Brenner

Well, the Nats are officially over. I placed twelfth, and the Contra placed seventh. The reason for the slight discrepency is because only the top eight masters flyers went on to the final, and while I didn't qualify, Dave Snow did. However, Dave had a midair in the sixth round so he needed a plane for the finals, and he graciously accepted to fly the Contra in the finals in place of the plane he lost. So, it turned out that the Contra made the finals, even if I personally didn't.

All in all the Contra performed flawlessly all throughout the Nats. It chugged through nine contest rounds, and maybe a dozen practice rounds. It made weight in the finals, (4999 grams with stock Integral wings & stab ...) and passed the sound check three consecutive times. The lowest sound reading was 93db, and the highest reading was 94.5db at 3m over concrete. The reason for the high and low sound readings was because the wind was changing all throughout the day. The low reading was in the morning when the wind was calm, and the high reading was during the last round when the wind was blowing the hardest.

Lots of people commented about the distinctive "Contra" sound whenever I was flying. Apparently it's a sound that carries quite a distance because lots of pilots on site three told me that they could always tell when I was flying on site one due to the sound carrying from site one to site three. However, all in all I think the concensus was that the sound wasn't so objectionable as it was distinctive. It didn't sound excessively loud in the air. It just didn't sound like either a glow engine, or an electric motor. Also, during the sound check the sound wasn't much different from any other electric plane setup.

In addition, I think I was able to prove definitely that the Contra doesn't draw any more mah out of the battery packs than a single prop setup. I typically drew about 4200 mah, and Dave drew about 3900 mah for a complete masters flight, including takeoff, procedure turn, landing, and in Dave's case, a sound check. The reason Dave drew slightly less was because he has much better throttle management than me. I still haven't totally figured out what that left stick is for yet.

I was also fortunate to receive lots of feedback and advice that will all go into our next design iteration, but before that the grand experiment will continue, until at least the end of the contest season. My flight scores during the later rounds were noticeably higher than my scores during the early rounds, and I'm going to try and continue this trend moving forward, and if at the end of the season I can be shown to be scoring consistently higher than I did when the season started, I will give credit for my improvement to the Contra. Also, I know this will be a very fair test because I know for certain that guys like Mike Meuller and Bobby Satalino are going to do their very best to make it as challenging as possible for me to score any better than I do now.

Brenner

Attached is a table that shows relative performance of various Contra prop setups. I'm showing it because there was a general concensus amongst pilots at the Nats that I needed a setup that had more speed. The way that Dave Snow explained it to me was that just because you are capable of flying fast, doesn't mean that you have to fly fast, but it helps to have the extra speed in reserve for those days when the wind is strong, and it's a ninety degree crosswind. It was actually like that on the Sunday before the start of Nats competition, so I'm fully aligned with what he's telling me.

Anyhow, I thought that this table would be interesting to show, because it shows some of the benefits of going to a Contra setup from a single prop setup, and it shows the various trade-offs for different Prop diameters and pitches.

First off, the general conclusion seems to be that bigger is better as far as diameter is concerned. This is because it looks like efficiency is increased without any real trade-off with respect to other performances. Secondly, there seems to be a direct trade-off between prop stall speed, and maximum level flight speed. Also, it seems that there are diminishing returns with regard to steady state vertical rate of climb, with the maximum being reached with a prop pitch somewhere between 21" and 22".

All of this suggests that the best combination might be either a 22X20, or a 22X21 pair of props, or somewhere in the vicinity. However, all of this is a very quick first pass look at the trade-offs. I plan to do quite a bit more work before I make a final choice for our second design iteration.

Derek.Koopowitz

Brenner,

The sound of the motor is definitely unique and you flew it very well... congratulations! BTW, I was very impressed in seeing you grease the gears on the gearbox and it took you all of about 10 minutes total - very slick. I look forward to seeing you improve it going forward - technology improvements are wonderful.

Brenner

Dave Snow sent me the motor constants for Neu's f3a motor, and I've re-run the numbers using this motor. This motor is a lot more powerful than the Hacker C50-14XL. The attached table shows a direct trade-off between performance and current draw from the battery pack. Obviously running this motor without good throttle management is going to cut the flight short before the end of the sequence. All of this aside though, it looks like all of the same conclusions apply, except that now it looks more and more like a 22X20 prop combination is the best choice for both the C50-14XL, and the Neu f3a.

Regardless though, it seems clear that the combination of the Neu and the Contra Drive are going to make a plane go like stink.

wattsup

My Feedback: (1)

Brenner, I have a Neu F3A motor that's unemployed a this point. You are more than welcome to use it/fly it in conjunction with the future development of your gearbox etc. Note, you flew with my flying partner Kirk Sutherland at the Evansville contest a couple of weeks ago. I'm reluctant to give you my email address or home phone number via this forum but if you are interested, I live in Franklin TN and my real name is Everette Carpenter. Talk to you soon___Everette

ExFokkerFlyer

My Feedback: (4)

I'd definitely be interested in giving it a try on my Neu as well should it become available!

Tom M

Brenner

Hey Everette,

Thanks for offering the use of your Neu motor. In actual fact, Dave has already lent me one of his Neu motors. That's one of the reasons he provided me with the motor constants. I'm going to try Dave's motor with my existing setup to verify my predictions, and then probably move on to a second design iteration that will work optimally with both the Neu and the Hacker.

wattsup

My Feedback: (1)

Brenner, you are welcome. I'm sure there are a large number of Neu F3A users that anticipate your future success__Everette

joenella

Any news?

Brenner

Currently we are designing what we consider to be the final (hopefully) version of the Contra Drive.

We have received an incredible amount of useful input from lots of top flyers. Also, we now have several hundred flights on our prototype, and based on all of this we are making the following enhancements:

1/.. We are increasing the pitch of both props in order to increase the straight line top speed and the vertical upline speed.
2/.. We are also changing the gear ratio to optimize power absorption with the new props.
2/.. We are adding a labyrinth seal behind the pinion gear to seal grease inside the gearcase.
3/.. We are changing how the rear shaft bearing is retained so that it can also take reverse thrust loads. This is needed because of the higher than expected reverse thrust being generated on downlines.
4/.. We are increasing the face width of the gears to increase the gearbox rated capacity to 3,000 Watts. This is so we can also use the Contra with the Neu f3a motor.
5/.. We are reducing wall thicknesses and adding lightening holes in order to save 2 oz of weight.

When we are finished we hope to have a Contra Drive system that can make the following claims:

1/.. The speed and vertical climb will be optimized for f3a p, & f3a f patterns, which should also work for all other patterns as well.
2/.. We hope to significantly increase the number of flights between re-greasing the gearbox. Initially the first re-greasing will be sooner because of break-in, and then after that we hope that the interval can be increased.
3/.. The Contra Drive should work with either the Hacker competition motors, or the Neu f3a motor.
4/.. The total system weight, including both props, should be no more than 2 oz more than the equivalent single prop system.
5/.. It will be possible to disassemble the gearcase for service without removing the motor from the plane.
6/.. We hope to be able to increase gearcase durability to 3 times that of current gearcases.

In addition to all of this the current Contra Drive design is still going strong. I am currently competing with it in the American Midwest in the AMA masters class, and it this level my scores have increased about 30 points or so from when I was flying a single prop Integral. At that time I was scoring about 950 against top Masters flyers in the Midwest, and now I am scoring about 980 or so, and I've also started taking the occasional round as well.

joenella

THX a LOT for the update.

1 question I have an crazy idea can I use this EDF motor? http://www.megamotor.cz/v3/script/de...ge_id=lang_eng
I know it is pushing the limit a bit further, but who will not like some xtra poooowwwwwweeerrr

Reko

Brenner, first of all thank you for sharing this work on RCU. It has been very interesting to follow your progress. Fantastic engineering!

I’m curious about the analysis you have done (post 52,54). Could you perhaps explain shortly how you have modeled the propellers? I have done similar design optimizations for single rotors, and are aware of how much uglier it gets for a contra prop. I’m curious what simplifications you have done, and if you have used any specific software propeller analysis tool. Did you for instance treat the contra prop as an equivalent 4 bladed rotor, or have you found any good theories for how to optimize the blades of a contra prop?

Best of luck with the continued work.

/David

Brenner

Hey David,

Good questions. I won't bore the other posters here with a too long explanation, but what I can do is give you a simplified outline.

Initially when I was exploring the basic concept behind replacing a single blade propeller with two propellers I looked at what the performance would be with a single prop, and dual props. However, this was done without considering any interaction effects between each of the two propellers. I did this work using Motocalc software. Doing this I was able to show that significant benefits accrue just from going from a fully loaded single prop to a pair of props that each have half the loading of the single prop. Motocalc allows you to input motor characteristics, plane characteristics, and a gearbox gear ratio. It also lets you directly specify the number of propellers, which makes this type of comparison quite easy. I derived the kinematic equations for a planetary gearbox separately so that I could convert actual gearbox ratios into the gearbox ratio needed by Motocalc. This derivation was done in MathCad.

Then I used a program called Javaprop that is freely available from this website:

http://www.mh-aerotools.de/airfoils/javaprop.htm

I used Javaprop to actually design the shape and profile of the propeller blades. This website also has a set of airfoil profiles specifically designed for propellers, which I used.

Then in order to take into account the interaction effects between the front and rear blades, an iteration process is required. The general idea is that airfoil characteristics are first assumed, and then calculations are done to predict performance, which is then converted back to airfoil characteristics that can be compared with the original assumptions that were made. This process is iterated until the predicted airfoil characteristics are the same as the characteristics that were assumed. An example of this kind of analysis is shown in the attached paper.

Brenner

Joenella,

I've tried your link, but it's not working for me. Could you tell me what the model number of this motor is?

Thanks, Brenner ...

joenella

It is an EDF motor their 22/50/2 which is quite powerfull

Brenner

I took a look at the motor you suggested, and I'm not so sure that it's as powerful as the Nue f3a. It lists the max amps as 100 A, but I think it would take larger props to push it close to these limits.

Currently the motor constants for the Neu, the Hacker, and EDF-120 are as follows:

EDF-120 1120 rpm/v
Hacker C50-14XL 1160 rpm/v
Neu f3a 1300 rpm/v

In a system designed to work with the Neu and the Hacker, I'm concerned that the EDF-120 wouldn't be loaded enough.

Reko

Brenner, thank you for explaining! I realized my question could require answering with a smaller essay, but your simplified outline was more then satisfying.
Interesting paper! I recently stumbled on a website with a software using, what I believe, that iterative procedure together with an older blade element code “Xrotor� to analyze counter-rotating propellers. If you are interested I can PM you the link when I get back to work tomorrow.

/David

Brenner

Hey David,

Yes, please send me the link. I would be very interested in looking to see what they have.[/right]

Brenner ...

joenella

Any progress we should know about, like when? and maybe a budget price?

David Bathe

Can I ask you Counter R' prop guys a question?Are you able to eliminate right trust engine offset using CR prop set up?In theory, the spiralling air stream should be more of less equal on both sides of the body when using the CR prop system...thus striking the fin on both sides as apposed to the left side as with a normal prop.Is this correct in practice?

Brenner

Yes, It's true. I am currently running with zero degrees of right thrust, and as far as I can tell there is no spiraling slipstream of air over the fuselage.

As far as progress is concerned, for us the pattern season ends this weekend with the D4 district championship in Hebron Kentucky. After that I am going to have a lot more time to devote to finishing our version 2 Contra design, and I hope to be able release a drawing / CAD package to Mike by the end of this month. (September ...) After that we will build parts and assemblies, and if the design is sound, we hope to have a small initial batch of drives available early in the new year.

As far as price is concerned, we won't have final pricing until we can properly cost our version 2 design, but initially we are going to have to start with small production runs because we don't have the money to front the cost of materials and CNC machine time for large batches. This means that the initial units will have to be more expensive than later batches where we can spread CNC setup costs over larger numbers of parts. Given this, my expectation is that our first units will be in the $800 to $1200 range, with sets of props running in the $150 to$200 range. However, keep in mind that these numbers are completely subject to change when we get proper costing information.

David Bathe

Thanks for the reply.What excites me is that you now have aircraft design opportunities that just haven't been possible because of the mono prop slipstream.Rudder/fin shape position being and obvious one, as you stated, you rudder response improved measurably, presumably because of the twin slip streams cancelling each other out and creating a far smoother flow of air towards the rear of the aircraft.I wonder who's going to be leading the charge?Exciting days.

serious power

Hi David,
In Austria,during practice, Seba often took off in a hover/prop hang.
His helper would hold the model pointing vertically, he would throttle up a little and the guy would let go.
What was striking was that it wound just hang there with the ailerons and the rudder at neutral/centered.
No torque roll while hanging or climbing up and out.
I think his/M Ramell's contra is a different design. They use differential type gearing (lay man term).
My understanding is that the props will always equally share the load.

Brian

Brenner

This is the same concept that we are using in our Contra Design. We use a planetary gearbox that shares the available power evenly between the front and the rear props. I have also been able to hang my Contra on it's nose as well, and you're right, it's much more stable than a single prop plane. The difference is probably similar to the comparision between helicopters with a single set of blades, and helicopters with contra rotating sets of blades. Also, this is what makes stall turns such a pleasure with a Contra Drive. You can essentially go into a hover at the top of your upline, and apply rudder when you feel like pivoting the plane about the center of the fuselage. This same effect also seems to keep the wings and tail "in plane" during the turn as well.

serious power

Hi,
I'm sure the observed effects are the same/similar.
However I'm told that the engineering principal is different.
I may have been misinformed.
As I understand it yours will have the props running at the same speed/rpm as each other regardless of the load on each.
Theirs can run the props at different (to each other) speeds depending on actual load.
If this is the case they do not have to exactly match the blades to thrust load as they will self adjust to share it.
In Austria we saw another system, involving belt drives.
This was how Akiba might interpret yours, with belts instead of gears.

Brian