EA 81 - Douglas Redrive

[Brian]

Yes, loads are another problem with tuning a gyro without a variable speed prop. We tuned mine tied down in the back of my traytop [which I use to transport it around on] but found once in the air, that the load range would jump to the next level ie from 50 % to 60 % and the engine is now out of tune at that load. Also, on mine, we were not able to tune above 4000 revs cause of the prop starting to churn the air and making awfull noises. Next gyro will have the computer it came with when the engine was in the car !! Far easier to tune and a lot less expensive than a new Wulf 3 D!!Brian

[asra]

Hi again folks. Glad if it helps anyone. Let's see, John - thanks for the email, I've been a bit busy with the rellies lately - family gathering at the weekend was fun. More this week, with two birthdays coming up...[]The Siamesed Port issue on the EA81 With two exhaust valves sharing one pipe there is no doubt that twice the (very hot) exhaust flow will increase the exhaust gas temperature. But by how much? Not really sure. This is where a bit of test and measurement can help set the baseline for an engine.In the previous post I allowed a slightly higher gas speed than I normally do for a 4-port exhaust, based on the observation that EGTs did not show a large difference on a BMC A-series (Mini/Sprite engine) exhaust which has a central siamesed exhaust, two siamesed inlets, and two separate exhaust ports. How large? Sorry I can't remember exactly, it was 35 years ago now. If I recall rightly, inside the port itself was distinctly warmer, but the pipe temperature was not a very big deal.My thinking at the time was that it was cooling issue - the exhaust gas emerges at pretty much the same temperature regardless of which cylinder and exhaust valve it uses, but once out the valve it starts to cool - the rate of cooling will be less down a siamesed pipe than on a normal one because it is heated twice as often.The pulse response is another matter, and here's a timing diagram sketch that shows all four cylinders. The firing order here is 1,3,4,2 but 1,2,4,3 amounts to the same thing with the cylinder labels changed. Image Insert: 9.22 KBThe previous post shows the same thing much magnifed - just one exhaust valve on one cylinder. We are still looking for a pipe length that will reflect back the 'pop' from the opening valve as a 'suck' when overlap occurs.The next sketch shows two siamesed ports together - 1 & 4 in this case, but it's the same for 2 & 3 in conventional notation.The time 'T' is the delay time we still want to happen.Image Insert: 6.3 KBThat's an overview of what's going on in our siamesed pair of exhausts, but it will be easier to follow if we remove the inlet timing information and look at what might happen to a reflected pulse from the other exhaust valve of the pair.Image Insert: 5.28 KBHere the time 'T' is shown starting at the point when the 'suck' reaches the exhaust valve (hopefully). There is a possibility of some of this being reflected back down the pipe again (valve is closing after all), and reappearing a time 'T' later as a pressure pulse. What effect might this have?On the diagram, it arrives whilst the exhaust valve is fully open, so it can disappear into the cylinder which will severely attenuate it.The pulse will already be attenuated by travelling down the pipe and back twice, and being imperfectly reflected at the partly-open valve, so we can usually ignore this. John - does your question imply an organ-pipe type of resonance? Double the pulse frequency and halve the length? My feeling is that there is too much damping in the system for this to be very significant - any leak in the pipe will damp resonance. You'll know what I mean if you played recorder at school and could not get one of the holes closed with your finger. In any case, the residuals from the first cylinder pulse are met by the powerful pulse pressure of the second cylinder.A Siamesed port definitely runs hotter, and seems to benefit from a longer pipe to compensate for the higher speed of sound. Evidence is available in the "Long Centre Branch" pipes for Minis, Coopers and Cooper 'S' engines. These almost universally work better than a 3:1 on the A-series engine. The centre branch is on the siamesed exhaust port, and is usually sevaral inches longer - then it joins the other two that have already been combined into one. Bit like a 4:2:1 system.Think that's it...?All the best, BenEdited for PS: Increasing the (siamesed port) pipe bore was a way of reducing the gas temperature - important that it blends into the port well, but sometimes that can be enlarged usefully. Failing that a tapered pipe - 8 degrees of taper has worked for me. Increasing the diameter, reducing temperature, and reducing speed of sound should allow a shorter pipe with similar effect. Might even be lighter? Ben

[asra]

Hello Bob, yes, I do 2-stroke pipes. It's where I learned about resonant exhaust tuning for competition as well as for aircraft use.The 447 should come with a standard set of plumbing - which works well enough. If I recall Nigel Beale rightly (Skydrive - UK Rotax importers), this is not an engine that responds well to tuning, it needs too many things done to it that add up to "Buy a different motor to save money".So do take care with your precious Rotax. We had the importers and factory behind us when we were busy tuning the guts out of Fuji engines, but we still burned the odd piston or six. You will need a box of jets to re-tune the carburation afterwards. If you can't afford a set (and who can?) it's worth doing a deal with your supplier if you can - sale-or return? Worth a try. You need to be able to set up the engine without accidentally blowing a piston due to incorrect mixture, or persuade a friend with the right experience.If you still need a mild pipe designed, I'll give it a go, but will need the exhaust timing and the desired RPM for the power peak. Don't seem to have a 447 in the files for some reason.... Ah yes! Rotax were the competition! .[]I'll be a bit busy now so I'm not saying when it'll get done.Best regards, Ben

[asra]

Thanks Ben ! Yah thats my take on the rotex 447 myself ... built to last but not for great HP ... the exhost pipe alone tells me that . if its anything like the old yamaha DT1's then its like you say and not worth the bother ! it needs too much to get it right ! ... I suppose I might could get 50 hp out of it if I realy tried real hard... you know , port and polish , 2 new bigger carbs , 2 expansion chambers ... and maybe shaveing the heads down ... but its not realy worth it if the realibility will go out the window as well ... thanks for the insight ! Bob......" Momm'a alwayse told me , Son the impossable is only a little bit Harder... and ya know I do believe She was RIGHT ! "

[NiqueNaque]

Somne pictures after todays effort at being a naughty boy and not going to work...It works - at last! This is a picture of the engine in the going condition with the redrive.. Of interest is the tracking position of the belt, which seems to like the back flange on the small pulley. I'd like for it to track in the middle but this may not be possible ever due to microscopic flexing in the plate and some misalignment which would be impossible to avoid. Rick may have some suggestions but I don't think it to be an issue to be worried about.Image Insert: 80.64 KBWith the machine in the bach (classical) yard for the last 6 months there was a little contamination in the carbie... you can see the deposits around the accelerater pump and the jets, this stuff was in every orifice in the carb.Image Insert: 80.95 KBMr cleaner fixed it... I got a short section of stainless steel cable and put it in the drill as shown, the drill had to be run in reverse so the cable didn't untwist itself and go everywhere. The cable is reasonably fine 3.25 or 1/8" 7/19 construction which is the fine supple stuff. I controlled the end with my fingers and light pressure to no ill effect. Make sure you wear glasses if you do this. The cable was a good solution as it would go into the ports and in the jets and did a good job of cleaning out the deposits, a shame I didn't think of this with the last carby before I pulled off it and gave it to Wal Flaks. If you need to go into a smaller hole just take as many strands out as will make it fit.Image Insert: 79.54 KBfixed it, ran as sweet as... But it won't idle below 1400 RPM without coming to a shuddering halt, as a direct drive it would idle at 500 RPM, I think the redrive belt having some leeway causes it to hunt as it snatches and grabs at these lower revs, I'll try putting a little more tension on it to eliminate the slop and smooth out the idle at lower revs, might be stuck with it running at 800 when it is all said and done.Cheers,Nick.

[asra]

Hay Nique Naque ! I am courious as to how much that raised your thrustline ??? or did you drop the engine as well in the proccess ? By golly it looks like it works ! Good job ! Bob......." Momm'a alwayse told me , Son the impossable is only a little bit Harder... and ya know I do believe She was RIGHT ! "

[asra]

Onya NickLots of fun aint(hill billy) it?[8D]Cheers SonnyIntelligence is not a privilege,it is a gift and should be used for the good of mankind.

[NiqueNaque]

Bob, Sonny...Thanks,The thrustline was at the last set of photographs set at about 4" high, I didn't like it and moved the engine down by 5" near enough, this took the thrust line down but it also took some of the weight with it, the seat remained in the same position. I put the new Oddessy battery above the engine to move its weight up and this I hope will bring the thrust line to near CLT, my guess is that it is still 1 to 2 inches high thrustline, but that is better than 4". I'll do a balance test on in the back yard and get Alex to take a photo, then I'll see how effective the move has been, I'll certainly have the cranks if it only moved an inch.One thing I discovered with the flat plate stabiliser that I put on was that it vibrates or flutters when the engine is running, could be a problem as it is in effect acting as an elevator when it does so and could result in the inducement of PIO, I've got one brace on it but I may have to add two more to steady it up.This could be a theory on the inducement of PIO ..... flutter on the stabiliser causing the gyro to pitch up and down, no doubt it has been though of prior, but how seriously?Cheers,Nick.

[Brian]

Nick, you still have some of that stuff in the carby, I had a belt drive and it idled as sweet as just ticking over. Did you find all the jets ?? Cause I dont think you did Brian

[asra]

Interesting thought Nique ! viberateing the HS to cause lift..... I suppose it probly would but i doubt it would be enough to cause PIO.... that would take quite a bit of force for that if your HS is the proper size ...... now the FLEXING of your HS might well be a bigger factor ! at flight speed and all if the HS is fluttering i.e. leading edge up and down a decentamount that could well cause problems ! .... stabelize that puppy and you won't have to worry about it ! sounds like ya got it close enough to CLT to me good work ! C ya !Bob......" Momm'a alwayse told me , Son the impossable is only a little bit Harder... and ya know I do believe She was RIGHT ! "

[Brian]

Nick, I have a stay coming from the keel out to midway on the stab on both sides, its a flat stay and while the rudder shakes a bit, the stab doesnt move. Hope that helps.Brian

[NiqueNaque]

Brian,Here is a shot of the underside of the carb.There are two ports below the butterfly - circled - and one above which as I understand it forms the idle circuit. I cleaned all of them out with the wire but you are most likey correct as the orifice into the carb is usually drilled from a larger hole in the casting and the casting is then plugged off. It did start today and idle at 800 for a while then it picked up to 1200, so it is coming down, just needs some serious work I suspect. I may have to drill out the plugs and clean the ports from the outside. This time I'll tap them and plug it off with screws so it can be cleaned out later. You wouldn't know of anyone who has a cross sectional detail of onw so I can work out how to clean it?Image Insert: 57.86

[asra]

Nick the two jets in outside of carby one for each throat are also idle jets blow compressed air down the primery one with idle mix screw out & clean this jet & to eliminate vav leak spray water around all possable leaks eg.welds & gaskets .Did you put new plugs in as you'v had the exhaust off and is it points and if so are they clean & have you cleand out the fuel tank as our new fuel is stuffed after 3months

[NiqueNaque]

Steve,Thanks. I'd like an appreciation of what you're saying..The carb is a Weber DGV series off a two litre ford cortina, I bought this one new from a carby mechanic and he set it up, with fuel injection on cars he's gone out of business, so no help available there.The fuel would be an issue as it is most likely more than 3 months old, I'll put it in the wife's car and get some fresh stuff into it for a trial.Now that you mention 'vav' leaks, by which I think you meant 'Vacuum' leaks, I had a Nova at one point and it wouldn't idle very well, tracked it down to a dodgy inlet manifold gasket, it was a bugger of a job to fix it but it worked well afterwards. So I'll have a look for a vacuum leak and fix one if found. I noted that at idle the mixture was extremely lean because is was sucking extra air in through the hole and this had to be offset by upping the revs to smooth it out.The plugs in the machine are new, about 15 hours on them which equates to 2000 km of car use.. are they affected by the removal of the exhaust manifold?Thanks for your input, I'll look forward to some ideas..Cheers,Nick.

[asra]

condensation your gyro's outside one exhaust valve could have been open and rust will form on plugs new or not probably just check em.with mt ytping scill cumbynd wif me spelin im just happy you can reed any of wot i rite