What a difference a little more power ma

Well boys and girls,
I took the Akromaster out flying yesterday, in frankly unfavourable conditions. However, due to a damaged main bearing in my PAW 149 I put back the OS 10 FP that had been removed due to NVA problems. This time I had the original R/C carb on, and left the muffler off.

By golly the plane was a wild thing! The line tension was almost scary, and the speed...! With the muffler the OS behaves much like the PAW (just about able to do loops and eights), but without the muffler, that extra bit of power just pushed the Akromaster over into the "full tilt" territory, and I was once again able to do anything I wanted with it. Fantastic.

I don't quite know why there should be such a big difference.

The other thing this means is that I'm finding the R/C carb locked open to be just a million times better than the OS venturi and stunt NVA, even when the latter is on muffler pressure, and the carb is not.

This has me perplexed. I sealed that critter up good and proper, and even tried choking it down, but I worked out the choke area on the venturi to be less than on the carb, yet the venturi hunts like nobody's business when you throw the model around.

Back to the workbench I think.
Anyway, I'm glad my Akro has come back to life!
Now I'm just wondering, would my wonderful PAW 15 be a sensible engine for an Akromaster? I'ts 1.5 times the weight of the OS, but slightly more power.

You have learned two things.

1) The OS muffler is too restrictive. Replacing it with something less restrictive can give you a muffling effect and minimal loss of power. Try a small tongue muffler if you can find one. Less weight and less back pressure.

2) Using a smaller venturi is like holding the carberator at less than full throttle. I don't know what the opening on the carb is or the venturi size that you were using, but OS is not known for putting perfectly sized stunt venturis on their engines, and venturi size definitely does effect power. It is just like opening and closing the carb. Just remember, it can be too big as well as too small. But putting on an appropriately sized venturi will give you the same effect with less weight and comlication.

If the PAW is 1.5 times heavier than the OS and you did nothing to adjust the CG between engines, it was likely nose heavy with the PAW which would make it much less responsive, possibly to the point that it would not turn well enough to do even basic maneuvers.

My one and only Akromaster/OS 15 baffle was full pattern capable-the whole enchilada- though it was pretty fast through the maneuvers and needed quite a bit of speed to maintain tension. Best prop was 8-4 APC which might be pretty good on the FP 10.

>If the PAW is 1.5 times heavier than the OS and you did
>nothing to adjust the CG between engines, it was likely nose
>heavy with the PAW which would make it much less responsive,
>possibly to the point that it would not turn well enough to
>do even basic maneuvers.
>
Jim....EXACTLY!
Tony

Tony:
I may have missed something. Is the PAW 149 and the PAW 15 the same engine? I am not a diesel person(not being into masochism, self flagelation(sp)or russian roulette)so didn't recognize the difference.

Paw 149 approx 1.5cc, .15cu = 2.5cc

Regards Frank

>Paw 149 approx 1.5cc, .15cu = 2.5cc
>
>Regards Frank

I'm begining to sound like an echo here.....Exactly right.......
What He said.........Tony

>Tony:
>I may have missed something. Is the PAW 149 and the PAW 15
>the same engine? I am not a diesel person(not being into
>masochism, self flagelation(sp)or russian roulette)so didn't
>recognize the difference.

Jim, displacement question, answered by Captured Rad...

Feareth not yon Diesel, Thou may abohr fuel Stench, forthwith..
.....Use your Combat starter glove, for pain reduction, and develop
a quick prop flip.....Tony

Jim,
the PAW 15 is 1.5 times the weight of the OS10. I was using the PAW 149 (that's the old cc number, so it's actually 09) which is very close to the weight of the OS.

I was considering moving up to the 15 for the extra power, but I appreciate I may need to do something with the balance if I do.

EDIT: Oh, I see you guys worked it out already!
I have always disavowed Imperial measurements, but this hobby has sneakily changed that. The other day I found myself thinking in terms of ounces, which meant nothing to me until recently.

Sorry, I was a bit slow getting back to this thread.

Thanks Len for your reply. I switched to using the OS muffler without the baffle, and I have a vague memory of that being better, but I guess it's the "tailpipe" that's restrictive.

The issue I have with the venturi is the fact that it nearly cuts out when I manoeuvre the model. It's impossible without muffler pressure, and just very bad with it. Using the carb I have no need of muffler pressure, it just runs very sweetly all the time. Very perplexing.
I may have another go at the venturi, but I'm running out of things to try. I made sure there were no leaks, I used some balsa tochoke it down... what else can I try??

>Sorry, I was a bit slow getting back to this thread.
>
>Thanks Len for your reply. I switched to using the OS
>muffler without the baffle, and I have a vague memory of
>that being better, but I guess it's the "tailpipe" that's
>restrictive.
>
>The issue I have with the venturi is the fact that it nearly
>cuts out when I manoeuvre the model. It's impossible
>without muffler pressure, and just very bad with it. Using
>the carb I have no need of muffler pressure, it just runs
>very sweetly all the time. Very perplexing.
>I may have another go at the venturi, but I'm running out of
>things to try. I made sure there were no leaks, I used some
>balsa tochoke it down... what else can I try??
>
>Jon

You appear to be going the wrong way with the venturi, Jon. It sounds like you need to open it up, not choke it down. You already said that the choke area on the carb was bigger than the venturi. You are probably running the engine too lean with the venturi and that is causing your problems. What is the inside diameter of the venturi that you are using and the cross section of the needle body? Also, what is the diameter of the carb (and then we will assume that the venturi nozzel goes about half-way in.)?

For reference, my box stock venturi is 6mm dia.

Okay, sounds like I need some hard facts and measurements. I'll get the details and get back to you.
Thanks guys.
Jon

Okay, I have the facts ma'am!
Venturi: Inside diameter: 6mm, gives area of 28.2mm sqr
NVA is 3.5mm stepped down to 3mm! Gives area ~18mm sqr
So, choke area is 10.2mm sqr

Carb: Throttle barrel I/D: 4mm, gives area of 12mm sqr
Spigot is 2.2mm diam with about the same protrusion into
flow, so area ~4.84mm sqr.
Choke area is 7.16mm sqr

So it looks like the venturi is the right one, but the choke area is quite a bit larger than the carb, which correlates to the effects I'm seeing in climbs.

Now, my needle looks like it's wrong. I checked the hole in the side of the venturi, and it's 4mm diam, which tallies exactly with what Geoff says. The correct NVA should be a tight fit!

By my calculations, with 3.5mm NVA would give me an area of 21mm sqr, and a choke area of 7.2mm sqr. Perfect! There's my problem.

So, I'm not sure whether to get a new NVA or just try choking the venturi. Probably I should get one that actually fits!

Many thanks guys!
Jon

It does sound like the proper size needle would give you the same choke area as the carb. According to my calculations you could use a carb of about 8.2 square mm which is a little in between, but smaller than what you have now. And a little smaller wouldn't hurt. So maybe you are too big. Also, if you have the turned down needle, it may not be seating on the engine body or holding the venturi down tightly against the o-ring to get the proper seal. Lots of possibilities, all of which could be solved with the right needle.

Jon

I still think you should check the NVA. I seem to recall from last time that you have an OS NVA that is a rattling fit in the venturi. You may well be sealing the airflow by replacing the venturi.

The OS 10 venturi is 4mm dia, necked down to 3.5 mm across the throat of the venturi. There are other original equipment OS NVA diameters out there in the wild--3.5 mm for the .20 and larger and a 3.2 mm (.125") dia for the .15. I took all these dimensions from OS replacement parts that I have bought over the last 2 years.

0.5mm is not much but it could be enough to give you a leak around the NVA at the crankcase and venturi and at the o-ring on the venturi.

I changed my OS 10 from Rear Needle to standard and replaced the carb on a 20 FP with a venturi and NVA and I do not think I compressed the o-ring 0.5 mm to fit the spraybar through the crankcase and venturi on either engine.

Hi Geoff,
ah ha, you remember! I have access to a micrometer now, so I will measure the NVA, but it does seem to fit the bill. It's about 4mm and it does have a step in the centre.

However, it did rattle around in the venturi. I ended up sticking washers on the needle side of it to seal it off. I tried using silicone etc but that didn't work. Didn't solve the issue really.

I'm a little confused about your referece to O-Rings. I think I may have said that I tried pushing the venturi into the crankcase as I tightened the NVA, to try to compress the O-ring and seal the venturi to crankcase joint. However, the NVA doesn't get very close to that.
Could you clarify, just so I can put my mind at rest that I'm not missing something.

Jon

I seem to recall that you had a rattling good fit somewhere with the venturi/NVA setup.

When I did the change from the RN setup on my FP 10 and replaced the carb with the stock venturi and NVA on the FP 20, I found that the spraybar for the 10 was 4mm, with the middle turned down to 3.5mm. The spraybar for the 20 and all the bigger FPs is 3.5mm dia. The difference of 0.5mm is close enough to 0.020". To assemble both engines, if my memory serves correctly, I had to compress the o-ring on the venturi more than 0.020" to allow the spraybar through the crankcase and venturi.

It sounds like you have the right NVA now--the FP 10 is the only OS that I know uses a stepped spraybar but leaks may still be an issue. When I was running-in my FP 10, I had it bolted to a folding saw horse and after it had burned about 2 pints of fuel, it picked up about 500 rpm when I tipped the nose up and when I pinched the fuel tube to rev it up and get it hot per the instructions, it pulled the saw horse across my brick patio. I was impressed.

The engineering purist in me wants to find a solution to a problem but the easiest solution seems to be wire the carb open for the revs you want. It's not like we are talking National Titles or anything is it?

>The engineering purist in me wants to find a solution to a
>problem but the easiest solution seems to be wire the carb
>open for the revs you want.

I'm the same. The carb works just fine, though it is a little more expensive on NVAs when I crash (still learning) and I'd feel more *authentic* running with a venturi. Plus, it offends my engineering sensibilities to have something not work and not know why.

THe power idea is ok but the weight isn't. The little Akromaster was designed with the old .15 glows like the Fox .15X, K&B GH Torp, Cox, Max, etc. that weighted in right around 4oz. or so in mind. The PAW weighs around 5.5-6oz. as I recall. Not only are you increasing the nose weight, but the wing loading goes up as well- NOT a good thing.
J.

Ah, okay. I guess that makes sense actually, as the old 15s were much lighter, yes? A modern Diesel is going to be pretty much at the other end of the scale! Thanks
Jon

Hi All,

Well I finally got round to ordering my new NVA, and after a bit of confusion the correct part was tracked down. Thanks to geoffg for the part number.

The correct NVA is *totally* different to the one I had before. It's much more stubby, the handle isn't spring-loaded, and most odd of all, it screws into the inside of the spraybar, like a Fox NVA, rather than over the outside like every other NVA I've seen.

Anyway, it is definitely bigger diameter, fits perfectly, and I'm just hoping that its more compact size will counter the fact that it's not spring-loaded to prevent it snapping in a crash.

I'll let you all know how it flies.
Jon

Get yourself a TD 09, add some nose weight, and get some longer lines. Just a thought. I'm eventually going to have to build an Acromaster and see what it is all about.

TD09 is a Cox glow, right? I have some 60' lines now for the Streak, so I could try that.
Truth be told it runs really well on the OS10 with the carb and no muffler, but I wanted to be a bit of a purist.

The plane is pretty beaten up now, so there's probably a fair bit of weight to be saved by rebuilding it, without all that glue to hold the parts of the fuse together...

60 foot lines seem awful long for a plane of this size. The longer the lines, the more power you need to keep the lines taut. What lines do you use now?

Problem with the Tee Dee .09 is that they are expensive (does Cox actually make them any more?) and glow heads are hard to come by. The Norvel .074 might work better. Or there's the AP .09 or the Thunder Tiger .07. Or if you can find one (they're available in the U.K., and supposedly quite cheap), a SC .15, which weighs around 4 oz. (built on a .12 case, and it's even a 2BB engine!).

The Akromaster is a small plane for a .15 - some 1/2-A combat models, these days, are as big (though not as heavy). That really long nose plays havoc with the balance if you use a modern .15.

You might also consider the FP 10, TT 10, LA 10, and MVVS .09 diesel.
All are excellent engines. All have mufflers except the MVVS.

Jim, are you using a muffler with the TD .09? I can't seem to find mufflers for mine.

George

Edit: Forgot to mention, my Akromaster has a Fox .15 baffle w/muffler.