Dean Pappas

Hi Guys, Hi Doug,
I spoke to Michael Rammel some years ago when he first came up with this contra-rotator. He never introduced it for sale, though I cannot say for sure why that is. By the way, Michael is a member of the F3A technical sub-committee.

The gearbox is a torque-sharing differential, that will make you slap your forehead in amazement when you figure it out. No matter what pitches you put on each prop and no matter the airspeed (at least in steady state!) the rotational flow is approximately zero.

What's so clever? Take the standard planetary box, and "float" the normally fixed ring gear in rotation. Now attach the rear prop to it. Mechanically fussy but conceptually dirt simple.

I believe that the contra gearbox is now being made by Hacker, and when I asked of the U.S. importer, I got a similar 3-1/2 k-buck price. obviously we have a business opportunity! This mechanism cannot possibly be patent protected, it has been in mechanical engineering textbooks for eons!
best regards,
Dean Pappas

P.S. I sure am glad that the Team placements are determined after the Finals, this year ...

Jeff Boyd 2

Well, interesting . . certainly looks swept here

Cheers, JB

shannah

My Feedback: (5)

Yep. I wonder if that wing will be on the production model

huison2005

i have put one in order , really look nice ...........

tIANci

This pic shows it probably has a swept back TE.

MTK ... contra rotating or coaxial are not cheap to set up, two ESCs, twin motor unit ... I wonder if the advantage is that worth spending for?

Jeff Boyd 2

tIANCI . . it is a SINGLE Hacker C50-11XL with a special gearbox and only ONE ESC.

BUT . . close to US$4000 for the complete system including motor, ESC, Props, etc. Soooo, NOT cheap

Cheers, JB

tIANci

Oh yes JB ... someone mentioned about the clutch etc. Man ... USD4K? That's the price of a nice Oxai ... you getting one to try so you can tell me how it runs? Hehehehehee ...

Jeff Boyd 2

Short answer . . . NO !

Cheers, JB

Dean Pappas

Thanks for posting the picture of the system Jeff.
That gearbox and soft mount detail looks exactly like the setup that Mike Rammel designed some time ago.
I think the website was http://www.f3a-efactor.de
Regards to All,
Dean Pappas

MTK

The expense in Not worth it to me..... there are other ways to straighten the spiralling air stream. That's one thing I am working on.

But for the person who wants the performance and doesn't have the knowledge, skill or time to drive to a solution, it is a good approach. It's only money after all. Some have it and many more don't

A counter rotating set-up which would require two of everything but smaller, would be much less expensive than one co-axial set-up. Actually when sized right, cost for two smaller plants should be marginally more expensive than one larger conventional set-up. BUT I haven't done the legwork to figure it out nor do I intend to

MattK

Dean Pappas

Hi Matt,
The contra gearbox ought to cost marginally more than a normal planetary unit.
Let's say double ... that would make the gearbox cost under $500; but the $3500 quote is simply a way to tell potential business to go elsewhere.

Dean

cchariandy

Incidentally, sweep is normally specified at the 1/4 chord. The wings on the Xigris C2, CPLRs and Silvestri's planes are quite swept and will add lateral stability over a straight tapered wing. The modern designs have added area to parts of the fuse to improve lateral control authority, I'm curious as to why these planes need more lateral stability...I suspect there is an element of "looks" in the design. They remind me of 80's lawn darts. I hope all the other designers don't turn this into the latest pattern fashion trend.

Colin.

MTK

Not sure what you mean with "lateral stability". Do you mean yaw stability? If yes, more yaw stability is not desired in a pattern model...the modern model (or ancient model) has (had) plenty. What is not wanted is the spiralling air stream.

There is an amazing amount of air mass displaced by a 2 meter model to generate thrust. That air mass hits at various undesired spots on the fuse causing torquing that require offsets to the power plant to augment. These offsets and their unwanted effects are what we are trying to reduce/eliminate.

The contra rotating props do this quite effectively as Dean and I pointed out earlier.

MattK

MTK

YUP....how is the counter rotating twin coming?

tIANci

Funny thing is hardly anyone at the Worlds were using the contra rotating system. Just wonder, leaving cost aside, how much difference it makes to a plane.

cchariandy

Yes , yaw or lateral stability. So you think increased wing sweep improves the effect of the slipstream?

Colin

radek

Just thinking out loud.
Would bolting on, say a non rotating 6 bladed prop with the right pitch just behind the active propeller straighten the spiralling airflow or would that do more harm than good? As in create turbulence and loss of thrust?

Rendegade

You mean a stator?

there have been some experiments done on aircraft with a set of "canard" fins either side of the fuselage just behind the prop to straighten airflow. It did work but had the adverse quality of making the aircraft very difficult to snap, from memory.

radek

Yes I suppose a stator is what it's called. Didn't consider that would equal placing stabilizers up front [X(]

Rendegade

Well, no it doesnt', but I think the initial idea was to straighten the prop airflow, which in essence is what the stator is there for, correct?

JAS

My Feedback: (2)

What about a 2nd prop, not attached to the motor, but free-wheeling behind the active prop?

radek

Rendegade, not sure I follow you now.
Yes, the stator thought was for straightening the airflow, but if it made the aircraft difficult to snap I figured the stator must besides straightening the airflow also double as a stabilizer/fin infront of cg. That was just an asumption though, since I with my limited knowledge can't think of any other reason it would make an aircraft difficult to snap.

radek

I'm guessing, but just guessing, That would equal a rather effective airbrake.

Rendegade

I think it was an unwanted byproduct. Becuase they were so close to the propeller they effectively had a constant STRONG blast of air, which had the effect of stabilising the nose, making it very unwilling to snap. Of course I beleive there was an upside, that the aircraft tracked much truer everywhere else.

I beleive they were fixed, and perhaps a way around this is to have them connected to the elevator servo, so they will push or pull the nose into a direction to make it snap.

Food for thought perhaps.

Jeff Boyd 2

Hmmmmm . . I LIKE that idea, Jas. I reckon it would work to some extent, but I am guessing it would cost efficiency, so more power needed ??? . . worth experimenting with though. Not expensive to try . . [sm=72_72.gif]

I think you might have to test it though . . I doubt us 'plebs' will be able to tell the difference

Cheers, JB