Open Goodyear - Mr D

Open Goodyear - Mr D

Postby ash » Thu Jan 17, 2013 11:14 pm

In the spirit of information sharing and encouraging participation, I'm going to document another racer build here for all to see. I'll take photos as I go and occasionally post them along with relevant info to explain what's going on and why. Feel free to ask any questions, make suggestions or boast about your own GY model as we go.

Next in the build queue is an Open Goodyear model. I've built a couple of half-arsed Open GY models before, but it's a serious class and needs serious gear. There are possibly more really fast GY models around NZ now than there have ever been, so it seems apt to go for gold. More important than speed, though is reliability. For all the really grunty Goodyears around, the "medium" pace models often win by virtue of being the only ones to make it through the full 180 laps!

My pick of the traditional Goodyear designs is the Mr D - based on the 1951 Mayer home-built racer of the same name with racing number 74.
Dave Clarkson's legendary Aeromodeller original is easily the most popular design in NZ because of its various helpful natural attributes as described in the original article:

~ Low wing, large root chord (allows the fuel tank to be ideally placed)
~ Short Fuselage (avoids breakages on landing or in the catch)
~ High aspect ratio (nice glide, easy to catch)
~ Relatively large tail (easy to fly)

Image

Full size version here: https://docs.google.com/file/d/0BxQgvOg ... sp=sharing


But, Clarkson's Mr D was designed for British GY rules with 2.5cc diesel engines and short lines, so there is room for tweaking to take advantage of NZ rules:

~ 3.5cc glow engine on crankcase pressure (LOTS of power)
~ 26mm wide hollow fuselage (fully internal controls and a much stiffer and possibly lighter structure)
~ 17.69m lines (the extra drag will absorb some of the power and lower rotational speeds make flying a proper monster a little bit easier)
~ Refuelling at all pitstops (the British "Click n' Flick" pitting method with huge fuel tanks - no refuelling, just restart and go - is not allowed, so a smaller/lighter tank will suffice)

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At the 2012 Clareville Nats I nabbed a couple of Stels F2D engines (2.5cc glow combat engines) from Peter Williams and initially planned to use them in this model. The idea was to take advantage of the Stels very light weight and build a very light model that was reliable and very easy to fly. Having flown Rob Wallace's Go21 model at the Kirwee Nats recently, I realised that the heavy duty models are not as scary as I was expecting, so I decided to break out my collection of ex RC car engines and choose the scariest one. I long ago realised that you should never buy used engines from RC car or boat people. They're butchers and only seem to sell engines once they're wrecked. For some reason I put a low bid on a pair of fancy Go21 engines in contravention of my own rules and accidentally won the auction... no-one else bid! $50 each - and they're actually in pretty good condition. So, the scariest one - an MG66. It's an Australian designed version of the Go21 series, full of fancy extra ports, titanium nitride coated crankshaft, ceramic bearings, turbo glowplug and surely some other magic. They're highly reputed in the RC car world, so lets see what they can do in the air....

First order of business - making it airworthy. Today's accounting procrastination distraction has been to make a prop-driver and nut, and a venturi for it. The NVA is on order and should be here in a couple of weeks - just a standard Super Tigre style unit. Somewhere I have the cooling head, which I will cut down to help clamp the "turbo" head button more securely.

Behold - MG66:

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Taiwanese made Go21 MG66. It's a 3.5cc glow engine, supposedly 3 BHP @ around 32,000rpm. The MG66 version was re-designed and modified by Massive Mods in Australia for the top offroad car racing class.

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The APC prop won't be pushed that hard though, I have some carbon props moulded up for business time. More on those another day.

Late Edit:

Yesterday I finally got the ST needle valves and fitted one to the MG66. The pinch-bar hole is 6.0mm in this engine and the ST spraybar is the usual 4.0mm, so I made an adaptor 4.0mm ID and 6.0mm OD and soldered it over the ST spraybar. Redrill the 1.5mm fuel hole through to the insides and it's ready to roll.

Image
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Re: Open Goodyear - Mr D

Postby ash » Sat Jan 19, 2013 5:26 pm

Some good advice from Rod:

Rod Brown wrote:Hi Adrian, I see you have grabbed the tiger by the tail, that motor should go like a haunted shite house. Reliability is the key to fast g/y. Don't even think about rubber quick fill valves, the crankcase pressure from a good car motor will turn them inside out. On the tank side of things I have gone to mechanical quick fill, pressure dome with 1 mm hole thru to tank and a vent valve. With this set up you can fill the tank and pump up the tank also . When the shut off is reset after refueling you will get a carb prime .

ROD


Rubber Quick Fill equipped tank - seals itself after refuelling, tank pressure squeezes it shut from the inside:
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Self-venting filler valve. The valve feeds fuel in down the middle and air can vent out around the periphery of the valve stem during refuelling.

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From left to right the bumps are: filler valve - pressure dome - vent valve.
The pressure dome is an air-lock feeding crankcase pressure into the tank in uniflow fashion, but preventing fuel from burping back into the crankcase on re-starts.

Image
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Re: Open Goodyear - Mr D

Postby ash » Sat Jan 26, 2013 8:29 pm

Tail Feathers.


Tailplanes are extraordinarily important in Goodyear racing. The tailplane and elevator is your only means of flight control. Thanks to the legislated design restrictions, they're often a bit smaller than you'd like, a bit fragile and swinging around on the end of a skinny fuselage. All this has an impact on good things like stability, control, easy flying and long life. Typically you have the elevator only on one side. This makes life simpler and reduces drag, but it means that if things go wrong, they go wrong in a big way. Last year we saw several models go down due to tailplane failure, so I'm keen to avoid joining that queue.

Things I think are important:

~ 5mm thick tailplanes. The "usual" 3-4mm balsa isn't quite enough and doesn't make best use of the stiffness available from the fibreglass skin. A tailplane with a 5mm + root and 3mm tips will be about twice as stiff but much less than twice as heavy. Light weight tails are important, but not much use if they fall off in flight! It only takes a bit of unseen hangar rash, a pitting incident or an unfortunate vibration to turn a weak tailplane into a model-destroyer.

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~ Spruce or cedar spar. Most well known GY plans show a "soft" balsa core edged with 3mm spruce to provide some knock resistance to the thin leading and trailing edges. Firstly, really soft balsa is not only weak in the conventional sense, but also extra prone to cross-grain cracking and handling damage. Bad news for a tailplane before you've even made it! Quartersawn (aka C-grain) medium density balsa is my choice for stiffness, stability and toughness. I like to add a 3mm spar right down the hinge line from tip to tip. It's there for a few reasons - Firstly to provide added strength and stiffness. Secondly to isolate the front half of the tailplane (the bit that takes all the load) from the "notch effect" (look it up in an engineering glossary for a decent explanation) of the square corner where the elevator is cut out. Thirdly to provide a sturdy and slightly more fuel-resistant structure to mount hinges in. For this third reason the elevator is similarly edged with harder wood.

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~ Triple hinging. I've tried stitched hinges, mylar hinges, pinned hinges, brass tube and piano wire and currently prefer CA hinges for racer elevators, mostly because they're easy, clean and so far reliable. Whatever you use, I think having three of them is the safe bet. If something bad happens and one hinge ceases to do its job, at least there will be two more maintaining your critical control surface. Hinges don't do very well on their own, I have the DNF to prove it. I also like having a wire elevator horn pivoting through a firmly fixed brass tube to isolate the elevator from longitudinal pushrod loads... more on that later.

~ Don't operate the fuel shutoff from the elevator horn or another horn on the elevator. Lots of people have being doing this without problems for longer than I've been alive, so I'm sure that many will disagree, but it seems risky to me to apply unnecessary impulse loads to your elevator hinges. I prefer to attach the shutoff trip wire to an adjustable clamp (wheel collar or similar) on the pushrod or to a dedicated fitting on the bellcrank horn itself. That way you're not yanking on the elevator every time you fire the shutoff. It also makes for a more positive firing action and smoother controls.

~ The usual basic requirements - Straight, symmetrical section, smoothly shaped. Twists, warps or lifting sections are probably bad news for your flight characteristics, drag, predictability, glide and groove. The wing and tail need to be fitted at zero degrees with respect to the engine centreline, more on this later.

Tailplane carving time begins with the razor plane. Taper the blank evenly both sides so that the tips are about 3mm thick and the root remains 5mm thick.

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I draw a centre-line around the cedar linings and carve the edges down to it. About 20 degree bevel on the LE and maybe 5 degrees on the TE to start with, then remove the soft balsa between there and the high-point. This helps avoid scalloping out the soft balsa in between the harder cedar.

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120 grit sanding WITH A SANDING BLOCK smooths everything out. Take care with your pressure and direction so as not to eat away the soft balsa between the cedar strips. Sand to 240 grit and round off all the edges before glassing.

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Not an aerodynamically ideal section, but it is structurally best to make the spar the high point, or at least to extend the high point back to the spar.

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Elevator, fin and canopy (1.5mm ply) ready for business. Plans often show the canopy and fin in balsa, but they are too prone to damage in racing conditions with the inevitable hard landings, flip-overs, ragged catches etc. Remember, even a jettisoned bit of balsa (or anything) is an instant DQ, so tough 1.5mm ply is the answer for fin and canopy. At this stage the tailplane/elevator is 10 grams. Should be under 24 grams with hinges, horn and glassed.

Image
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Re: Open Goodyear - Mr D

Postby DH100 » Sun Jan 27, 2013 8:29 am

Excellent Ash.

Some of those out there who want to know the basics of building will learn a lot from this thread. The real advantage of models like this is that it is a short program ...no long drawn out deal like some builds ... one I know well in particular! The end result is in your hands completed almost before you know it.

I get a great kick out of Sportsman Pylon but had an argument with a fence post last time out. John Danks has been keeping the pressure on to get back in the air ....and I will, but all my attention has been on my Vampire which you know, has been going backwards as well as making progress at the same time.

Great to see this and I will watch with interest.

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Re: Open Goodyear - Mr D

Postby ash » Sun Jan 27, 2013 3:39 pm

Thanks Brian. Most of the racers already out there will know all this stuff inside out and have their own ways, but it's the new recruits, retreads and converts that I'm aiming to help with this stuff. Articles like this by Dave Clarkson in Aeromodeller are what got me into this stuff in the first place. It's hard to find good info about making stuff from scratch in this polystyrene world.

Good luck with the Vampire and Pylon repair projects. Inspiring stuff!

A.
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Re: Open Goodyear - Mr D

Postby ash » Sat Mar 02, 2013 12:11 am

A lot has been done since the last post, wings have been carved (yes WingSSS! More on that later), the fuselage is mostly built and glassing has commenced. Rather than leap into that lot, I think I'll stick with the tail bits for now and move on to the other things once the tail appendages are done.

In our last episode the tailplane was carved and sanded and the ply fin and canopy silhouette cut out. Today I glassed them.

I used 0.73 ounce/sq-yard glass cloth from Aerospace Composite Products in the USA because I was getting some special stuff from them anyway. It's the nicest cloth I've seen and much better than the very cheap HK stuff doing the rounds. It is however, not very cheap. You pays your money, you takes your pick... Airsail, 436, Jambo and others probably have decent glass as well. Glass cloth weight is possibly more of a factor than its cheapness or otherwise. 0.73oz is equal to 24g/m2, which is very light. In order to get it any lighter there tends to be quite a lot of space between the weave that will ultimately get filled by epoxy or some other compound. That's weight not doing much for strength. The next grade up is 1.4oz or 48g/m2, which is less than double the thickness and woven from similar tow, but with less air space in between to be filled up with epoxy. More strength from not much more weight. We like that.

So, 24g on the tail. It's strong enough and with a marginal weight advantage, so I'm ok with it. The heavier cloth would have been fine too, given the beast of an engine up front tail heaviness is unlikely. On the wing and tail I'll use the heavier stuff.

How many layers? Just one is fine - one piece over each side. Sometimes I use two pieces on each side overlapping in the middle to give a triangle of double thickness to reinforce the centre. I forgot to do that this time, but with the extra wooden spar in there I'm not worried about strength. Don't bother with using multiple layers of light glass where you could use one layer of thick glass. It's very inefficient in terms of strength/weight ratio. The only reason to do that is if you don't have the right cloth for one layer or if the curves in the part are too tight to get heavier cloth around.

How to put it on. Press it! That's the best way, I reckon. You could also vacuum bag it or just leave it to set in the wide open.

My press, as most are, is a pair of MDF panels lined with firm foam sheet. Mine is a couple of layers of reject bed roll from Para Rubber. Other people use couch cushion foam, but I think that's a bit too soft. EVA bed roll is possibly a bit hard, but I didn't find anything in between. The perimeter of the MDF panels is laced with long coach bolts and M5 blind nuts to apply the pressure. You'll see in the pictures how it all goes together.

Alternatives - small items like these could be pressed inside an old phonebook with weights on top. Vacuum bagging is good once you're kitted up and have some experience, but that's easier said than done and you'll still need a press to keep the goodies straight while the epoxy cures. Plenty of resources for that on the interwebz, so hit Google if you want to go that way. The old-school way would be to simply brush or squeegee thinned epoxy over the glass cloth as if you were doping silk onto the balsa and sit it flat to set. Works fine, but needs a fair bit of sanding to level out and is consequently not as strong or light. Beggars can't be choosers etc. Speaking of dope, it is often recommended to paint the balsa with a coat of thinned dope and sand lightly before glassing in order to prevent the balsa soaking up too much resin. In my experience this just harms the adhesion of the glass/epoxy layer to the balsa, so I don't do it. That may also be because I can't be bothered finding a suitable brush and waiting for it to dry... try it, you might like it better than I do.

So... here's my press with three tailplanes, a fin and canopy laid out with their respective sheets of glass cloth. Everything is cut out and lined up before the epoxy is mixed. They're all laying on a waxed (mould release wax - probably not really needed) clear mylar sheet (thanks to Andrew Robinson) which will serve are the release layer, foam protector and surface mould:
Image

I use ADR epoxy resin from Adhesive Technologies in Auckland. Fantastic stuff. ADR240 resin plus ADH28 hardener: http://adhesivetechnologies.co.nz/adrseries.html
West System is commonly recommended and easy to get, but ADR is much better. Lay some on and spread it out. Ideally you'd weigh the glass cloth and use an equal amount of resin, carefully spreading it over the glass on the waxed mylar sheet to make sure you're not adding excessive resin and unwanted weight. And here are the wee panels all tucked up in sheets of delicate glass lovingly smeared with the finest epoxy resin applied by a rubber squeegee:

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I find that in practice your spreading out procedure will effectively eliminate most of the excess and leave the weave of the cloth not completely filled, but not completely dry. You want to be able to see the weave of the glass through the thin layer of resin evenly over the whole piece. Any weave or pin holes are only going to get filled with paint later anyway, so it makes insufficient difference to weight in the end to get too frantic about it. What you want to avoid, however, is having great glistening pools of resin floating over your cloth. That may not completely squeeze out and you could add a whole lot of weight and have the glass cloth swimming over the balsa surface rather than pressed onto it. If your resin is too thick on a cold day, or on a hot day it starts to set before you've spread it, you're not going to be able to get rid of the excess and disaster will occur. On a cold day you can thin the resin with a small amount of acetone - 5-10%. On a hot day, spread your resin out in a shallow dish so that it stays cool. Epoxy generates heat when it sets and that heat just makes it set quicker which makes yet more heat. Here's a picture of the measuring cylinder I used to mix the resin today. It was slightly hardener-rich and sitting in an insulated plastic tube on a hot day. Chain reaction coming right up! It got hot enough to boil itself and melt the plastic tube. It has set hard while still in a boiling state leaving this brittle foam cylinder.

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So... avoid that. Regarding glass/resin ratios and whatnot - ideally you want about 40% resin to 60% glass fibre to get the optimum strength/weight ratio. That's near enough to the mathematical maximum possible. In practice it is not so easy, so you should be happy with 50/50. Weigh your part before and after. Weigh your glass and resin. Calculate what they should be based on the surface area and see how close you're getting to the ideal... or not. It's a good idea to weigh the parts at various stages and take notes so that you can predict the final weight and make changes before it's too late. For the record, my tailplane was 9g bare and is 13g glassed, so that's about 2g of glass and 2g of resin. Calculations predict about 1g of glass, so there is in fact a lot more resin in there than the theoretical ideal! I will have to be more careful with the wing, because it's so much bigger. Two grams on the tailplane won't keep me awake at night worrying.

Anyway... lock that press down tight and come back tomorrow:
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Peel open the mylar release layer to reveal these freshly encapsulated gems:
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There it is, flasher than Michael Jackson:
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Well, we can't have that, so the flashing gets cut off:
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And the edges get sanded down and rounded over smoothly:
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And there we have it... tailplanes for the year:
Image
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Re: Open Goodyear - Mr D

Postby DH100 » Sun Mar 03, 2013 9:45 pm

Wow, that's a neat covering idea for that style of build. I'll file that away.

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Re: Open Goodyear - Mr D

Postby ash » Tue Mar 05, 2013 10:16 am

Fuel tank technology has come up again on CLNZ. Here is the discussion:

Dave Ackery wrote:Open Goodyear can be attractive if you want to go fast for not much money.
When they go the models are amazing but it can be frustrating too, there are many ways thing can go wrong, (let me count the ways,,) however the biggest issue is restarts from pitstops.
Some smart people have it sorted, and some people have some progress to make.

Some questions
1 ) Tank size –
Bback in the day,, the strategy was a huge 100 lap tank. Make two quick stops, then fly out the laps never adding fuel to the tank, and never releasing the tank pressure, just reset the cut-out in the pit stop and restart. This was know as ‘click and flick’, and was quite reliable for a heat, ( in the final you did need to have one stop where you filled the tank).
But the current rules require fuel to be added at each stop, which negates the click and flick strategy. So these days do we need to the huge 100- 130 cc tanks ?, what size tanks are the smart fast guys using ?.

Should the tank be rectangular, wedge, front feed , rear feed,, ?

Are good stops possible if you run the tank dry, or is it necessary to still have fuel left ?

2) Stops - What goes wrong and messes it up and leave people flicking in the pits for ages ?.
What is the key to reliable stops, restarts ?.

I am assuming that we have two things in place first,
1) no pressure leaks anywhere , none, nil, nothing

2) the cut out fully shuts off the run AND the pressure line, (easy to say)


Any good ideas, comments or suggestions on the subject are appreciated.

cheers
Dave


Adrian Hamilton wrote:I have been thinking about these things a lot lately and picked up the following clues from people who know:

Tank Size - Big enough to fly the race out if you unexpectedly have to make both pit stops early in the race. Small enough to keep your wet weight well under the maximum.

Tank shape - I have observed all three shapes in the hands of successful experts. I'm leaning towards front feed or mid feed to keep fuel lines short and close to the CG.

Shut off - Running the tank dry loses any tank pressure and can make the engine run lean or erratic for perhaps several laps, losing you time and perhaps overheating the engine and killing your chances of a fast re-start. Not to mention the uncertainty of where in the circle it finally stops.

Stop/re-start errors - Overheating due to lean settings, leaky tanks, erratic shutoff, long runs, loading up etc will kill the hot restart. Flooding during re-filling due to bad set-up, too large a pressure feel hole, refuelling with an open shutoff etc. Shrinking the piston relative to the liner with excessive exhaust prime. Insufficient priming. Dead, distorted, leaky or loose glowplug. Too much or too little glow driver power. Dodgy glow driver wires and connections. Insufficient compression. Unfamiliarity with the engine and fuel system.

Speaking of Open Goodyear, I have several 21 car engines in the process of being converted for CL use. Will consider selling some of them to people who show that they will use them in competition. I may also have a spare glass Mr D wing as I made two for my current project and don't have a use for it right now.

Persons interested in converting their own engines will be interested to discover that HobbyKing have Go21 and 25 clones branded "EG" for under $60USD. An absolute bargain for Open GY or Open B.


Ian Thompson wrote:my take on GY tanks.. about 75cc for a .21 glow should give around 50 laps hence a margin for error should something happen like a shutoff activated on takeoff etc..
Stops are more reliable when there is fuel left in the tank..when the shutoff is opened the engine is primed due to the residual pressure in the tank.
Shutoff should be closed during refill to stop flooding..
If you have a separate vent valve the tank can be repressured using the filler bottle..stops the engine sagging as it builds tank pressure again..
Find what works in practice & follow the same routine everytime!!
My tank is a 1/2 coffin shape with the pick up about about 1/3rd down the tank.
All tank joints should be lapped & tank should be made of tinplate or stainless steel..brass shim work hardens & splits at the worst possible time..Use good solder preferably bar solder not rosin cored & neutralise any flux used..
hope this helps
Ian


Rod Brown wrote:For my 10 cents worth , When using rubber quick fills the procedure is . Shut off motor with fuel still in tank ( shutoff shuts off fuel supply and pressure line). At the pitstop pilot holds down elevator . Pitman resets shutoff and releases . Pilot then moves elevator from down to level /up . Pitman refuels ,starts motor .
I now use a spring loaded tank filling valve , spring loaded valve on refuel bottle, pressure dome on top outboard side of tank with a 1mm hole thru to tank (outboard as far as possible). Also a spring loaded overflow valve. This procedure is shut off fuel and pressure . At pitstop pilot holds level/up elevator. Pitman starts refueling first , opens overflow . When tank full keep refueling , release overflow, withdraw refueling bottle keeping pressure in it ( this pumps up pressure in tank) Reset shut off, start motor. I don`t use a non return valve in pressure line , but I will try it again in the future. I like to power up glow plug before resetting fuel fuel shutoff , don`t know if it makes a difference .
One thing I believe is very important . The uniflow point on the tank must be outboard of the point where the fuel meets the air in the carb. I like about 5-7mm .
If you don`t do this you will have trouble cold starting unless you open the needle. Hot starts / takeoff wont be good unless you have a really tight motor.
My tank is around 70cc . With the extra range it will give, you can pick your pitstops when the ground is clear. These models are not good at overflying the segment before your pitman. Also a big tank allows more nitro and be able to run on the rich side to liquid cool your motor

Hope this is of some help ROD

ps slow goodyear is another kettle of fish . Anybody having trouble ? Ask and I will write my set ups for diesel and glow
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Re: Open Goodyear - Mr D

Postby DH100 » Sun Mar 24, 2013 2:03 pm

Hi Ash....so what's happened to the racer?

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Re: Open Goodyear - Mr D

Postby ash » Mon Mar 25, 2013 12:00 am

Progress has been continuing Brian, but I haven't had time to post it here yet. Pre-contest repairs for other classes take precedence.

Since you asked, here's some wing construction detail...

I'm not one to follow instructions very well, and I tend not to follow plans exactly either. I can't resist "improving" or customising. As well as tweaking the Mr D design to take advantage of the broader rules here in NZ, I'm trying a few structural alternatives to, I hope, capture a net gain in power utilisation from the monster Go 21 MG66.

Goodyear wings are traditionally carved from light 3/8" (9.5mm) balsa and covered in light glass cloth for stiffness and toughness. One of the keys to speed in CL racing is vibration reduction. Stiffness plays a very large role in minimising vibration, so anything we can do to increase stiffness, especially at the join between the wing and fuselage, will result in more of our power being turned into airspeed rather than vibration. The extra wide fuselage and cheek cowl visible in the CAD plan above cover half the equation. I figured that the wing needed to be beefed up to match, but not at the expense of weight. There are two ways to increase the stiffness of the wing. 1 - use stiffer materials. 2 - make it thicker (get the strength further from the centre).

Carbon fibre instead of glass cloth covering would have been one good way to make the difference, but I'm far too cheap to spend $80 on spread-tow carbon for a GY wing. I'm also too cheap to let my stack of medium-heavy 1.5mm balsa sheet go to waste, so I put two and two together and decided to try a blue-foam core sheeted with balsa and glass cloth, much like a pylon racer wing. The foam being so much lighter than balsa gives me the chance to make the wing slightly thicker in the middle and also to use harder balsa without a weight penalty. I estimated that the resulting wing would be twice to triple the stiffness of a conventional wing. As it happens, the unglassed wing was five times stiffer than the conventional wing I made in parallel.

To the bat-pictures!

The blue-foam core was hot-wire cut in the usual manner, 1.5mm balsa sheeting is glued to the foam core with epoxy. The TE is taped down to stop it sliding around in the press:

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The balsa/foam laminate is compressed inside the foam shucks to ensure proper alignment, straightness and adhesion while the epoxy cures:

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The leading and trailing edges are cut back to receive balsa and spruce reinforcement. The foam core isn't enough by itself to survive the impact of the pitman's catch and the TE is too thin to hotwire reliably.

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LE and TE balsa strips are to be added one at a time.

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Good old $2 shop masking take makes a great self-releasing clamping system for awkward shapes. Stretch it over and rub it down.

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Spruce, cedar or fir edging takes the knocks.

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Wing tips are laminated from thin ply or veneer. What you can't see there is the 17 grams of lead tip-weight inside the outboard wing tip and the leadout channels inside the inboard wing.

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Since this wing is already largely shaped by the foam core, only the add-on parts needed to be carved. I found it much easier to do the aerofoil shaping with no tip edging and then to add the tip afterwards, so I have done the same on all wings and tailplanes made since. It's much easier to use a razor plane when there aren't contrary grain directions to deal with.

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Here it is after a bit of fine tuning with the razor plane. The light balsa strip on the near side is the hard 3/8" reinforcement for the catch at pit stops.

Image

So at this point, as I said earlier, it was five times stiffer than the conventional solid balsa wing I made at the same time. Most of that stiffness comes from the fact that it is 20% thicker all the way along and that the surface material is harder and stiffer than the light balsa of the conventional wing. Unfortunately it is also 20% heavier than the conventional wing, so the density balance didn't quite work out to plan. Some people might fear that the thicker wing is a drag penalty, but in the context of 17m of steel wire hanging off one wing tip and an exposed engine and tank, the wing only contributes about 3% of total drag. I suspect the power conversion advantages of the stiffer wing will far outweigh a possible 0.6% extra drag. Time will tell whether it is a worthwhile experiment. We may never really be able to tell what it is doing for speed, though, unless I build another almost identical model with a normal wing. Whether it breaks or not is the most likely determinant of success. For all the extra time and fussing, I'm not sure that this is a useful innovation in practice, but it certainly satisfies the urge to deviate from the published plans!

Next time, more glass pressing and maybe the spare wing.
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