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Contra Rotating Propeller Drive for f3a 2m Pattern Planes

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Old 11-07-2012, 09:38 AM
  #951
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

hi, brenner
I made a contra rotating on my cnc milling machine, a lota work, but its really good . I heared that you are interested in contra rotating, you can watch my video on youtube under mikebassam1, If you like the video then we can work things together, my contra rotating mechansim is a promising bussiness for the RC world,

mike
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Old 11-07-2012, 11:44 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Well Mike, that looks like a good strong first effort on your part to provide a propulsion system for a 40 size sport plane. Is your unit attached to an outrunner motor? Regards,Everette
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Old 11-07-2012, 12:19 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Mike,

Your Youtube clips only show the front view. Can you show the side view as well?

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Old 11-08-2012, 09:56 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

wellEverette, yes it is an outrunner, I believe itis obvious from the three cablesthat coming from the front of the motor.
and thanks for your interest in my contra rotating setup, as far as its good for a 40 size well I can make them to be install on double this size, actually I can make them to be install on any RC airplane size.


regards mike,
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Old 11-08-2012, 10:11 AM
  #955
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

hi Brenner,

I hope you liked my set up, there is nothing to see on the side of the motor , its just an outrunner motor and the gears, BTW im in the prosess of making a larger one I will post it as soon as im finished.


Mike,
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Old 11-08-2012, 11:43 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

For those pilots who are using Neu f3a motors, we have encountered an instance of the motor rotor being assembled incorrectly, causing the drive splines to be recessed about 2.5mm into the motor housing. The problem here is that this will shorten the engagement of the rotor spline in the pinion gear of the Contra Drive, which in turn can cause premature spline wear.

The problem seems to have been caused by swapping the two brass spacers that locate the motor rotor axially in the motor housing, causing the rotor to be located too far back into the motor. What appears to have happened is that on the motor in question, the two brass spacers were swapped when the motor was dissassembled to replace the bearings.

I'm attaching some pictures that show what I mean. The first picture shows the motor rotor with the two brass spacers, which have unequal lengths. The second and third picture show what the rear of the motor looks like if the rotor is assembled properly, and if the rotor is assembled improperly.

If the rotor is assembled correctly, the end of the rotor shaft will be flush with the back of the rear bearing. If the rotor is assembled incorrectly, it will stick out about 2.5mm. This 2.5mm represents lost spline engagement.

We don't know if this is an isolated instance, but it is something that can happen easily when the motor bearings are changed, so I suggest that pilots who using a Neu f3a motor check the rear bearing to see if they have a problem.
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Old 11-08-2012, 11:57 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Good find Brenner!
I'm 90% certain mine stick out that 2.5mm. I'm 100% certain it came that way new - I've never opened it our had it serviced.
Dan
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Old 11-08-2012, 01:04 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hi Brenner,

Is it a simple job to swap the brass spacers around?

There's a few of your egg beaters flying down under now... Very unique sound for sure.

Regards,
Jason.
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Old 11-08-2012, 01:22 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Well Mike, I did not notice the 3 cables sticking out from the motor that would indicate it was an outrunner motor. My Neu F3A inrunner motors have 3 cables exiting the rear of each motor. So that would not be that unique. What made me suspect and ask the question was the sound that I heard coming from the motor on your Youtube video was like unto a "Turnigy" unit from Hobby King! I'm familiar with the Hobby King outrunners via asssociation with sport flyers at my field. Am I correct? Regards, Everette
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Old 11-08-2012, 06:40 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Jason,

It is if the fan can be removed from the rotor shaft, but if the motor is new, it's likely that the fan is still pressed on pretty tight, and in a case like this it's probably a good idea to ask Neu for advice.

We are actually working on a modification for the pinion gear that will substitute an 8 tooth splined bronze insert for the current 4 tooth steel spline that the Contra currently has, and this insert will be sized specifically to accomodate the change in engagement length that results from this problem.

The only downside is that separate inserts will be needed to accomodate the Neu motor and the Hacker motor, and the older version of the Neu motor that was sold before the current version will no longer be compatible.

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Old 11-08-2012, 07:28 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hi Brenner,

I just cheked a brand new motor. The shat is flush with case so came from factory correct. I never had chance to install your contra this year. Working now to get it done for next season.

Best,

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Old 11-09-2012, 01:18 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hi Brenner, sometime early in the year I told you about the splines on my Neu motor starting to wear and we decided it was due to lack of greasing while assembling the motor to contra. I did notice at that time that the rotor shaft stuck out a bit at the rere of the motor, but thought nothing of this. I have recently sent my motor to Neutronics to have the rotor and bearings replaced as the spline wear was getting very bad. I have just checked the returned motor and lo and behold the shaft is flush at the rere bearing.
Hopefully based on what you have reported in your post the splines will now last much longer.
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Old 11-09-2012, 05:53 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Brenner,
Great news on the new adaptor/insert - sign me up when it's ready. Will it fit in my version 2 (I think) drive? (I'm not really interested in upgrading to the latest version) Will it be a user-replaceable part?

I've long thought that some of the Contra noise was from clatter between the pinion and adapter. If nothing else, it clicks/clacks a lot with the motor cogging when turning the drive by hand.
I'll have to check my pinion for wear and probably get the shaft replaced and properly positioned by Neu.

Thanks, Dan
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Old 11-09-2012, 09:04 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Just checked my new Neu, the rear of the shaft is flush with the rear bearing TG. This V3 is the smoothest yet for me, very little cogging.
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Old 11-09-2012, 12:08 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes



Well Everette, you were right about the motorwas a Turnigy, but the sound in the video is not from the motor its from the air thats been torn in between the two propellers. but I still give 4**** for predicting that. I wish all turnigy motors make sound like that. BTWdo you use neu contra motor? how many RPMcan they stand? and how much is it? please. my motor with gears can stand over 30000 rpm tested full power, what you saw in the video was half of its power which was 12000 rpm honest, measured it with a tachometer. I used ceramic bearings in the motor.
I installed on my airplane and flew it in Chicago and the speed was over 90 mph battery 4000 mah 11.1 volt.


Mike,

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Old 11-09-2012, 04:12 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Well Mike, BINGO! I felt certain I was correct. To answer one of your questions, I do run the Contra Drive/Neu F3A inrunner motor combo and have been very pleased with the results I've had with the combo in 2 different pattern planes. Finally it's important that you realize that me trying to compare your Turnigy outrunner set-up to my CD unit/Neu F3A set-up is like unto comparing a Yugo automobile to a Ferrari. With all due respect, there is NO comparison. I wish your well in your future efforts, Everette
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Old 11-26-2012, 07:36 AM
  #967
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hi Brenner,
With regard to the wing fence / tip-let discussion I sent an email to Martin Hepperle to see if we might get his views. ( http://www.mh-aerotools.de/ )
This is his reply;


Brian,


In the past I have seen winglets, wing tip fences, slotted blades etc. proposed for propellers.

None of these thing was successful in the long term, even if full scale aviation tries all means to reduce fuel consumption and noise.



Such a wing tip device could improve efficiency if

- the diameter of the propeller is limited (a small increase of diameter is usually more efficient and simpler), and

- you can make sure that the tip fence is properly aligned and profiled according to the local flow.



In my opinion the second point is not manageable for model aircraft propellers. It is impossible to produce an accurate airfoil section on the tip of a conventional model propeller, even with CNC machined molds. Assume a desired airfoil shape accuracy of 0.5% or better to achieve the desired aero performance. With a tip chord of 10 mm this means that the shape of the molded propeller must be as accurate as 0.05 mm, after finishing, sanding etc. Now if the airfoil on the tip fence has only 5 mm or less, this becomes rather difficult.

Also the relative thickness of the airfoils must be in the order of 5% of the chord length, i.e. the 10 mm tip section would be as thin as 0.5 mm and the tip fence sections even thinner – with a proper airfoil shape on them.

Quite difficult to manufacture. Just cut a propeller in slices and scan them to inspect the true airfoil section.

If you flatten the tip fences to an increased propeller diameter you will probably have a better propeller and less manufacturing trouble (as long as the tip Mach number is not too high -> noise).



Because these devices are attached to the tip, they will have a big impact on power consumption – a small bit of additional drag at the tip will require some additional torque. In terms of noise it is similar – a small β€œerror” at the tip will have a much larger effect on noise than a problem more inboard. For low noise a low RPM is the best measure to take. Another, second order effect is to reduce the loading towards the tip by introducing some washout and/or reducing the chord length. The tip fences would actually increase the loading there as they reduce the natural lift loss toward the free propeller tips, like an end plate. There should be no radial flow along the blades, except if they are stalled (e.g. at static or takeoff conditions) and the tip fence cannot change this – it would have to be more inboards, e.g. at 70 or 80% of the radius.



Concerning the thick trailing edges, be it just thick, an added plate or a wedge like shape there is indeed a positive effect on the efficiency of the control surface. The dead band at small deflections is usually vanishing and the flow stays attached at larger deflection angles. So this is a useful feature, but it also creates some drag. SO good for aerobatic models, not so good for speed models.



Regards,

Martin Hepperle



Interesting !!!
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Old 11-26-2012, 10:14 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

So Brian,

The thing to ask him now is how we reprofile our existing prop tips to minimise noise which frankly I feel is a bigger issue than efficiency and what shape should a development prop be?

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Old 11-26-2012, 10:59 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hi Malcolm,
I think he has already answered that.
He is saying ,aside from a bigger diameter moving more slowly, to wash out the tip and reduce further the tip cord.;
' Another, second order effect is to reduce the loading towards the tip by introducing some washout and/or reducing the chord length. '

To my mind he is describing something very like the tips in the photo in post 902.
Noise and efficiency are closely related - if one is high the other will be low. Energy wasted by a prop,, will result in radial flow, noise and heat. The noise is measurable.

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Old 11-26-2012, 01:12 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Just to avoid overlooking the obvious. Should we be considering going to larger diameter props in the future?

My thinking is that as soon as the next generation of battery technology becomes available, and we get a step increase in specific power and energy density, we are going to hit the wall as far as passing the f3a noise test is concerned, which means that we are going to do something in tandem to be able to handle the extra power without increasing noise.

In my mind, this probably has to be a step change increase in prop diameters, which should let us absorb more power without increasing prop tip speed.

Another side to this coin is that in this future scenario, Contra Drive technology becomes quite a bit more relevant, because increasing prop diameter with a single prop setup, and at the same time absorbing more power, means higher motor torque, larger rotational flow, bigger gyroscopic effects, larger p-factor effects, etc. It's quickly going to become unmanageable.

Imagine a pair of 24x22 props spinning at 4000 rpm with an 8000mah pack. The performance would be unreal.

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Old 11-28-2012, 01:08 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Quote:
ORIGINAL: serious power

Hi Brenner,
Yes, in my view more taper to a smaller ,rounded and washed out tip would do a lot for noise.
The 'lost' pitch could be fed back the rest of the blade to maintain the same load to the motor.
It is a redesign though , but having said that it may be progress.

However if radial flow/ radial flow losses is/are significant due to our wing spinning the tiplets may still be the favorable method to getting an improvement.

Brian
Hi Brenner,
I have been giving all this some further thought - what is it about this stuff ??. BTW I'm working on the extended landing gear 2x24x22 Wow .
I do agree though re the 'next' batteries.

Anyway we have two distinctly different performance areas to consider ;
1; Static performance ;
The noise test is our only real concern here.
The high pitch/prop stall concern is in practice not an issue. It's likely that the two big slow props have less 'slip' than a single smaller fast prop and that this allows the contra to bite and accelerate effectively so take off and picking up after a snap etc is not of any concern.
So the question is what will help with the noise test and what won't.
2; In flight performance ;
Efficiency is the goal here.
If the prop design is efficient it can then be optimised for battery consumption and/or for more speed.

Picking up on what Martin Hepperle has said in his mail's I now think we should at least examine the tip region to see if there is scope for improvement.
He said, and I think he's got a point, that the tiplets can help if diameter is limited and if they are made very precisely but that this is very difficult at our prop size.
You ,correctly, have concerns about the durability of the tiplets and taking Martins comments on board they should be made very fine and so would be very delicate.
He also said that more diameter is more efficient and better.
He further said to wash out the pitch at the tip and to reduce the chord at the tip.

I now think that for the noise test the stalled condition inboard on the props is not likely to be a significant factor.
I think the load at the tips, which seem to have a lot of pitch currently, is likely resulting in a noise level that can be reduced with Martin's and your suggestion of introducing wash out.
If this is the case it is worth considering so as to get more margin for noise tests. (Note; a lot of other current F3A power plants fail without 'assistance' ! )
Assuming Martin is correct there should also be a significant in flight efficiency benefit to removing the pitch and or chord at the tips.
Again assuming Martin is correct my concern about radial flow in flight was not well founded and your own suggestion about wash out is 'on the money'. As is your one of extra diameter.
I have to admit that all this is far from my area of expertise.

I am now wondering about doing a rework on a pair of existing props as a starting point. I'm just not sure how much scope there is.
The 'clipping' that some did on the existing props was done for other reasons but it struck me at the time as introducing more pitch at the tip.
The tapers that were put on probably helped some.
Twisting ,with heat, is possible but is difficult to control - I'm open to suggestion.
Removing chord at the tip TE looks feasible combined with some sanding to restore an airfoil - this sanding would remove some pitch also.
Again open to suggestion ! (I might need some spare props !! ). It will be more delicate.

I think a single action that could help with niose tests and in flight efficiency has to be worth considering.
I know that in practice that 'it's easier said than done' - however 'nothing ventured nothing gained'.

Brian

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Old 11-29-2012, 08:22 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Brian/Brenner,

About 100 years ago when I was actively into control line teamrace we habitually used to washout the tips of our 7 x 8" Bartels glass props by removing some material from the last inch of trailing edge towards the tip in a triangle as viewed in plan and then using a flat sanding block thin back the trailing edge keeping the back of the prop as flat as possible and washing out the tip.

The result was more revs than the standard prop without losing dia and h99opefully an increase in efficiency. The problem I see with modifying a standard pair of props in this way will be an increase in revs and hence noise both static and airborne. I believe the only way this approach will bear fruit is with an increase in dia which isn't really desirable or a redistribution of blade area which might be worth experimenting with.

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Old 11-29-2012, 08:41 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hi Malcolm,
If a re-design was on the cards the load could be increased some inboard with pitch area etc.
Diameter could be increased either/as well.
I'm thinking something like in the pic,,.
10 mins sanding,
If the pitch at the tip was = 15degrees it is now 3 to 5 degrees.
I would have to twist the blade to get to 0.
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Old 11-29-2012, 09:20 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Brian & Malcolm,

The Contra Props are actually already washed out slightly along the full length of the blade. The washout is linear, and it's not much. I believe that there's pitch minus 0.5 inch at the tip, and plus 0.5 inch at the root. The labeled pitch is only exactly valid at the midpoint of the blade.

However, I don't think this is what we are talking about here exactly. From the discussion so far, I think it would definitely be worth trying to repitch the props right at the tip. I'm also tempted to try narrowing the blades, starting at the midpoint out to the tip, much like the eliptical distribution full scale prop that was posted a page or two back.

One consequence of narrowing the blades towards the tip, is that the effective pitch of the prop is going to increase slightly as the loads towards the tip are reduced. This will help to match the power loading and keep the rpm from increasing too much.

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Old 11-29-2012, 09:43 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Brenner,

Not sure I understand your comment about the effective pitch increasing by narrowing the tip. Do you mean due to the blade twisting under its pitching moment? If you narrow it from the trailing edge and sand to bring back its original thickness you will have decreased the pitch out there and so the average pitch will be decreased.

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