on a forty five, two of four on a forty five, three of four on an upline, etc...)

2/.. The next thing I noticed was how easy it was to do perfect stall turns, because there were no forces twisting the plane out of plane during the stall. This made stall turns especially satisfying to do. It also helps a lot with the figure m.

3/.. The tracking on vertical lines was absolutely sensational, because the trim of the plane didn't change as the speed of the plane dropped on long vertical uplines. Once you set the line, the plane just keeps flying up hands off. This helped me a lot on the figure M in masters, because it has 3 of 4 vertical upline rolls on both legs, and it's important not to lose heading during the rolls.

4/.. The tracking on vertical downlines was also helped, because there was no trim change from a powered upline, to an unpowered vertical downline due to the cancellation of motor reaction torque into the fuselage, so vertical downlines are plumb line straight with the wings locked in rock solid. This made the figure m much easier to do, especially on the last downline to the exit.

5/.. Also, the tracking on vertical downlines was helped because there was no trim change from negative reaction torque into the fuselage from motor braking effects. This is because each prop cancels out the braking reaction torque from the other prop. This also makes the figure m much easier to do, especially on the last downline to the exit.

6/.. The biggest surprise was the huge motor braking effect from the double props. This was a wonderful surprise, because it made it so much easier to match vertical upline speed to vertical downline speed. This also makes it much easier to score well on the figure m because the timing and rhythm of the rolls are now the same on the uplines as well as the downlines. It also helps everywhere else as well.

Then there was all the stuff that I did expect, like tracking through outside loops, tracking across the top of humpty bumps, etc.

Anyhow, my point is that, theory aside, there's no way that I'm going to go back to flying single prop setups now that I know how a Contra powered plane can fly.

Brenner ...

serious power

Hi Dave,
I was dashing out the door last night when I saw your post. I was on my way to meet friends for a few beers and a game of pool actually. One must get the priorities right .
I did not have time to get into this question then.
I will do my best using lay-man's English ;
Two matched gyros ,spinning opposite directions, do NOT maintain or add gyroscopic stability. They ,if everything is the same (moments/speed/weight) , cancel each other - completely !!!!
They do this because all the forces/vectors are exactly opposing.
The math is simple eg; 1 + -1 = 0 2 + -2 = 0 etc.
I will try to elaborate some;
' Every action has an equal and opposite reaction '
Assuming you have a steady state (already spun up to speed) single gyro and you apply a side load to the axis (a torque) the reaction is precession. It comes out at close to 90 degrees because the flywheel is spinning - the vectors add to produce this movement. The additional force (the side load) is applied to the shaft not the spinning disc. However the side load is an additional force and the disc IS spinning.
Now if you apply that same side load to a system with 2 steady state opposing gyros what happens to the 'additional force' ??
We are all happy to say it does not result in precession !!
So what happened to the 'equal but opposite reaction'
Does it speed up the spinning components - No !
Does it slow them down - No
Is there a mini black hole between them absorbing the additional force - No

The shaft displaces proportionally to the side load. That is all there is to it .

Hope this helps some.

Brian

Brenner

Brian's exactly right.

All of the gyroscopic forces, motor torques, braking torques, and p-factor effects are still there. It's just that they are all contained inside the Drive, so the Drive still needs to be strong enough to resist them, but the airframe no longer needs to.

This is one reason I'm concerned about using props that aren't stiff enough to resist the bending loads, because there is a potential risk that the front and rear props will bend into each other and contact during snap rolls.

Brenner ...

wingster

My Feedback: (26)

Brian

'"The shaft displaces proportionally to the side load. That is all there is to it."


Since the two props have a small seperation along the shaft, the two forces would result in a torque(not a twisting toque) on the shaft and therefor the airplane.

serious power

Hi
I am describing a ' hypothetical ' dual flywheel contra-rotating gyroscope. As if such a thing might exist. Geez !
I could have been put it better than I did though . It's not easy.

Brenner

There is a bending of the drivshaft due to the gyroscopic moment of the two props, but since the gyroscopic moments from each prop are the reverse of the other, they cancel out, which means that the driveshaft gets bent (figuratively ..) but the plane doesn't.

Brenner ...

wingster

My Feedback: (26)

I didn't mean the plane get bent, just that it experiences a torque which results in a change in pitch or yaw. Fairly small though, since the lever arm between the two props is small.

OhD

Thanks Brian and Brenner for your great posts describing performance of your planes with the Contra Drive. I also got a similar message in an email from Dave Snow. I don't know about everyone else but I am all excited about getting mine in the air no matter what the analysis says or doesn't say. It's going to be a great year.

Jim O

TonyF

My Feedback: (92)

Well, I know what I felt when I did the test running up the model then trying to move it in pitch and yaw. No precession but definitely resistance to movement. I'll let everyone else continue the discussion.

The Contra unit is IMO the next step in flying pattern. The advantages to using it have been said here many times and I agree with the majority of them. I do feel that we will not see the absolute best performance from it until a model is designed specifically for the Contra, then tweaked over several generations. Just like the designs have evolved pre-Contra. And it must be said that since pattern competition is completely subjective and not objective, there will be numerous paths that designers will take to come up with what they feel is the best compromise. Exciting times!

Jim, come on over to the Tailwinds any time and you can fly mine. I've been flying it a lot lately and it is always fun!

Angus Balfour

People reading this would be forgiven for thinking that there are no good planes already on the market that would suit the contra. I've been flying a Valiant since late April with the contra with NO modifications and it flys great as it comes right out the box - same plane/motor/contra combo I flew at the world champs. Obviously the Wind S also goes very well on a contra. The new Oxai Ray Bird with its tall fin and slender nose would also suit the contra well. I'm not saying for one minute that we should "make do" with what we have and not develop a model specifically for the contra, I'm just highlighting the fact that there are models already on the market that work extremely well in their stock format and I think this fact has been lost.

pippy

Hi all Contra enthusiasts, I recently acquired a Nuance model for xmas and I have installed a Contra in the front of it. I have never posted a picture before but here goes nothing.I hope the contra will suit this model. From preliminary weights I have done it will be quite a light model, somewhere in the region of 4800g(10.6lbs) and that's with a battery of 1180g(2.6lbs). Enjoy.

serious power

Hi Brenner,

Did you say someplace (I looked - but did not find ) that natural aluminum - anodized is an option.

Nice Paul !

Brian

Brenner

Hey Brian,

Natural aluminum is definitely an option, but it scratches much more easily than anodized aluminum. We can also use clear anodize, but it results in a washed out grey finish that I think would only be suitable if someone wanted to paint the spinner to match the plane. In this case it would provide a nice hardened surface that would resist scratches, and make a nice base for paint.

Brenner ...

jrpav1

My Feedback: (17)

How does the contra-rotating setup compare noise-wise to a conventional single prop? Has anyone made a side by side comparison with a reasonably accurate sound meter? Just curious as I've never seen (or heard) a contra prop in person.

John Pavlick

Brenner

Hey John,

There are two measures of sound performance. In the air, and on the ground.

On the ground, Contra Drive powered planes have been sound checked at the US Nats, and at the recent World championships, and in all cases to date they have passed the sound check.

However, it's useful to note that the actual measurement you get is going to depend on several factors. Here is a partial list:

1/.. The motor you use. The Neu motor is more powerful than the Hacker C50, so it's going to test louder
2/.. The props you use. Higher pitch props are going to sound quieter on the ground because more of the blade is stalled when the plane has zero airspeed.
3/.. The gear ratio you use. Higher gear ratios are going to spin the props faster, so they will test louder.
4/.. The batteries you use. Higher C rated packs will have less voltage drop at full throttle, so you will have more power, and you will also probably test louder.

In the air the sound is more subjective than anything else. While the sound is different, it's not louder than existing single prop inrunner setups. In fact, I've been at lots of contests were some of the inrunners were louder, and lots of times people don't even notice when a Contra powered plane is flying.

However, it's definitely true that Contra powered planes have a different sound, and if you haven't heard one before, you will notice the difference when you see one fly for the first time. Sometimes you hear a syncopating sound, but usually not. The 22x18 props might syncopate a bit more than the other props, but if you don't listen for it, you probably won't hear it.

Brenner ...

jnred123

Brenner
How do think the Chip style split rudder would work on an Integral with the contra.
You could vary the thickness of the TE by having it slightly open at neutral to get the on centre feel dialed in, or does it only work as I believe Chip uses it as a single sided movement with the other side staying centred.
Cheers
Greg
Australia

Dsnow

Hi Greg, Chip had a nice report in post #338 on page 14 and flew the Visa with and without the rudder split, must be nice to have that option.

Dave

Brenner

Hey Greg,

I think the the split rudder should work quite well. I wonder if the split could be mixed with throttle. Also, using just one side should go a long way to reducing rudder sensitivity and making the rudder more precise.

Brenner ...