Author Topic: Counterpaw - design, build, fly.  (Read 6581 times)

Offline Graeme Jones

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Re: Counterpaw - design, build, fly.
« Reply #165 on: 02, May 2017, 08:53:49 PM »
This counter rotating propeller business is more complicated than I thought. I've been doing some more research and found -


NA F82 Twin Mustang - props turning outwards at top, reversed to inwards at top to cure excess drag on take off.
Lockheed P38 Lightning - props turning outwards at top.
DH 103 Sea Hornet - Props turning inwards at top


Other aircraft such as the DH Mosquito and P61 Black Widow, which you might expect to have used CR's, didn't, both having the props turning anticlockwise looking from the front. Not a problem in the P61 apparently, with tricycle undercarriage, but the Mossie was notorious for it's swing on take off and landing.


Turning inwards at the top makes more sense in the case of an engine failure. If the left engine stops, the good engine on the right will yaw the aircraft to the left, but it's torque reaction, turning clockwise looking from the front, will roll the aircraft to the right. Depending on other characteristics of the design, dihedral etc. there will at least be some tendency for the two to cancel out. If I'd used inwards at the top rotation for Counterpaw's first flight, would it have flown successfully on the one good motor? Either way I'm considering reversing the rotation for the next flight attempt.

Offline Ashley Hoyland

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Re: Counterpaw - design, build, fly.
« Reply #166 on: 03, May 2017, 08:34:55 AM »


Just to follow my theory (and it is just my theory as far as I know) on prop rotation direction. If you leave the rotation with the top turning away from the fuselage I believe to establish directional stability an alternative solution to changing the direction of the propellers would be to position the rudder and fin upside down, (much more area below the fuselage centre line than above it) to keep a positive spiral flow on the largest part of the vertical stabiliser.


Could it be that the P38 'gets away with it' because it has twin fins and rudders in line with the propellers.


Ashley
« Last Edit: 03, May 2017, 08:39:33 AM by Ashley Hoyland »

Offline Graeme Jones

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Re: Counterpaw - design, build, fly.
« Reply #167 on: 20, September 2017, 02:44:41 PM »
Just to draw a line under this one, I'd better say why I've been unable to report any progress. To recap, flight one was very short because the right hand motor did not spool up correctly, leading to a severe swing to the right, 2 or 3 seconds of flight and wiping off the undercarriage. Flight two, after repairs and some adjustments, lasted about 30 seconds, with instability in pitch leading again to removal of the undercarriage.


I assumed that moving the balance forward would cure the instability, so I shifted the batteries as far forward as they would go in the nacelles, resulting in a 6mm movement in balance. Flight three proved me wrong. Much worse instability in pitch and now unstable in yaw as well. Well beyond my ability to get it down in anything resembling one piece, although I did at least crash it in the correct field.


Damage was extensive, the lower wing and nacelles being totally destroyed and the fuselage badly damaged but possibly repairable. At some point in the future I may do that, build another wing and try again with a more normal layout i.e. 1 motor in the nose.


The project hasn't been a complete waste of time, I have learned a bit about ESC's and the old-fashioned fuselage construction was a success. The use of transparent film for covering was worthwhile in terms of saving weight so I may use that again.


What I don't know is why it was so unstable. Bit of a worry if I ever complete my DH84 Dragon, which is of similar layout, size and weight.

Offline Malcolm Harris

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Re: Counterpaw - design, build, fly.
« Reply #168 on: 20, September 2017, 11:00:56 PM »
Graeme,
A bit late in the day these comments but having read back over your thread I'm struck by one particular pic and that is the full side view. The distribution of fuselage side area looks all wrong to me, not enough fin area by a long way.


My best guess as to your problems are severe yaw instability which in turn will cause blanking of the tail as it yaws relative to the line of flight and it's this that's causing the pitch instability.


If it was mine and I could return it to flying condition even as a patch up, I'd tape a larger thin balsa outline to the fin to increase its area by at least 50% and try again.


Hope you do!


Cheers,
Malcolm

Offline Malcolm Harris

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Re: Counterpaw - design, build, fly.
« Reply #169 on: 20, September 2017, 11:04:19 PM »
Just to amplify my last remarks. We know that twin contra rotating props move the neutral point forward so I have no reason to believe that side by side props won't do it by at least the same amount.


This coupled with marginal fin area to begin with is compounding the problem.


Malcolm

Offline Graeme Jones

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Re: Counterpaw - design, build, fly.
« Reply #170 on: 24, September 2017, 01:31:01 PM »
Malcolm - see pics. below. The fuselage side profile of Counterpaw, before addition of fin strake and rudder extension, is the same as my earlier Catspaw, except for the nose. Balance points relative to fuselage length are the same. Both versions of Catspaw have had their balance moved substantially rearwards to around 35% of the MAC, rather than 25% as designed. They will now spin (just) and flair for landing.


Graeme

Offline Malcolm Harris

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Re: Counterpaw - design, build, fly.
« Reply #171 on: 24, September 2017, 10:13:54 PM »
Hi Graeme,
I stick with my original comment. Both designs look under finned to me! Remember the twin has the destabilising influence of two big prop discs working against it!


Also I'm amazed that Catspaw won't spin or flair at 25% MAC. With enough elevator throw it's bound to. While not wishing to cast any aspersions on your trimming and setup abilities, have you had someone else used to setting up modern F3A models have a look?


The reason I say this is because the trend is towards forward CG to get a much more "locked" feeling and compensating for this with flight conditions to achieve spins and snaps and even a landing condition to give that extra control authority in the flair. This is why most modern designs pitch to the canopy in knife edge where their predecessors pitched to the belly.


Hope you don't mind me making these comments?


Cheers,
Malcolm

Offline Graeme Jones

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Re: Counterpaw - design, build, fly.
« Reply #172 on: 25, September 2017, 07:17:12 AM »
Malcolm - don't mind at all, I need all the help I can get! Not sure about the "two big prop discs" idea. Two fourteen inch props half as far forward as one twenty inch prop should cause less effect, not more.


No, no one else has flown any of this series of models, I'm the only pilot in my club that flies mode 1, and I'm alone in Wales in flying F3A. This series of designs was originally based, loosely, on the model that started me in my attempts at F3A, a Kyosho Osmose 70. That flew very well but was overweight (3 Kg) and also needed the balance moved back for the same reasons, although not as far. The weight and my flying skill resulted in removal of the undercarriage several times, so my original Deluge was much lighter (2 Kg) and  had an undercarriage more suited to my grass field.


Deluge+ and Catspaw were basically enlargements of the first design, with some minor variations. It's very difficult to compare them with a modern F3A design because they are all ARTF's, there are no plans available.


Counterpaw was a deviation rather than a derivation. There's obviously something wrong with it, but I'll probably never be sure what. It is my only complete failure in 50 years of designing and building my own models, so I'm not too unhappy about it.


Graeme




Offline Malcolm Harris

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Re: Counterpaw - design, build, fly.
« Reply #173 on: 25, September 2017, 08:48:23 AM »
Thank goodness there are still guys like you doing their own thing. Please don't stop and keep entertaining us by posting here.


I would definitely urge you to move the CG forward on your mono and increase control authority until it will do what you want.


I'm sure you will find a nicer flying aeroplane.


Kind regards,
Malcolm

Offline Yoda

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Re: Counterpaw - design, build, fly.
« Reply #174 on: 25, September 2017, 09:34:38 AM »
No, no one else has flown any of this series of models, I'm the only pilot in my club that flies mode 1, and I'm alone in Wales in flying F3A.


Hi Graeme,


Are you near the Rhyl or Wrexham clubs? There are a few guys there who could give you some advice. Mode 1, sorry, not sure.

Offline Graeme Jones

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Re: Counterpaw - design, build, fly.
« Reply #175 on: 25, September 2017, 12:02:21 PM »
Yoda - I'm in south Wales, so no. Thanks for the offer though, I'll try to visit next time I'm on holiday in north Wales.


Malcolm - I did try increasing the control throws on the medium size version, Deluge+, but after some experimentation I moved the balance backwards by moving the battery to the rear on the electric one and moving the Rx battery back and adding tail weight on the IC. Then I put the throws back as designed. By the way, even with the original, forward balance, Catspaw will flick roll nicely at normal flying speed, it just wouldn't stall properly when slowed down for spins or landing flair. More of a vague mush, bit I admit I haven't played about with increasing control throws yet. Not enough flying time.


I've also mentioned before that my piloting skills don't really do justice to these aircraft. I started this F3A lark to improve as a pilot, but actually I'm not as good now as I was a few years ago. Again, not enough practice.


I've found an interesting possibility for the problems with Counterpaw. The Biside design notes at Bondaero talk about suitable servos for the AMT. The servos I used for AMT and rudder fall outside their guidelines at 10 Kg cm and 0.07 s/60 (Savox 1257TG). Was I simply over-controlling? Pilot induced oscillation/thumbs lagging behind servos? It might explain why the rudder became more tricky only after I moved weight forward. I'll try one of them in another aircraft instead of my usual Futaba 3152.


Graeme


PS I never did get round to fitting the mass balances that I made for the AMT. That might have helped too.