SE5a CONSTRUCTION BEGINNING TO . . .

[Greyhead]

I said in my last post “A fully enclosed installation such as the SE5a isn’t the place to bed-in a new engine “. Of course it isn’t ideal for any air-cooled engine new or old so extra care has to be taken to prevent over heating; the full sized was water-cooled. To explain how I hope I’ve overcome the problem I’ve used some old photos as well as a couple of new ones.


The SE5a has some positive points; there is a hole directly in front of the cylinder head for cooling air to enter.





And there is a large opening in the fuselage behind the engine for heated air to escape.





However, things are not as straightforward as they seem; hot air wants to rise but the exit hole is in the bottom of the fuselage. Once the model is moving forwards the air flowing over the hole should produce a venturi effect and suck the hot air out, but this is where the SE5a designers throw another spanner in the works!


Below the engine shaft there is an even bigger air entry hole, air entering here will not help with the cooling but will have the effect of “pressurising” the bottom half of the engine compartment and restricting the outflow of hot air.


The easiest solution would be to mount the engine inverted but because of the position of the engine shaft this would mean either a much larger model or the cylinder head sticking out the bottom of the fuselage, so this is a non-starter.


The engine plate has a hole corresponding to the one in the fuselage, so the hot air can transfer to the bottom half of the engine compartment.





A template, this is where you’re glad you paid attention in all those geometry lessons, is drawn and stuck to litho plate.





When cut out and bent up it produces an air scoop that forces air entering below the engine shaft upwards into the top half of the engine compartment where it can help to cool the engine.





Also with little or no pressurising of the bottom half of the engine compartment the venturi effect can work and suck out the hot air; a win / win situation!

[Guest]

What method do you use to secure the flaps of this litho plate box? Are they glued together? I know you can't solder them!

[Greyhead]

Giles

The flaps are glued with 5-minute epoxy. It works well providing the mating surfaces are well “roughed up” and of course all the soap used during the annealing process has been cleaned off! Cyano also works well.

I’ve also used 5-minute epoxy to glue the finished air scoop to the former; in fact I use epoxy for every glue joint in the engine compartment, whether or not it needs it from a strength point of view, because it’s fuel proof.

Grahame

[Guest]

Hi Grahame, only found this thread yesterday and @ 04:30 I decided it was time to go to bed with still over half left to read, can't say any more than the guys have already but will say best of luck for your first flight. Some video footage would be something to cherish if you could post on the site when the time comes. When I grow up I wana be a scale modeler too

[Greyhead]

Graeme

Sorry to be the cause of sleepless nights but glad you’re finding the thread interesting!

I see from one of your posts that an SE5a is on your “to build” list; if and when the time comes let me know, I’ve got quite a lot of research material and photos you might find useful.

Grahame

[Greyhead]

There will be a short delay before I can fully test the exhaust.


We went down to Leicester over the weekend to visit my brother and as we got ready to leave I remembered that I wanted to borrow his rev. counter. Being super organised he knew exactly where it was so went to get it, his super organisation being as good as mine when he looked where he “knew” it was it wasn’t there! Sound familiar? He found it soon after we’d left and as he‘s coming “up north” in a couple of weeks for the TMFC Bring a Model Night we can do the tests then.


I’ve made a clamp to stop the extension being blown off the silencer; it’s similar in concept to the Laser carburettor and exhaust clamps.





It’s made from tin plate and I’ve used soft (lead) solder. I don’t think it will get hot enough to melt the solder but even if it does it won’t matter because once it’s clamped in position it holds itself together.





I’ll use a cable tie around the silicone tube.

[Guest]

Grahame, I was working on this same problem on my Laser at the weekend. I made up brackets on the side of the silencer, fixed using chemical metal. It worked just fine at idle but as soon as I revved it up they were blown off! I'm going to get my friendly local TIG welder to weld brackets on for me. I was amazed how the pressure was enough to unstick the chemical metal.

Your bracket looks much better than my first attempt, but beware - there is lots of power in these things!

[Greyhead]

There will be yet another delay before I can test the exhaust! My brother had intended to visit for the Teesside MFC Bring a Model Night and bring his rev counter with him, unfortunately he got the dreaded lurgy so couldn’t come.


Although I was feeling pretty rough myself I took the SE5a along; the lighting in the room left something to be desired to say the least and several people commented that it was very difficult to see the cockpit so I said I’d post a photo on the TMFC forum.


When I checked I found that I hadn’t in fact got a photo of the finished cockpit so I took one and thought I might as well post it on here as well!

[Guest]

Superb Work there Grahame,

Hang on whilst I find me hat, goggles and white scarf and I'll take her up !!

Seriously, that looks good enough to climb into and fly away, you could easily be looking at the real thing in this shot

Regards......Mark

[Guest]

lovely cockpit Grahame!!!

cant wait to see her in flight!

Richard

[Greyhead]

I’ve done the exhaust tests and the results are very promising. The clamp has held the extension in place and the silicone tube used as a seal has worked well. I’m using the AcroWot as a test bed and now I know that the basics work I’ll cut the silicone tube to the correct length and attach the exhaust extension well enough to fly with it to give the system a thorough test before fitting in the SE5a.





I tested the revs using 5 different props altogether, both with and without the exhaust extension; for scale models such as the Parnall Elf I have in the past used a 13x6 but I’m thinking of using a 16x4 for the SE5a


Prop 13x6 Revs without extension 9700 Revs with extension 9400


Prop 16x4 Revs without extension 7000 Revs with extension 6800


The revs have dropped somewhat with the extension fitted but not to any significant amount and although the revs are considerably less with the 16x4 prop the static thrust is increased. I have no way of measuring this but just holding the model it was clear that the “pulling power” with the larger diameter lower pitch prop was much greater. The theoretical flying speed will be less of course but then it will be better able to overcome drag, which the SE5a has in abundance, so the end result could be the same!


I’ll tabulate all the results in another thread (tutorial?) as it may be of interest to compare the effects of prop diameter versus pitch on engine revs.


“Tabulate” may be the wrong word because as I’ve just found out tables can’t be used in forum posts, hence the rather strange way I’ve presented the above!

[Guest]

you could always make the table in work/excell then just print-screen and upload the image?

Richard

[Greyhead]

My brother eventually made it “up north” so I assembled the SE5a so he could see it properly; he took some close ups, which I thought you might find interesting, so here there are.

[Greyhead]

As I thought might happen the exhaust did get hot enough to melt the solder joints for the strap, which used soft “lead” solder. It didn’t affect the working of the exhaust but I decided to silver solder it to eliminate any chance of “metal to metal contact”, which could cause radio interference.





I’ve included this photo, which shows the work before any cleaning up, just for Giles to show that my silver soldering isn’t really that neat!


Well I was a bit hasty silver soldering the strap.


I took the AcroWot out today to test the exhaust in the air and basically everything worked OK but the strap broke so I’ll have to replace it with one made from thicker steel, I’d used tin plate and obviously the vibration proved too much. I only had 3 flights but the silicone tube used as a seal shows no sign of deteriorating and the reduction in revs had no noticeable effect at all.


I’ve decided to include all the prop size / revs data in this thread as it is relevant to what follows.





For the first 2 flights I used a 12 x 7 prop, which gives a theoretical top speed of 64 mph. I then tried a short flight with the 16x4 prop, being a new engine I didn’t want to run it for too long with such a large prop. With a theoretical top speed of only 26 mph I expected to see a vast difference but in fact there was very little difference when flying into the fairly strong wind, with the wind the difference was noticeable but not excessive. As the AcroWot isn’t a particularly “draggy” design I’m very surprised by this but it bodes well for using a 16 x 4 prop on the SE5a.

[Guest]

Pictures of her flying please!!!!!