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Contra Rotating Propeller Drive for f3a 2m Pattern Planes

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Old 07-02-2010, 02:27 PM
  #26
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Anthony,

Thanks for the congratulations. I've got you on the list. The next steps are to get flying experience with the Drive, and also evaluate reliability. Then, if there are measureable benefits, (ie; how close can I get to Bobby Satalino, and Mike Meuller ...) Mike Gaishin and I will finalize the design and make plans for a first production. run.

The design we have now is expensive because it's all CNC machined. If the demand is there we would really like to get the cost, and maybe also the weight down as much as we can. The idea is to have something that people can use with their existing airframes, and with their existing motors. (if they have a Hacker C50-14XL that is ..)

I've been flying the Contra all week, and I can now report that with the 22X18 front and rear props I am drawing pretty much exactly what I draw from my single 21X14 prop Hacker/Integral. In the future we might experiment with different gear ratios in an attempt to bias the performance more towards lower battery consumption rather than vertical performance. The neat thing is that this is the kind of thing that is possible when you have control over the design of the gearbox. Right now my calculations are showing that the Contra Drive should have about a twenty percent performance gain in vertical performance over the aforementioned 21X14 single prop setup, and this is with similar power draw from the battery packs. So, there is some potential design margin available to reduce battery consumption and still have more flight performance than current systems. My thinking here is that if the draw from the battery pack can be reduced enough so that a 4350 mah pack can be used, then it would be possible to put together a system that is several ounces lighter than current single prop systems.

Some people might question how it's possible to actually have better performance with the same draw from the battery packs. The answer is actually quite straight forward. Propeller efficiency goes down as a prop becomes more heavily loaded, and goes up as a propeller becomes more lightly loaded. This is why wind turbines have such huge blades, and why human powered aircraft use huge diameter props that turn at very low speed.

Anyhow, when I go from a 21" diameter prop to a 22" diameter prop the propeller efficiency increases, and then when I go from a single prop to two props, the efficiency increases again because now each prop is only loaded half as much as the single 21X14 prop was on my old Integral. Then, since each prop only has half as much load, it spins at lower rpm, and doesn't need as much structure, so you also save weight. (this is on a prop by prop basis. In application you actually end up gaining weight because now you need two props instead of one)

Another interesting thing is how quickly I'm adapting to the new system. When I first flew the Contra Drive, one of the things that struck me was how different the feel was between the Contra and my single prop Integral, even though the airframes were identical. The overall feeling was that I was flying a significantly larger plane, because all of its behaviors seemed to be happening slower. Now it feels perfectly natural. I'm still working on changing a few habits though. One persistent problem I have is that I sometimes unexpectedly climb during my rolling maneuvers because I keep applying rudder and elevator too early. The problem is that I still don't trust the plane to maintain altititude during the roll. It'll just take a little bit more work.
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Old 07-02-2010, 03:08 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Wery exiting this project!!!
how is the sound/noise on the setup so fare???
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Old 07-02-2010, 08:10 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

The noise is actually quite interesting. On the ground when the plane is being held it doesn't sound much different from a typical 2m elctric pattern planes, but as I begin a take-off roll, and the plane picks up speed, I begin to hear a synchopating whir sound from the two propellers as they pass each other at 7,000 plus rpm. This noise gets louder as I advance the throttle and the plane picks up speed, and I when I turn around and do a high speed pass down the flightline it sounds the loudest. However, when I turn around and enter the box, this synchopating sound drops off, and when I pull to vertical line with full throttle, it doesn't sound very noticeable at all.

My current thinking on all this is that when the plane is being held staticaly the props are stalled along a significant portion of the blade length. This means the airflow under these conditions is very turbulent, and this turbulence dampens out any synchopating rhythm sound. Then as the plane picks up speed the props become unstalled and as the flow reattaches to the blades you start to hear a synchopating rhythm.

This synchopating rhythm sound is very noticeable when the plane is flown close to the flight line, but not so noticeable when the plane flies out at typical pattern distances, so I'm assuming that it's a component of sound that doesn't carry very far, just like when you can't hear any of the higher frequencies when a couple of kids drive past playing a huge boom box.

Another interesting thing to mention is that often times when I land I come in at steep angle with the throttle at idle, but the overall impression is that I'm diving at the field under power. This is because even though the motor isn't driving the props, the airflow past them is, and they make enough noise by themselves to give people the impression that the plane is coming in under power. Which leads to another interesting thought. One possible way to minimze flight noise is to use the Brake feature in my ESC to stop the props whenever they aren't under power. I think I'll try this.

Regardless though, I think that right now I am going to be able to pass the sound check at the Nats because the plane happens to be quietest when it's held staticaly. If I need to reduce the sound further, then I think my next step will be to try higher pitch props, and higher gear ratios in the gearbox. This should reduce static rpm, and stall a higher percentage of the propeller blades, both of which sould give me a significant noise reduction.

Oh, by the way, I've checked the sound on the ground with a friend's sound meter, and I'm getting 92 db over grass at 3 m.
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Old 07-02-2010, 08:19 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

I just disassembled the gearbox after about twenty flights to check for any evidence of wear, and all of the gears look great. In fact the gear teeth still have oxidation on them from when they were heat treated.

One interesting thing that I saw was that the centrifugal force of the spinning gearbox housing is causing grease to pool at the outer rim of the gearbox housing, which suggests an interesting possibility.

What I'm thinking is that with the right combination of holes and passages, it should be possible to set up a continuous circulation of grease over the teeth of the gears. The planet gears would pick grease up from contact with the ring gear and then transfer it to the pinion gear where it would be flung off and returned to the ring gear. If this works it should be possible to get a lot of flights before the gearbox needs regreasing.
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Old 07-02-2010, 10:15 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Very nice explanations.

On further reading on the subject, it seems to me that normally the two props are chosen such that the front is smaller in diameter considerably, probably to allow less cancellation of each prop to the other. Could the noise be reduced if two different sized props are used?
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Old 07-03-2010, 09:25 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Yes, I think that making the front and rear props different sizes would help reduce the synchopating rhythm noise. However, making the front prop smaller would also increase its rpm, because the planetary gearbox automatically shares the power going into each prop so the rpm of the front prop would increase in order to compensate for the reduced diameter, which could possibly have a negative effect on noise.

Also, one of the primary benefits from using a contra rotating prop system would be to have each prop cancel out the gyroscopic effects of the other, and this effect might be reduced if the two props were different diameters. I'll have to look at the equations for gyroscopic moment and power to see if this true, or if the increased rpm of the first prop equalizes the gyroscopic moment regardless of differencs in prop diameter. (within reason of course ..)

My current thinking is that noise can be affected in the following ways:

1/.. Mismatched prop diameters. (see above..)
2/.. Reduce rpm, and increase stalling of blades when plane is held statically by increasing prop diameter, pitch, and gearbox ratios. (reduces ground clearance and reduces performance at slow speeds)
3/.. Reduce rpm, and increase stalling of blades when plane is held statically by increasing pitch, and gearbox ratios. (reduces performance at slow speeds)
4/.. Increase separation distance between front and rear props. (reduces counter whirl effect, and distance is limited by need to keep the length of planes under 2m)
5/.. Mismatch prop pitches. (This would mean that each prop has a different operating point, and would spin at different rpms, both of which might reduce efficiency)

Right now I'm thinking that the thing I can most easily trade off is low speed performance, because with the power available I currently have no performance issues at low speed, so this leads me to think that increasing prop pitch and the gearbox gear ratio is the best place to start. I figure I can drop about 200 rpm by going to a pair of 20X20 props and still maintain prop efficiency and flight performance in the air.
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Old 07-03-2010, 12:32 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Brenner,

I believe that most setups like this use a higher pitched prop in the rear as the front prop has accelerated the air. What you may have it the rear prop cavitating in the wash from the front prop.

I would try leaving the 2018 up front and the 2020 in the rear.

Good luck, eagerly watching this,

Chuck Hochhalter
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Old 07-03-2010, 01:08 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

As mentioned, measuring rpm with an optical tach is problematic as it won't separate the individual props. Likewise, the planetary gears will balance the load - so rpm may be different even with the same props (as in Chuck's scenario). One way to sort this out is to measure the rear prop rpm using an EagleTree Hall effect sensor and motor rpm electronically. With those data the rpm of each prop can be calculated. Add an ET pitot tube and also obtain in flight airspeed.

Does it make a difference which prop turns which way? While gyroscopic precession is removed (balanced) and probably P-factor also, spiral airflow is still present (maybe less) as the rear prop will dictate this. Would be interesting to determine the effect of each on flight.

Lots of things to investigate. If you need winter testing time - something can be arranged.
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Old 07-03-2010, 10:31 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Chuck,

You are right about about the airflow over the rear prop being faster than the airflow into the front prop. I was considering this when I originally designed the Contra Drive, and I calculated that in a steady state vertical upline the flow over the back prop would be about 10% faster than the flow over the front prop.

When the plane is held statically there is a much larger difference because the flow essentially accelerates from zero to the blast of air that we all experience when we try to hold a plane at full throttle. During a steady state high speed pass the change in flow velocity is much smaller, probably less than 1%.

My original thinking was to try and optimize the front and rear props so that the pitches are matched during a steady state vertical upline, because this is the flight condition where we spend most of our time at full throttle, and under these conditions the best combination should be the 20X18 on the front, and a 20X20 on the rear like you said.

However at the time all of this was just theory, and we had genuine concern about low speed performance with high pitch props that was harder to predict because the props were going to be partially stalled until the plane got up to speed, so we made the decision to stick with a single pitch and use it as a starting point. Of course in hindsight, now that I'm flying the Contra, all my concern about low speed performance has turned out to be a bit foolish, but regardless, using a 22X18 on both the front and rear has turned out to be an excellent starting point, and I am extremely pleased with the performance of these two props so far.
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Old 07-03-2010, 10:50 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Earl,

I really like your suggestions about the Eagletree system. The only thing that's missing is a way to quantitatively measure spiral airflow. I can probably measure it on the ground with streamers, but measuring it in the air is a different sort of challenge. Regardless though, I think your suggestion about the Eagletree system is something that would be very helpful to use before we decide on what we'll make for our second prop.

The things that are affected with a Contra Drive are as follows:

1/.. P-factor from prop blades having unequal angles of attack is cancelled out.
2/.. Motor torque into the airframe is divided by the gearbox gear ratio.
3/.. Gyroscopic moment is cancelled out
4/.. Spiral airflow over the wings, stab, & rudder is straightened.
5/.. Propeller efficiency is increased by 10%
6/.. It makes a funny noise when the plane flies. (not loud, just funny...)
7/.. It's much harder to stop the props from digging into the grass when you drag your plane back to the pits.
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Old 07-04-2010, 12:29 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

The funny noise probably is from high order harmonic sound waves and could be relatively high pitched (see [link=http://www.rcgroups.com/forums/attachment.php?attachmentid=2815700]this URL[/link]). The cited increase of noise level could be 30 dB at the axial direction. Decreasing RPM probably is a good idea to conceal the extra noise out.

Since the detail of the drive has not been revealed, this question may be ir-relavent. Would it be possible to try different combinations of aft and/or foreword props on the noise etc. design parameters?
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Old 07-04-2010, 02:10 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Excellent reference; a lot of of what's described in this paper confirms what I'm currently seeing with the Contra Drive that I'm now flying.

One thing to note however, is that the 10% increase in propeller efficiency that I cited is what I calculate I'm getting by increasing the prop diameter from 21" to 22", and dividing the available power between two props. Currently I am not making any attempt to estimate the potential increase in efficiency from the recovery of energy from straightening the spiral airflow.

I had hoped to see this show up as a reduction in power draw from my battery pack, but I'm not seeing this. For all intents and purposes my power draw appears to be unchanged from when I was flying my single 21X14 prop on my old Integral. However, this doesn't mean that that the spiral airflow isn't being straightened. It just means that the energy recovered is either too small to show up, or it's being used up in some other way. For instance, it might be lost because the prop pitches are mismatched. If this is the case I'll see it as either an increase in performance, or a reduction in battery power draw when I try a 22X20 prop on the rear.

As far as the spiral airflow is concerned, there is some indication that it is being straightened. This is because of the very significant increase in rudder effectiveness that I'm seeing. In order to reduce this effect and make the plane more flyable, I've had to reduce the rudder travel to less than 30 degrees, and increase my rudder expo to 75%, and I might have to do more.

As far as noise is concerned, I believe that the concerns that are expressed in the literature are comparing two counter rotating props to two contra rotating props. In this case the power loading for the props in each scenario doesn't change very much.

In my case I am going from a single prop to a pair of props that each have half the power loading of the original single prop, and what I'm experiencing is really a change in sound quality, rather than in increase in sound power. Regardless, I still have to be able to pass the sound check at the Nats, so I can't have a system that is dramatically louder than what we currently fly.
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Old 07-04-2010, 08:40 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes


Quote:
ORIGINAL: Brenner

Hey Earl,

I really like your suggestions about the Eagletree system. The only thing that's missing is a way to quantitatively measure spiral airflow. I can probably measure it on the ground with streamers, but measuring it in the air is a different sort of challenge. Regardless though, I think your suggestion about the Eagletree system is something that would be very helpful to use before we decide on what we'll make for our second prop.

The things that are affected with a Contra Drive are as follows:

1/.. P-factor from prop blades having unequal angles of attack is cancelled out.
2/.. Motor torque into the airframe is divided by the gearbox gear ratio.
3/.. Gyroscopic moment is cancelled out
4/.. Spiral airflow over the wings, stab, & rudder is straightened.
5/.. Propeller efficiency is increased by 10%
6/.. It makes a funny noise when the plane flies. (not loud, just funny...)
7/.. It's much harder to stop the props from digging into the grass when you drag your plane back to the pits.
Hi bernner,

You write: "Gyroscopic moment is cancelled out",

I know that people prefer larger diameter props (heavier) in purpose to increase the gyroscopic effect in aim to increase the plane stability, especially in gusty winds.

Can you refer to that ?

Best Regards,

Isaac Najary
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Old 07-04-2010, 09:01 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Isaac,

Interesting thought. I've always though the the primary problem with gyroscopic moment from rotating propellers is the yawing effect on the aicraft when you pull or push with the elevator. The idea is that when the plane is rotating about an axis, gyroscopic moment manifests itself as a torque that's applied to the airframe along an axis ninety degrees from axis that the plan is looping around. This is the effect that I'm referring to. I've never considered that there might be a stabilizing effect in heavy winds.
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Old 07-04-2010, 11:59 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Quote:
ORIGINAL: Brenner


As far as noise is concerned, I believe that the concerns that are expressed in the literature are comparing two counter rotating props to two contra rotating props. In this case the power loading for the props in each scenario doesn't change very much.

In my case I am going from a single prop to a pair of props that each have half the power loading of the original single prop, and what I'm experiencing is really a change in sound quality, rather than in increase in sound power. Regardless, I still have to be able to pass the sound check at the Nats, so I can't have a system that is dramatically louder than what we currently fly.
I was surprised as well the noise increase from CRPs could be 30dB. Probably the noise got cancelled in certain spots in SRPs. Just need to make sure your measure covers the axial direction, which is the worse spot.
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Old 07-05-2010, 08:10 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes


Quote:
ORIGINAL: Brenner

Hey Isaac,

Interesting thought. I've always though the the primary problem with gyroscopic moment from rotating propellers is the yawing effect on the aicraft when you pull or push with the elevator. The idea is that when the plane is rotating about an axis, gyroscopic moment manifests itself as a torque that's applied to the airframe along an axis ninety degrees from axis that the plan is looping around. This is the effect that I'm referring to. I've never considered that there might be a stabilizing effect in heavy winds.
Concur that is an interesting thought. I can't say as I have ever noticed a positive effect from gyroscopics though. It seems to me that IF the plane flew through some turbulence oriented in the pitch axis, a larger gyroscopic effect might translated to the yaw axis (when using large diameter / heavier props)? It is very clear to me that the larger diameter / heavier single props do increase gyroscopics (negatively) in both pitch and yaw.

Regards,
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Old 07-05-2010, 07:34 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Brenner,

Nice work and congratulations! A nice in flight streamer test for flow around the fuse might be well documented with a keychain camera...I think Earl used one of these at one time inside his airplane (with streamers) to get a visual of cooling air flow through the cabin during a flight.


-mark
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Old 07-05-2010, 07:59 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Thanks for the congratulations Earl,

I agree, some sort of in-flight streamer is probably the way to go. It'll have to wait until after the Nats though.

Brenner ...
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Old 07-06-2010, 07:18 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Actually that was Mark, but he makes a good point. Swan makes a cool video camera with good resolution and plenty memory that weighs only 20 grams & is about the size of a pack of gum. Li battery charges from USB. Pretty handy.
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Old 07-07-2010, 09:11 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Here are some pictures taken at the flying field. Of particular interest in the picture of my rudder. This shows the rudder at full deflection. I also have 75% of expo on rudder as well. I figure it's about half the throw, and 30% more expo than what I had on my single prop Integral.

If anyone wants to ask me why, I have to admit that right now I really don't know for sure. There's something about using the Contra Rotating Drive that has made the rudder much more responsive.
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Old 07-08-2010, 07:22 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

I would love to see your weight and balance spread sheet. How heavy is the prop/gearbox, etc., and where is your cg? i've never seen an electric with the battery pack that far back.

Jim
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Old 07-09-2010, 09:55 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Jim,

The plane that I'm taking to the Nats is going to have the weight comparision shown in the attached graphic.

However, one thing I need to mention is that this is our first prototype, so it's not what we consider to be our final design. Our design target is to get the total system weight to something below 28 ounces. How successful we are will depend on what we can do to replace CNCed parts with tooled parts.

The CG that I currently fly at is 6.5 mm behind the leading edge of the wing tube. I currently have two battery trays. the one show in the pictures, and another tray that is fitted over the landing gear plate. The one in the picture allows me to move the battery pack all the way back to the front of the wing tube so that I can try different CG positions.

Another reason for me experimenting with the plate shown in the picture is that the Contra Drive moves a lot of it's weight further forward. The actual motor is pushed forward by about 20 mm or so, (note that you can't actually see the back of the motor in these pictures...) and the gearbox is actually integrated into the spinner. Also, the second prop hangs way out front on the tip of the spinner assembly.

Currently my system balances the way I want it with the plate shown in the pictures, and with the batteries pushed to the front of this plate. One side benefit is that the batteries are now much easier to install and remove from the plane. Also, I can get easy access to the wing bolts by undoing a wing nut on the front of the plate and hinging it back.

Brenner ...
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Old 07-09-2010, 11:43 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Thanks for the rundown. Makes sense and having the battery pack further back is good in some ways. Keep up the good work and I hope you knock 'em dead at the Nats with this setup. Then you'll really have to go to work on it so we can all have one.

Jim
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Old 07-11-2010, 10:51 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Thanks Jim,

I took the Contra to Evansville this weekend for it's first contest, (we got all the rounds in on Saturday so now I have an extra day to prepare for the Nats..) and I am very pleased. When you fly at the practice field it's all too easy to just imagine that you're flying better, but flying actual contest rounds and being scored by your peers is the final arbiter. Based on my experience at Evansville, I think I can definitely claim the following:

1/.. Spins and snaps are much improved. For as long as people have watched me fly, my spins have always been an achilles heel, and I can honestly say that now I don't know what I'd do to improve them further. Same thing with my snaps. The Contra makes the actual spin more controllable, which makes it easier to hit my exit, and now I can actually do a spin entry like it's described in the rule book.

2/.. My half cuban with two of two is improved, because now it's easier to maintain my line when I do the two of two.

3/.. My half square on the corner with half rolls is a lot easier to fly because now I can make the middle inverted corner without losing my line. I ascribe this to better vertical, and no trim changes when pushing inverted.

4/.. It's now easier to make the eight sided loop look good because I can fly each of the sides at constant speed. Now all I have to do is count off in my head, and each side comes out perfect. With my single prop integral I would slow down on the uphill sides, and speed up on the downhill hill legs, which was a lot harder to fly.

5/.. It is easier to do the vertical upline on the reverse humpty bump without needing a visible correction. I ascribe this to no trim changes between idle and full throttle.

6/.. Stall turns are much more fun. I really enjoy letting the plane slow down to a full stop in the upline, and making everyone think I'm going to flop the stall turn, and then just pinwheel the plane around with the wings perfectly in plane, and no wiggle on the down line. (and this is with only the rudder throw that I showed in a picture earlier in the thread...)

7/.. The figure M is easier to do for the same reason.

8/.. Point rolls and slow rolls are easier because now I can delay applying top rudder.

I also asked around and asked if anyone thought that the Contra was too noisy, or too slow, and no one thought so. I'm also going to poll everyone when I go to the Nats as well.
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Old 07-11-2010, 11:19 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Brenner, my flying partner Kirk Sutherland flew in the Evansville contest yesterday and spent some time with you. He was impressed by the Contra's constant speed and it's ability to stay on line, both horizontally and vertically throughout your flights. As you recall, he is and has been a YS flyer for years and is not easily impressed! Well, that all changed yesterday. Sorry, I could not make the contest but I lost my canopy during a practice flight earlier in the week. I look forward to ALL of your updates in the future. Thanks__Everette
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