Another Parnall Elf

[Greyhead]

Thanks everyone for the favourable comments and I agree with you Barry; the “banking right” photo above is about as aerobatic as it gets with the Elf but that’s as it should be.

[Greyhead]

When I started the Laser at the recent “photo shoot” it was quite noisy but having travelled 160 miles to get to the flying site there was no way a bit of noise was going to stop the flying! During the session the noise level gradually increased and on the 4th start exhaust fumes started to exit via the cockpit; those of you who have followed this thread may recall that I made a remote silencer that up to date had performed very well but something was obviously amiss.

The exhaust system consists of about 6” of stainless steel flexible pipe connected to the silencer by a short length of silicone tube reinforced by braided stainless steel. As you can see from the photo the silicone tube has completely failed, I assume the missing bits must have been blown out through the silencer.



The hot exhaust gasses eventually burned a hole right through the firewall, hence the fumes from the cockpit



Both aerobatic and funfly models use these silicone tubes extensively to join the exhaust to the “tuned” pipe but not enclosed inside a cowl; the reduced airflow and the resulting increase in temperature obviously proved decisive.

A re-design of the exhaust system is called for and a fair amount of repair work to the firewall so no flying for a while (with the Elf at least) but it could have been much worse, just behind the firewall there are a lot of wires for the radio equipment and of course the fuel tank; the Elf could have come to a spectacular end!

[wonwinglo]

Grahame you were lucky there ! strangely enough mu Elf caught fire also,I had an engine backfire and flames licked at the cowl with scars to show it,it is horrible when you hear the crackling of a glo fuel fire,like meths you cannot see it and before long it takes hold,I once lost a brand new control liner this way in my youth,Nitrex 15 and all that.Always carry a fire extinguisher around with you when you go flying,it is no joke when your model ignites.Some tricky repair work there Grahame.

[Greyhead]

The damage is not as bad as I’d first feared, what I thought was completely degraded wood turned out to be the remains of the anti-vibration foam. Yes there is a hole through the firewall (an appropriate name as it happens!!) but the main structure is still sound. There is some delaminating of the ply but having removed all the charcoal and giving it a thorough soaking with super-thin cyano it’s now as strong as ever.



The only thing inside the fuselage is a bit of scorching of the wood and the insulation on the power lead has turned brittle. Luckily it’s not the battery lead itself but the switch harness lead so that’s easy and cheap.



This repair shouldn’t take long, just a case of gluing a patch on both sides, but figuring out what to do with the exhaust system to stop this happening again may take a bit longer!

[Greyhead]

The damage is not as bad as I’d first feared, what I thought was completely degraded wood turned out to be the remains of the anti-vibration foam. Yes there is a hole through the firewall (an appropriate name as it happens!!) but the main structure is still sound. There is some delaminating of the ply but having removed all the charcoal and giving it a thorough soaking with super-thin cyano it’s now as strong as ever.



The only thing inside the fuselage is a bit of scorching of the wood and the insulation on the power lead has turned brittle. Luckily it’s not the battery lead itself but the switch harness lead so that’s easy and cheap.



This repair shouldn’t take long, just a case of gluing a patch on both sides, but figuring out what to do with the exhaust system to stop this happening again may take a bit longer!

[Greyhead]

Well I’ve managed to modify the exhaust, as you can see from these photos it’s not the most elegant of engineering jobs.



Modifying is nearly always more difficult than starting from scratch, in this case the silencer inlet and the flexible exhaust are slightly different diameters; this wasn’t a problem when they were joined by silicone tube but I’ve had to use 3 different diameter tubes to join everything together.



As it is impossible to fit the exhaust as a complete unit it is in 2 pieces, once again not a problem when they were joined by silicone tube. The “moveable” joint is a reasonably tight fit and has been sealed with car exhaust paste. On the test runs it’s sealed OK, just hope it stays that way when in flight.

[Guest]

hey hey

not been around awile just caught up weith this topic

love the shots of it in flight hope your silencer holds out this time

[wonwinglo]

Grahame,its belt and braces but who cares it has done the job,judging by those scorch marks you were very lucky to not have a totally burnt out airframe,I hope that it holds together for you,they used to rag me about my fire extinguiser in my field kit but it saved another persons model once,get one of those small powder extinguishers from a car accessory shop and put some brackets on the side of your field box,you do not want this to happen again.

[Greyhead]

Haven’t been able to test the modified exhaust in flight as yet because I’ve “pulled” my back (again!!) and can’t carry my flight box. The way the weather’s going it may well be next year before I can report back about that but here’s a couple more photos that show the very distinctive shape of the Elf in the air.





and a "fuzzy” shot of a landing; it was just too far away from the camera!

[wonwinglo]

Lovely flying shots Grahame,join the club I have had a bad back these past couple of weeks as well ! probably an old injury rearing its ugly head,very painful.

There is just something about the sit of the Elf whilst in the air,quite unique with that warren girder strutting arrangement,thanks for showing these reminders of what a fine biplane she is,mine sits with flat tyres in my hangar having not flown now for some three years,a real hangar queen.

[Greyhead]

At the end of last season this thread was left a bit “up in the air”, paradoxically because I couldn’t get up in the air!

I can now say that the exhaust works very well and there are no signs of the paintwork failing around the undercarriage.

One thing of note is the aileron control arms; these originally used a 2mm nut and a closed loop adaptor.

Nuts are intended to be locked, either against another nut or some hard surface, but in this instance I’d used the nuts as a bearing and vibration etc. has caused considerable wear.

I’ve replaced the nut with a length of snake inner cyanoed into the tube and then tapped 2mm for the closed loop adaptor. I was a little worried relying solely on glue to hold the tube and snake together but when testing the result it was impossible to pull the closed loop adaptor out of the tube.

[Guest]

Grahame,

Just a quick tip on the exhaust tubing if I may,

I recently had a similar problem on one of my Nephews R/C Cars where the silicon tubing had melted joining the manifold to the tuned pipe....and this is on a Nitro Engine. Wanting to do a quick easy fix without travelling miles to a model shop or waiting for mail order, I went to Halfords and got some car radiator hosing.. ok, it cost me just under £5 but I have plenty spare and as it is quite thick walled stuff it wont melt easily. Given the sort of temperatures it is designed for in a proper car, I don't see this stuff burning away just yet. The High temp Fuel tube as sold in Halfords would probably do just as well for your model as it is slightly small diameter and still thick walled.

Just use a couple of small hose clamps to secure and it's done.

So far my quick fix on the car has worked well and has had several runs, the Tubing is still as good as when it was fitted.

Ps......Glad you managed to get some flying in, she looks fine in the air.

Regards......Mark

[Guest]

Your article in the mag inspired me to look closer at the Elf II, (straight wings) and I have been working on a CAD plan for a 70" with an RCV 91. I didn't find any plans anywhere except for a 3 view of th Elf 1, some what different to the Elf II and looking as if it was a 5th down the line copy. I visited Shuttleworth last year and was kindly allowed to get a load of photos when it happened to be in the workshops. I am hoping to visit Shuttleworth in the next month or so which is a bit of a journey ( I live in Cyprus). I have to thank you for some of your ideas which have been very useful, the wing locking mechanism for one. I am currently at a 90% finished stage with the plan and am getting pretty good with CAD drawing and have collected quite a few bits and produced the parts for both wings and hinge and lock mechanisms.

In the mag You didn't mention wingspan or engine size but I have now found this site which would have made my life a lot easier if I had found it a year ago but I am retired so have a bit of time available. The local club here is pretty good (safety is a bit dubious though). I will post some pics when I get to that state. All the best

[wonwinglo]

Hello 'R' good to hear that there is still plenty of interest in the Parnall Elf design,my own model is now an hangar queen with flat tyres but she has given me many happy flights with an OS.91,the plans I used were done by Chris Hughes a friend of mine who goes under the name of Modelplan,he is based in Coventry,UK.

My own model was the protype built for Modelplan to test the design build,I also flew her at Old Warden Scale days before they introduced those silly B-Certificate regulations in the name of safety,and they would not let me fly her,my theory is if you can take off or land in a few designated feet then that is what makes a pilot.

I am not interested in throwing a model all over the sky,safe steady scale flight is more important to me.

Best of luck with your own build,you must be keen to travel all of that way to get scale fidelity !

[Greyhead]

Glad you’ve found the thread useful; when I wrote the article for RC Model World I included the web address but the editor in his wisdom deleted it before going to press. A great pity in my opinion; I’m sure, as you’ve found, that it would be of interest to many modellers.

When you start your build I hope you’ll also start a thread here about it; “Yet another Parnall Elf!” we’ve already got one member who is hopefully going to start one.

Anything you think I might be able to help you with you only have to ask.