Laser engines exhaust pressure nipple retrofit (for pressurized tank)

[Simon Bulk]

Hi Laser enthusiasts.

 

Unfortunately the "Laser - Technical questions"- thread was closed until further notice, so I try my luck with opening a new one. Here's my problem:

 

I got a Laser 70 engine (made at the beginning of the 2000s, 2 head screws, black carb, old main needle without spring), which runs truely perfect. The engine is mounted on a 140cm selfmade spitfire and fits beatifully to that model. The airframe is very light and not equipped with any landing gear, so it must be hand-launched, which generally is absolutely no problem at all with the loads of power the laser provides. Now comes the problem:

 

In that moment the model is launched, the fuel flow seems to be cut due to the launch impulse, no matter how soft it is launched. This causes the engine to stop abruptly. Unfortunately the success-rate is very poor: I would say more than 50% of the launches end with a deadstick landing directly after launch. 

 

Tank has already been lowered, so perfectly in line with the carb as Jon has always described.

 

All of my smaller, also handlaunched models, with OS engines (.15 - class, all with a pressurized tank) have no problem at all. So, as first time in my life I am considering to retrofit a pressure nipple on the Laser 70 silencer. The silencer seems to be made of aluminium cast. Unfortunately I cannot reach the interior of the silencer to tighten the nipple by a counter nut.

 

I've seen some laser engines with that mod. Is there any experience, whether a cut thread in the aluminium is a suitable solution? I hope this message finds Jon and he can maybe help me out what nipple type was used at laser and how they have been fixed.

 

Thanks in advance and best regards

Simon

 

 

 

 

 

 

 

[Jon H]

The exhaust is made of 3 machined parts welded together. The wall thickness is insufficient for a nipple and it will simply vibrate out in short order. 

 

Do you use full power when launching? if so throttle back to a lower setting and give that a try. You have so much power even half throttle is likely to be enough to get it moving. 

[Simon Bulk]

I tried with all lower settings and, indeed, got better results with the throttle 60% open. But still, lots of fails…

[RICHARD WILLS]

Jon knows more about engines than Ive forgotten . So I would never question his wisdom in the Laser department or indeed any other IC motor.

However , being of an inquisitive mind , and having had a few hand launch heart stoppers myself in the past , I wonder what is actually happening . 

The first gut reaction of course, is fuel shortage caused momentarily by the shove forward .

But surely the fuel in the tank would be pushed against the rear of the tank as the model is shoved forward causing momentary high pressure of fuel right where the clunk pick up is . The weight of fuel in the line already would be of no consequence compared to the 8oz of fuel being "pressurised " up the line . 

So could it be a rich surge , not a lean cut ?

A bath time experiment may reveal the truth ! 

If any of this musing has a basis of fact then perhaps a form of baffle might resolve the issue ?

Where is Barnes Wallis when we need him ?

[Ron Gray]

Use a dolly!

[Simon Bulk]

Interesting thoughts, gentlemen.
 

But with space left to extend in the tank and with the vent completely open, why should the fuel press against the fuel line? 
 

My observation when dead-sticking at launch is very close to an engine dying due to a lean cut…

 

But of course I might be wrong with my assumption.

[RICHARD WILLS]

I suppose one way of checking this theory (you're going to like this ) ,is to half fill a tank with coloured water and then attach some string to the front of the tank . 

Swing the tank around you like a lasso ,so that the centrifugal force adds extra "G" to the liquid in the tank . 

If the rich cut theory is true then the weight of the fuel would have to pump fuel towards you via the fuel feed and consequently the tank should lose fuel . 

I would recommend trying this in the living room when your wife is watching her favourite soap in order to reap the full benefit . 😕

[John Rickett 102]

Are the tank vents properly positioned, facing into the airflow?  

[Alan Hilton]

How about a Perry pump the vibrating one ,they work with  4 strokes 

[Jon H]

a lean cut caused by the strong acceleration of a hand launch is not uncommon. My 15fp 2 stroke suffered from this even with tank pressure and it was only a change in launch method (a run and release rather than a throw) combined with a reduced throttle setting was the only real cure. 

 

Not a fan a pumps, they cause more problems than they solve and a strong acceleration will stop the pump anyway. 

 

You are as lean as can be on your slow run needle, and while this is normally not an issue it might be a factor in this specific case. opening it a bit and then trying a half throttle launch might do it. 

 

To be honest, i would either fit wheels or make a dolly as Ron suggested.....or, you can launch the model inverted. Having the model inverted will cause the tank to sit high, making the engine run slightly rich and likely offset the acceleration issue. You launch inverted and then roll the right way up once away. A chap my dad flew with did this with with a funfighter spitfire way back when. Its a high risk strategy mind you if you forget up is now down. 

 

 

[Simon Bulk]

Very good hints, thank you very much.

 

I wonder how Jon figures out that I am on the extreme low end of the slow run needle…That might indeed be the case. 
 

Is the backfire an indication?

 

Launching the model inverted is definitely possible, I’ll try that after experimenting again with the slow run needle.

 

A fixed landing gear is not really in scope, but the dolly thing is something I’ll might give a try.

Another interesting fact I must add: the problem has gotten worse over time. This year the problem has become way worse than it has been the year before…

The only thing I’ve changed this year is the airscrew, now I got a slightly heavier load (13x8 G-Sonic, still revs very good at over 9,000 rpm). I guess that contributes negatively to the “sensitiveness”. 
 

Edit: vent tube is facing into the airflow.

Edited August 10 by Simon Bulk