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Dec 22, 2018, 09:17 AM
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Discussion

CAP 2020. Ex EAR 2019 Mercury Marvin, EP & RC


Some of the old(er?) guys here prodded me into thinking about this model, that Iíd been wanting to build for over 50 years, but never got around to.
Been looking at the plan on OZ, and the instructions sheet provided by nickharp (now on OZ too, since yesterday).

Here are some observations/thoughts.

1. Flies at 50mph. Or 75fps. On 40í lines., or a 250í circle. That is 3.3 secs per lap. Was it really so fast?
As far as I can recollect after 30-40 years, my OS20 powered Peacemaker couldnít have been under 5 sec on 50í lines.

2. 75fps on a 6Ē pitch prop means 150 rps, or 9000 rpm, or 10500rpm on 5Ē pitch. Probably ran at much higher rpm to drag the lines along

3. July 1956 Model Aircraft review (Sceptreflight.com) shows 0.1 bhp (80w) at 10500rpm, and 1.2 bhp (100 w) at 14000.

4. Weighed 10-12oz

So it could do the pattern, lines and all, at say 150 watts/lb...

Without the lines, perhaps 100w/lb would suffice

That AM10 weighed 3oz, so with tank and fuel, say 150gms

So an 80gm 2s 1000mah lipo, with a 30 gm motor plus Esc and 2-3 9gm servos may be enough?

Opinions, please.

Iím inclined towards enlarging to 36-40Ē, extending the tail moment a bit, reducing throws drastically, flapperons coupled to elevators (not sure if my Optic 6 supports this) etc.
Last edited by k k iyer; Jan 03, 2020 at 11:40 AM. Reason: Title change
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Dec 22, 2018, 02:00 PM
I like real wooden aeroplanes!
Sundancer's Avatar
100 watts per pound would be PLENTY. I would use the flaps as ailerons driven from two servos - one in the aileron output one in the spare 5th channel and mixed so that you can then couple them to elevator with another mixer if desired (remembering of course that they go DOWN when the elevator goes UP - square loops here we come!) - but, as you say, cut down the movement very drastically. I think I would prefer to split the flaps and make the outer portions just ailerons and the inner portions just flaps, in this case these could be mixed with elevator and also operate as landing flaps vis a mixer to switch them to another, positionable, channel. Personally I wouldn't bother with rudder on a model like this, with no dihedral it will be strictly a "bank and yank" type machine. My 14 ounce 34 inch Square Dancer design (below) is set up this way and is wildly aerobatic on 80 watts including control-line type square inside and outside loops, top hat and triangles in it's repertoire .

And yes, I would want to lengthen the Marvin tail moment a tad.
Dec 22, 2018, 03:39 PM
B for Bruce
BMatthews's Avatar
Depends on how you want to fly it.

For keeping it control line but electric powered I'd suggest try to hold it to around 16 oz and 100 watts for the original size and for 40' lines.

Depending on how you tweaked the needle of your OS on the Pacemaker it may have flown slower in a rich level, lean maneuvering stunt model style. And that would explain the 5.5 second slow lap times. But that same model tweaked to peak and flown at more of a combat model like pace would be turning roughly 3.3 second lap times on 50' lines too while running at around 60 mph.

Scaled up for CL still to maybe 40" span and probably around 20 oz I'd still go for 100W/lb or roughly 125 watts running power. Prop pitch as suited to the speed you want to fly.

Changing to RC flying? OK, we COULD alter the design but I'm not seeing anything going on which would stop it from flying just fine as an RC conversion. We could make it "better" with a tail moment increase. But then it would look different too. And it's not too far off the moments and areas that were seen on a lot of the old fun fly style designs from the 90's. So I'd be inclined to keep it rather stock and only make internal changes to wood sizes and structures to adapt it towards lightness and electric power RC flying. And perhaps alter where the hinge line is located on the wing to make the "flaps" larger and better as strip ailerons. Keep the overall outline the same but alter how you scale the airfoil to a lower value such that the hinge line is shifted forward so the ailerons become roughly 18 to 20% of the chord at the middle of the wing panels.

And while I'm not normally a fan of massive power levels on a model of this sort I'd say an exception is well within proper order. It SHOULD be able to effortlessly climb up into wing overs or perform vertical 8's from horizontal flight I'd say you'd still want something over 100 W/lb. I'd aim for around 130 just so it's not TOO extreme. And then likely only use full throttle when the nose was pointed up sharply.
Dec 23, 2018, 08:06 AM
It must have a machinegun
Scaledown's Avatar
When I was a kid my dad brought down from the top cupboard an almost complete control line stunter that his younger brother never finished. My brother and I were amazed by the coupled flaps that we had never heard of. It was too damaged from a house move to fix, but my brother (probably about 14 years old) decided to clone it. With an Enya 09 on the front it stunted like nothing else we had flown. Only very recently I trawled through Outerzone in search of that plane, and I'm certain it was a Mercury Marvin. So I'm keen to see this build progress.

I think it would fly acceptably with almost no modifications to the structure. If you have plenty of power and enough pitch to cruise fast, I feel the flaps as they are now should give a decent roll rate.
Dec 23, 2018, 09:04 AM
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@scaledown,
Thatís a good looking plane!
Could you post a profile view please?
Thanks and regards.
Dec 23, 2018, 09:37 AM
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@sundancer,
@bmatthews,
Thanks for the inputs, boss.
Lots to think about.

First of all, I think I better scale it up to 36” or 39/40”

Then, as you say, lengthening the rear fuselage will make it look different.
Perhaps keep the fuselage profile and just move the stab back...
And reduce the percentage width of the flaps/ailerons/flaperons...

Regarding power, I have a vintage (no silencer, no throttle) OS Pet, a NIB OS 10LA, and (amongst others) an L2210 1650kv.

That (250w) 40gm electric motor flew my 36” foamie Spitfire (pic on OZ ) like crazy with unlimited vertical.
Hard to believe it’s half the weight and twice the power of the glows...
Maybe it’ll sober up on 2s instead of 3s.

I fear that my glow engines may have to remain boxed, as the electrics are so much easier.
I could even use dope and tissue...
Last edited by k k iyer; Dec 23, 2018 at 09:55 AM.
Dec 23, 2018, 09:45 AM
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I guess this Bede BD8 that I started planning several years ago can wait for 2019 Build Off #2...
Dec 23, 2018, 10:04 AM
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@sundancer Sir,
Simplest is just smaller width ailerons with one servo.
Next, with two servos for flaperons,
And then maybe my Hitec Optic 6 can mix flaps/elevator on flaperons...

Separate flaps and ailerons may be an overkill on such a small model...
Dec 24, 2018, 12:48 AM
It must have a machinegun
Scaledown's Avatar
Quote:
Originally Posted by k k iyer
@scaledown,
Thatís a good looking plane!
Could you post a profile view please?
Thanks and regards.
Unfortunately that pic was taken in the 70's. It was the only photo. The plane has long since gone to the hangar in the sky.
Dec 26, 2018, 11:32 AM
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This Phoenix Magic is the closest coupled model I have.
13 oz with 4 servos
Motor, prop, esc 4oz.
Lipo 3oz
Total 20 oz
Span 39Ē, avg chord 9Ē, wing area ~2.4sqft
Hence wing loading ~13oz/sft

Now this is actually a 3d model, though I only fly it in 2d.

Wanted to compare with a Marvin of similar span, to see if it would fly more or less the same.

Surprised to find that the horizontal tail vol coeff on the Marvin seems to be only ~0.19 vs ~0.98 on the Magic.

So in converting from CL to RC does the Marvin needs a substantial increase in rear fuselage length and tailplane area?

And I doubt if I could build a reasonably strong 40Ē Marwin under 12oz for finished bare airframe.
The Magic is close to being flimsy, has to be handled with real care...

So, in sum, the questions are
1. Will an RC Marvin need a significantly higher horizontal tail vol coeff
2. At 40Ē span, with maybe a longer fuselage, will it need significantly over 100w/lb?
Dec 26, 2018, 11:37 AM
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Another pic to show that the Magic has very very little wood!
Dec 26, 2018, 03:25 PM
B for Bruce
BMatthews's Avatar
The biggest effect of the horizontal tail coefficient does is it sets where the overall aircraft neutral point is located. And by connection it sets the rearward limit for your CG location.

It does other things too. A higher tail volume gives a stronger pitch for a given elevator input. And it damps out any oddball oscillations more rapidly.

But if you're flying more for fun and sport a lower tail volume coefficient is not a deal breaker by any means. And it's not like there are not a lot of other very short tail moment models out there. Look at Andy Clancy's Lazy Bee design. And Great Planes Slopoke.

And you want short coupled? How about this model I did many years ago which uses a wing design ripped off directly from the old British diesel combat models with ailerons added on. They don't get more short coupled than this. And then there's the legions of Sig Wonder models out there. Models of this sort loop and stunt and then settle down just fine. And the Marvin would be roughly half way between these sort of examples and the Magic you show.

So while a Marvin would need a slightly more forward CG location than your Magic it will still fly just fine. Especially if you don't need to have the pitch authority to drive the model from flying to nose high stalled and hovering in the snap of your fingers.
Dec 27, 2018, 08:58 AM
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Bruce sir,
Thanks for you reply. You give me hope!
(BTW Iíve never even seen a CL combat model, far less consider flying one)

Guess (for RC) Iím psychologically drawn more towards a layout like Dave Plattís Contender than his Marvin.

This seems to be a throwback to circa 1978, when I found it far more comfortable and effortless to fly the Schweizer 2-22, including hands off (though with 10kg lead in the seat cushion), than the 1-26 that had noticeably better performance!

Diagnosis: Inadequate piloting skills, inadequate courage, slow reaction times

Nevertheless, Iíve wanted a Marvin for 50 years.
So hereís my plan
1. If the 2019 Buildoff #1 is CL themed, Iíll build an RC Marvin as per plan (though maybe 120%)
2. If not, a stretched fuselage version for RC
3. An original size CL for my vintage unsilenced OS Pet, even if Iíve to get lines from UK. (Or HK!)
Actually this is what I want! Though if it really laps in 3.3 secs, Iíll probably be down soon after the reverse wingover.

Incidentally, I learnt to go inverted, not from an extended loop, but from a reverse wingover, thanks, if I remember, to Dick Mathisí Excalibur article in MAN...

Wish me luck, despite internal contradictions in the options!
Regards
Dec 29, 2018, 03:35 PM
Culper Junior
I tried going inverted Ďonceí from an inverted wingover with my McCoy 19 powered Jr. Flite Streak. Had alfalfa stains on the rudder before I caught it.
Dec 29, 2018, 11:21 PM
B for Bruce
BMatthews's Avatar
You flew control line but never saw any combat flying? You've led a sheltered life ! ! ! !

Here's a rather nervous video of a classic diesel engine powered combat. I did a bunch of this years ago and it's just about as much fun as a person can have while flying any sort of model airplane. Hard on the equipment though as the mid air at around 2 minutes in suggests. I got tired of all the repairs. But DARN was it a blast to do ! ! ! !

The flyers get points for air time as well as streamer cuts so that's why they bandaged up the model and got it into the air again after the crash.

Scampton Vintage Combat 13/4/14 (5 min 49 sec)


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