Author Topic: Re: Contra-Rotators (Read 1194 times)

Igor Burger · « **on:** April 04, 2007, 02:45:17 PM »

>>>contra-rotator<<<

and what you think I actually work on ...

Dean Pappas · « **Reply #1 on:** April 11, 2007, 12:45:56 PM »

Hi Gang,
I would have just sworn that a bunch of you would jump all over Igor's fascinating idea.
Okay, I will.
So how do you control this thing Igor?
Is the RPM matched? Do you need to adjust the pitch of the rear prop to be higher/lower than that of the front?
My guess would be to slave one motor controller to the other so that both motors make the same torque (same current) all the time.
Maybe that will keep the spiral airflow cancelled at all airspeeds?

Regards All,
Dean pappas

Mike Palko · « **Reply #2 on:** April 12, 2007, 03:53:53 PM »

Igor,
The idea of ground adjustable pitch is neat (at least it looks that way from the picture), but the assembly looks heavy

.

If this is for a C/L model (it may not be) I think conventional carbon props would work better for you. Just repitch them until you get the desired pitch and save the weight of the adjustable hubs.

Can you share more info? My assumptions made above may be very wrong depending on the size and application.

I started looking into contra-rotating props about a year ago. I have an idea for a very unusual PA model, but I need to do a LOT more research.

Anything contra-rotating is neat in my book.

Mike

Igor Burger · « **Reply #3 on:** April 13, 2007, 02:40:59 AM »

In reality I work on something else actually, because European championship is comming.

But I am prepared to use it this way:

1/ The motor is combo of two PJS 1500 http://www.pjs.cz/produkty/informace/500-1500/index2.htm
It is low rpm motor, co I cout with battery of 5 cells up to 4Ah. Ithink it will be 2x20A. My xls says the usefull prop will be graupner 9x6 (available left and right) and it will need 19A for 10000. Power on shaft will be 2x210W and efficiency 75% ... it is not the best, but usefull.

PJS can make twins with almost any motor ... here is exmaple of smaller twin available as regular product: http://www.pjs.cz/produkty/informace/2x550R/index2.htm

2/ the prop on picture is Variprop which is available in many combinations including folding prop and 5 blade prop, the middle piece widhts 11g (1/2oz), and yes it allows on ground pitch setting (every blade independet, so it allows tricks with unequal pitch or so) ... but I thing it is not the best choce for my use, but looks great
http://www.ramoser.de/home_e/varioprop_e/varioprop_e.html

3/ ESC will be separately for each motor, I will use Jeti SPIN modified for stunt (little quicker responce in governor mode and brake)
http://www.jetimodel.cz/eng/spin44_en.html

It allows to set RPM directly by something called Jeti Prog Box:
http://www.jetimodel.cz/foto/jetibox2.jpg

it allows to set all parameters of ESC including gear ratio, number of motor poles and governor mode rpm (directly like rpm, not any percentage or so)

so I do not plan to play with pitch, I will rather play with independently adjustable rpm setting ... they go the same battery, so the power difference is not important

but I do not think it will need exact adjustment ... on on prop models we have 0% counter rotating, so counterballancing up to 90% will be great improvements and on the other hand, my primary goal is to cancel gyroscopic moment and that needs equivalent rpm ... I do not know, we will see what will work best

4/ since ESC allows rpm setting, both ESC can work with the same impulse width (throttle) so I will use one timer:
http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=212908&mesg_id=212908&listing_type=search

Dean Pappas · « **Reply #4 on:** April 16, 2007, 10:37:06 PM »

Hi Igor,
The more I think about it, the better your idea sounds. In Stunt, we fly in a very narrow speed range throught the maneuvering portion of the flight. The torque sharing can be set up once, unlike a flight profile that requires operation over a wide speed range. Takeoff and landing are obviously not an issue. I can't wait to hear about the first flight tests!

later,
Dean

bob branch · « **Reply #5 on:** April 18, 2007, 08:08:54 PM »

At the Toledo RC show last weekend Axi displayed 3 dual motor counterrotating powerplants. One was the large one they showed last year which is now a production item. Also this year there were a small version and an intermediate version. The small version is in production the intermediate one which looked close to our size was still in development. I talked about using the system in a stunt application to one of the factory reps who also flies F2B. He didn't think it was a very workable idea from a stunt pilot perspective. They have found that the power systems are quite inefficient due to the back prop working in disturbed air. They are finding that they can help maters by making the front prop smaller than the back prop. He felt the system was too inefficient for competative stunt use. I'm not trying to speak pro or con about the idea. I'm just trying to covey what I was told and offer it up for discussion as well. I think the opportunity to cancel out P-factor in maneuvers is an interesting prospect. The findings Axi has made on the prop efficiencies is an issue though.

Bob Branch

Mike Palko · « **Reply #6 on:** April 18, 2007, 09:13:27 PM »

Bob,
I read into contra-rotating (CR) props quite a bit in the past and came across some of the same information you shared. I realized just how many drawbacks there are to CR setups, but for every drawback there seemed to be an advantage. I guess it's like anything else, you can't get something for nothing.

When someone starts to apply CR props to PA models we will start to learn a lot more about it. The advantages just might outweight the disadvantages. Even if they equal out (or even a little disadvantage?) IMO it will still make awesome fictional and semiscale designs possible!!

Mike

Igor Burger · « **Reply #7 on:** April 19, 2007, 01:38:28 AM »

>>>inefficient due to the back prop working in disturbed air<<<

Contra props are MORE efficient ONLY in case that the power train is LOADED, means slot of slippage. It is not our case and 4 blades are always less effective than 2 blades. That is why contra props are succesfully used mainly on boats with small high pitch water screws.

Anyway, I do not think we need to hunt for any big efficiency as our prop themself work in VERY low efficiency mode. If you calculate drag of moddel, its speed then you will see that the efficiency is very very low. That is principle of our flying system, we convert unnecessary power to heat in 80% of flight and only in vertical segments we come to higher efficiency.

For me it is more toy then seriouse contest weapon and main issue is to see what can cancelled gyro effect bring. We will see. It is still possible to lock both rotors and use only one prop (I hope that governor will not make too much troubles

)

bob branch · « **Reply #8 on:** April 19, 2007, 06:56:52 PM »

Igor

I for one am very interested to see what happens when P-factor gets eliminated. I look forward to your results.

Bob Branch

Dean Pappas · « **Reply #9 on:** April 19, 2007, 08:16:49 PM »

Hi Gang,
I'm with Igor on this. CRs are supposed to be more efficient in the flight mode that matters: climbing.
When a normal prop is loaded, the spiral airflow gets worse. The energy tied up in swirling is wasted.
Curiously enough, the full scale guys who have done CRs have either slightly lessened the span of the rear prop, or washed out the very tips of the rear prop. It sounds like the guy you talked to at Toledo never got the relative loading on the two props correct.

The development curve will be a bear, but I have no doubt that killing off GP will be a boon to Stunt.

Dean