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Old Nov 29, 2011, 09:43 AM
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Build Log
Frog Build Off Oct 2011-April 2012 - MAMBA

Well, after a week or two of fiddling about with the plans and a bit of thinking, I can at last get the build log started.

The Mamba is a swept wing "jet fighter style" rubber model of 19 inches span, broadly following the construction and philosophy of the better known Senior Series models. The plan carries the initials "CTB" in very small letters bin the bottom right, from which I surmise that C. T. Buffery, one of the regular Frog designers, was the man responsible. I downloaded the plan from the excellent House of Frog web site, unfortunately, probably due to some peculiarity of my printer, it printed slightly under size as confirmed by the check ruler on the plan, so scaling up for my working drawings had to be by an awkward 2.13 factor - thank the Lord for calculators!

My version is a double size electric model for aileron and an all moving tail plane for pitch control. A model like this needs to fly fairly quickly to look right, so this will be a fairly sturdy airframe - sheeted leading edge with hardwood (lime) surface spars, carbon fibre centre reinforcement etc. - with a target weight of 20 to 24 ounces and a fair amount of power - a 200/220 watt outrunner running from a 3 cell lipo.

Most of the initial thinking has been in devising the structure and linkage set-up for the AMT, which will be driven by a Hitec HS85MG servo, as proper operation and adequate strength of this is central to getting the sort of performance I am envisaging. Personal experience with a lot of AMT equipped slope and thermal soaring gliders has highlighted the fact that they work beautifully PROVIDED the linkage is solid and slop free and the ratio of area in front of and behind the pivot is correct - I used to use 25% in front of the pivot on the gliders, but have opted for 20% on this model.

It was therefore logical to start with the AMT and the fin with it's drive linkage to make sure this crucial area was adequately sorted before going on to the rest of the airframe. The wings being swept and tapered require a bit of work to develop the ribs and also to incorporate the outboard ailerons (to be driven by two 9 gram servos) but are otherwise straightforward, using a favourite "D" box structure from my E400 and 2 metre soarers. The fuselage should pose no problems, slab sided in 1/16" sheet with longerons and a turtle deck, very much like my Tom Tit x2, with a top hatch and pop on nose cowling, but at the moment I am still undecided between a bolt-on wing or a one piece model. As for the undercarriage; I intend this model for flying on my adjacent rough grass field, so a fixed undercarriage is out, apart from which it will look far better in the air without the wheels dangling. However, I will probably make a drop-off tricycle unit for flying from good runways.

So here is progress so far on the AMT, plus a glimpse of the drawing for the fin, construction of which is the next task after cutting the custom AMT drive bell crank from 1/16" GRP board.
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Old Nov 29, 2011, 10:30 AM
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This should be a fun build - I've been wondering how these deltas and swept wing jet style aircraft will scale up. Silly question already - AMT=All Moving Tailplane?

Along for the ride

Owl
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Old Nov 29, 2011, 10:50 AM
RFJ
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Good start George Funny how we all have our favourite building techniques. I would be reaching for a sheet of 1/4" balsa and a razor plane for that part but your way is obviously much better - lighter and stiffer. Are you retaining the tailplanes on the tubes just by friction.

Will be following along with interest.

Ray
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Old Nov 29, 2011, 04:00 PM
I like real wooden aeroplanes!
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Owl, quite right AMT = All Moving Tail, sometimes incorrectly called AFT - all flying tail, which is a somewhat different thing.

Ray; tailplane retention will be a single 22 g wire clip running in the same cut-out in the fin that the drive rod runs in, same system I used to use on all my thermal soarers.

You are right about pet building methods, when we get round to the wings you will see another one of mine - thin hardwood spar booms incorporated in the "D" box sheeting and a full depth slotted spar creating the webbing, cap strips and a split 1/16" sheet TE with hardwood cap. Similar to the wings of my 66" Mucha standard "Profile scale" (i.e dead accurate scale outlines but a fuselage just wide enough to hold the radio gear - an inch or so), except in this case the TE is solid.
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Old Nov 29, 2011, 04:42 PM
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Interesting looking project! I think you're right to make this one a 'hot' model

I have a question... I don't profess to be an expert on AMT's and for all I know you may have had success with this set up in the past, but...

To me the pivot point looks very far forward? I may have missed something obvious but I thought that the pivot point of an AMT is should be 25% of the MAC (Mean Aerodynamic Chord) rather than the ratio of areas? The 25% MAC position is where the pitching moments of the airfoil are zero - ie minimum servo loads. On your swept tail the MAC will be around the middle rib - in other words the pivot will be close to 0% MAC... very heavy.

I understand why the area ratio rule works too - in fact with an unswept surface it translates as exactly the same thing... but I'm just a little concerned about it where there is so much sweep.

I don't wish to sound critical but I'd rather pipe up and be wrong than sit on a query that might spoil the model.


Jon
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Old Nov 30, 2011, 09:39 AM
I like real wooden aeroplanes!
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Hi Jon

Thanks for thinking to bring this up, I cannot fault the theory but all I can say is that practical experience was behind my choice of pivot point.

This is based on a number of models. My beloved Hi-Phase (I had four!) has a swept back AMT, albeit less sweep angle than on the Mamba, and the area disposition is 19% in front, 81% behind. Plotting the position using the 25% MAC method would put it a lot further back. All I can say is that in many, many hours of flying these models, including some extremely rapid descents from big lift to get down before slot end, the pitch handling was impeccable. I also checked the plans of two PSS models with swept tails I used to fly way back when, an F86 and an F100, both with more sweep than the Mamba and the figures were 21 and 23% in front of the pivot, again using the 25% MAC the pivot is considerably further back. Both these models were flown in very strong slope lift at high speed and again the handling was without fault. The last model is my EDF Mig 15, again with more sweep than the Mamba and here the area is split 24/76, again, no handling problems.

So I am happy to go with the set up I have chosen - we will find out if there is a problem in due course, and if so I will be only too happy to confirm that theory is right and my approach wrong!

Incidentally, one other important aspect is to make the vertical leg of the drive bellcrank longer than the horizontal one, in the case of the Mamba the ratio is 3:2, again a figure I have used before.

With the pivot in the forward position I do expect the elevator response to be less snappy - i.e. "heavier" - than if it was further back, but I think this will be better for a swept wing fast flying model which will look best doing wide fast figures rather than 3D type aerobatics.

But as I said, thanks for the thought and never hesitate to chip in if anything strikes you.

BTW, how are your builds going?
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Old Nov 30, 2011, 11:46 AM
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That's good then George! I didn't doubt your competence I think the issue here is more about servo loads rather than 'will it or won't it fly'. I would imagine the servos would be up to the job and on these smallish models the loads are low enough not worry too much about flutter. And the forward position should reduce 'snappiness' as you say.

My build s are going rather slow I'm afraid but I'm making small bits of progress. (A stringer here or there ) I've currently got 3 other models on the go (!) including a pistachio Yak 11 racer that I have to complete by the end of Dec for a free flight cook-up. It's a bit smaller than yours!

But the Frogs will happen! As soon as I have anything interesting to post I'll start a dedicated thread for them...

Jon
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Old Nov 30, 2011, 11:58 AM
RFJ
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Must admit, with just a quick look at the parts pictures and not doing any sums, I would have reversed the drive bellcrank - pivot at back, drive at front and with the lower drive arm swept slightly forward if necessary to fit into the fuselage but..........Mr Foss knew how to design models and his Phase series of gliders were great performers so who am I to argue I'm sure George's arrangement will work perfectly OK.

Quote:
But the Frogs will happen!
Good to hear that Jon. Just about to start my next one.

Ray
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Old Nov 30, 2011, 01:38 PM
I like real wooden aeroplanes!
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Quote:
Originally Posted by Yak 52 View Post
I think the issue here is more about servo loads rather than 'will it or won't it fly'. Jon
Quite right Jon, I have always used fairly hefty servos on AMTs in the past, this one will be a metal geared HS85MG which should be well up to the job in a 24 ounce model, even a fast one!

Pastaschio eh - too delicate for my clumsy fingers! I am full of admiration for anyone who can build an 8 inch span scale model AND get it to fly!
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Old Dec 03, 2011, 04:28 PM
I like real wooden aeroplanes!
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Steady progress this week, I've been working quite slowly and deliberately as getting the AMT set up right is going to be crucial to the performance of the model. The tailplane halves have been carved and sanded, complete with joiners they weigh 20 grams (3/4 ounce).

I cut and filed the tailplane drive crank from 1/16" thick fibreglass PC board and bushed the centre with a 1/8" length of tubing. The fin is basically 1/4" thick with 1/16" sides and diagonals so there is 1/8" of space in the middle for the crank and drive mechanism. This means that to be sure of clearing the clevis which connects the Goldenrod to the crank, I have cut a slot in each side of the fin which will be covered with a 1/32" ply plate.

The build sequence was to first make one side of the fin, then fit the crank and drive snake, retained by the pivot rod, then add the other side of the fin structure. 1/16" ply patches have been let into the fin side around the location of the pivot rod. Everything fits together nicely with no wobbling and zero slop, and the control movement is nice and smooth without any binding.

Next job is to fit the tailplane stubs to the fin, these are 2.5 mm thick, an inner layer of 1/16" balsa and a facing of 1 mm lime, the same material which has been used to face the ends of the tailplane halves. The stubs will be carved to shape to match the tailplane roots, then fitted loose over the pivot rod. With both tailplane halves fitted everything is squared up carefully by using two 90 degree triangles cut from 1/4" balsa, and only when properly aligned are the stubs cyanoed to the fin, followed by final carving of the fin to section. Once that is done it will be on to the wings.....

As shown in the photo, the assembled fin/tailplane, with joiners and Goldenrod drive. weighs 36 grams, which is quite an acceptable weight.
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Old Dec 03, 2011, 04:49 PM
Scott R/C Time Pilot
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Very tidy George. I surmise that your Mamba will be elevator and Aileron? I didnt see a provision for a rudder on your drawing.

Cheers
Scott
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Old Dec 03, 2011, 04:53 PM
I like real wooden aeroplanes!
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Yep, that's right Scott, no rudder on this one - it's intended for high speed jet fighter type zipping around the sky, hence the target 130 watts/pound power loading, and a rudder is generally pretty superfluous for this kind of flying.
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Old Dec 04, 2011, 09:41 AM
I like real wooden aeroplanes!
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Well, winter has finally arrived here in the Haute Vienne, it is blowing half a gale and p****g down outside so nothing for it but to spend a little quality time on the building board. Don't you just LOVE proper model building, what do all the guys who only fly RTFs do when the weather is like this, sit and look at their models?

Prior to getting on with the wings I wanted to completely finish the AMT/fin assembly, which involved fitting the tailplane stubs and the little fairing at the back, carving and sanding the fin to finished shape and fitting the covers over the elevator drive connection, photos below.

With a thin washer slid over each side of the pivot rod to give 1/32" inch clearance between the stubs and the tailplane operation is smooth and there is no binding or slop at any point throughout the full movement (which I suspect is quite a lot more than I will ever actually need to use in flight). I was going to fit a wire retaining clip to hold the tailplane halves on, but the fit is so good that you have to give quite a pull to start them sliding off, so I will think about that, it may not be necessary.

I'm glad I elected to build this assembly first, getting this dead right has given me the impetus to crack on with the rest of the build.
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Old Dec 04, 2011, 01:28 PM
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Lovely work George!
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Old Dec 04, 2011, 04:24 PM
Scott R/C Time Pilot
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Cool, very slick, the tail looks like an 80's sloper, ala Son of Savage(Rebeck)

Scott
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