Dsnow

Hi Malcolm, I really don’t want to open up the old discussion of gyroscopic effects with the two props canceling each other out but my best guess is that since one prop was much heavier than the other the gyroscopic forces are back into effect. The experiments that you mention used two motors of the same weight so the forces were canceled out, I’m sure if I went back and added some weight to one of the motors it would act more like a gyroscope.

I really doubt anyone flying the contra will notice this effect with the current props since the weights are well matched. I have some first generation and current generation props and just picked out mixed generations to make a prop combo (dia and pitch) that I wanted without thinking the weight was an issue. If anyone has a better explanation I would really like to hear it but in the past whenever I’ve changed prop combos, with the same generation props, I never saw any trim change as significant as this, rudder went from straight to 3/32” right trim with front prop heavier than back, and just wanted to pass the info onto Brenner.

Dave Snow

Brenner

Hot off the Press!

I just heard that Gernot Bruckmann won another FAI World Cup in Spain over the weekend. (I think this is three in row now..)

Congratulations Gernot!

Brenner ...

MTK

The theory of different flywheel effects of the heavy and light props has teeth.

To prove or disprove, is there a secure way to add 20 grams to the light prop and repeating the experiment?

Brenner

Yes, but you will have to add the weight permanently.

You can inject epoxy into the blades through the holes in the hub.

Brenner ...

Dsnow

Hi Matt, I think this is an experiment that can be done on any single prop setup: trim the plane with a heavy prop then switch to a prop with approximately half the weight/mass but the same diameter and pitch, this should cause the plane to yaw and require retrimming of the plane. If anyone tries this please let us know how it goes.

Dave Snow

MTK

Dave, true enough. With so many e-prop offerings over the years (meaning weights) this should be a straight forward experiment. I don't play big E so don't know for sure, but I'd bet there are otherwise identical props around excepting the weight.

I am interested in that result too....

BFoote

Of coure different weight props will have a gyroscopic effect. DUH. The fact of the matter is that even with exaxt same weight propellers, one still has not canceled out the gyroscopic effect of the electric motor. Contra simply cancels out the vast majority of the gyroscopic effect.

The greater mass out the radial diameter of the extra spinning propeller will have a greater effect. distance x mass x velocity. Its the velocity squared function that is the killer as velocity increases with the square of the radial distance. The fact of the matter is that nearly all that extra weight is probably very close to the hub with a different propeller unless it is a thicker chord prop.

Bottom line, if you put that extra heavy prop in the same spin orientation as the electric motor you will notice the effect to a greater extent. Your plane will become more controlable on the other hand if you place that heavier spining prop OPPOSITE the spin direction of the electric motor. Well, will require less offset rudder.

PS. Your rudder 'problems' are due to Rudder flutter at neutral as RC planes don't balance their rudders and can get away with it as they are all preloaded in a standard single propeller design. Balancing the rudder, a thicker symetrical airfoil on the rudder, will get rid of rudder neutral wobble. Make the top/bottom of your rudders all moving. This will likewise lighten the load on the servo making it far more precise at small movements, even for digital servos with variable current circuitry.

Malcolm H

So if this effect is gyroscopic it will only apply forces to the model when the angle of the spinning mass is changed with respect to the direction it was travelling. i.e. it will precess.

If this is so how does rudder trim help? Surely you only need to counter this effect when pushing or pulling and therefore a neutral rudder and a fist full of rudder stick as you go round the corner is needed?

So here is another test, if fitting a heavier prop causes no change I'm level flight but does whole pushing or pulling the effect is gyroscopic but if it occurs in level flight it is aerodynamic in some way.

Malcolm

Malcolm H

Sorry last post typed on phone late at night here. Should have read:

"In level flight" and "while pushing"

Malcolm

Wasson

Malcolm,

As you know, I am not a contra owner but for whats it's worth I would agree - both with the theory and the practice.

I have flown my Gaudius extensively with both a Mejzlik 20.5 x 12 WE (97 grms) and a Mejzlik 20.5 x 12WEL (43 grms) of identical profile.

As expected, I have noticed no difference whatsoever in terms of rudder trim. To be honest I haven't detected any noticeable difference in terms of gyroscopic effect when pushing, pulling, or yawing either.

I accept that in theory such an effect must exist, but since F3A pilots are continuously making small corrections to all 3 of the principal control surfaces, I very much doubt whether any such effect would be really meaningful in practice.

Bob

serious power

Hi,

Bob, maybe it's time for you to try one.

Does anyone know if Gernot is representing Austria at the worlds ??
I checked the WC website and I can't find a listing.

Brian

atglenn

No he will not represent the team at the Worlds. He is the alternate I believe. He tends to come in 4th at the team selection every year for some reason.

serious power

Hmmmmm,
And he beat Seba this past weekend - and he will place top 10 ten ,likely top 5.

Brian

Malcolm H

You don't see many contra powered full size aircraft flying these days but what I had forgotten about the Gannet was its small additional vertical surfaces on the stab. I wonder if the designers underestimated the de-stabilising effect of the second prop and had to add these later?

https://www.facebook.com/photo.php?v...type=2&theater

Malcolm

OhD

Anyone in the world using a Contra on an Axiome +? It is a good flying airplane but I suspect it might be better with a Contra.

Jim O

TonyF

My Feedback: (92)

That's pretty much what I have with the Contra Episode. I am extremely pleased with how that is flying now. I've been doing a lot with the 2015 schedules and it has no problem with anything. The pilot on the other hand, has some issues with some of F-15! But we're getting there!

OhD

As I recall you had to make many mods including increasing the effective vertical stabilizer to get what you wanted. How would you compare it's performance to your Contra/Nuance? I'm thinking the latter might be better for the Masters program.

Jim O

TonyF

My Feedback: (92)

Well, you might have to make the same mods to an Axiome + with a Contra installed. What I did to the Episode was to change the incidence of the wing, add more downthrust and add an additional stab fence over stock. I'm not really certain the incidence needed to be changed but it is working where I have it now.

The way my Episode is flying now I think it would work great in Masters. You certainly don't need the KE performance it has for that schedule but it won't really hurt anything. The Nuance w/Contra is also very good. The higher side force available with the Episode does make it a little different to fly then the Nuance. I find it better now but it does take a little getting used to it. I guess you can take your pick!

Stuart Chale

I think all (maybe most) of the full scale airplanes using contra drive props had additional yaw stabilization. Additional vertical fins on some, much larger vertical fins on others. When I discussed the contra with a local aeronautic engineer, he said right away that it would make the plane less stable in yaw.

Stuart Chale

I can't answer in regards to the Axiome +, but I would suspect that it would take some combination of additional sfg's, added vertical fin area. wide rudder etc. to make it fly like you would like it.
My 2 cents after reading as much as I could, talking to people flying them, talking to an aeronautical engineer and flying the contra on an Episode and now a Beryll. The contra has some clear advantages, one disadvantage is decreased yaw stability. Much less rudder is required and or increased rudder expo is needed. Many newer planes are designed to already be less stable in yaw. This is needed for the knife edge looping and integrated rolling loops. The Episode is such a plane and I assume the Axiome is as well. Putting the Contra on an already yaw unstable design will make it even more so. My first flights with the Episode were hard to keep on a line after each turnaround. I tried different fixes and ended up with the 2 SFG's on each stab and a wider rudder. That helped a lot. I am not flying FAI so I decided to go a different rout. My thoughts were to try the Contra on one of my older planes that was not designed for all the knife edge looping, my Oxai Beryll. I still added SFG's on the stab and with them I am not sure if it can do a knife edge loop. Certainly not with the authority of the newer designs. In fact it feels pretty close to the way it was with a single prop in regards to surface sensitivity. I like the way it flies better with the Contra though. I am flying it in Masters.

I think that in order to take full advantage of the Contra on a successful design you will have to do things to the plane to bring back the yaw stability to the way it was with the single prop.

Stuart C.

OhD

I think you nailed it Stuart. The new designs including the Axiome + are less stable in yaw and the Contra would make it worse. What we really need is a plane designed for the Contra. I'm inclined to forget about experimenting with a $5000 airframe that is nice enough to go in a museum and put the Contra on something else, because I agree it has its advantages. Thanks for telling me what I probably already knew down deep. We always hope someone will tell us something like all you need to do is install the Contra and it will be the best plane on earth.

Jim O

wattsup

My Feedback: (1)

Hi Jim, somewhere way back in this thread I recognized this need and I tried to encourage the designers and manufacturers to come up with a 2m pattern plane that was designed and built specifically to accommodate the Contra Drive unit. I suspect this is why we are seeing these difficulties associated with a "factory retrofit" to these existing designs currently available. Note, I'm on my third 2m/CD retrofit and have done the "surgery" on all three! Hopefully, someone will recognize the need and step up like Brenner and Mike & Andy Gaishin did when they designed and started manufacturing the Contra Drive unit 2 +years ago. Nothing is more frustrating than to be so close, yet so far! Just my thoughts..........

Stuart Chale

I really like the Contra, I like its benefits and I think I am flying better with it. Is it a game changer, is it required No of course not just look at what people are flying at the worlds. Will it make you move from the bottom of the pack to the top? Not without a lot of practice
After playing with it and getting to fly the Episode with it, I think people need to be careful about buying the latest and greatest when they are not flying FAI. The new sequences require a more yaw unstable plane that top pilots can make look great with ease but the rest of us might have less success with in the lower classes. "We" would benefit form a more stable aircraft.

wattsup

My Feedback: (1)

Hi sc204, I could not agree with you more.............."You Still Gotta Fly It!"

serious power

Hi Brenner,
The props on this look interesting ;