Laser Engines - Technical questions

[Frank Skilbeck]

Unusual problem with my 150 today, first flight saw me run off the end of the runway, as one of my flaps wasn't working, into the buttercups. Flap fixed but the engine was playing up at low speed, the engines was hunting and then stopping. Cowl off and flush out the carb, tried a different plug, check the tank but no better. Tried leaning and richening the low end, richer seemed to make things better, and on these new settings it seemed it was back running well and on a subsequent flight ran the tank down to a few mm left of fuel with a really good tick over and pick-up.

 

I'm guessing I got some unwanted material in the carb, but it was an unusual problem., will maybe take the carb off tomorrow and give it a full check.

 

 

[Vince Findlay]

Just a quick question about running Model Technics Low oil 5% in Lasers. I’ve been running my 180 single on Optifuel four stroke mix for a couple of years without any issues. Same plug and didn’t touch the carb in that time. I tried the MT fuel and it started 1st time and sounded great on the ground and never quit but sounded rich and felt underpowered. The next couple of flights after that I leaned the carb a quarter of a turn each flight to see if it made any difference, but didn’t. I reset the carb and tried three fills of the old fuel and the engine is pulling like a train again. Any advice guys?

[Denis Watkins]

for information, Optimix 4 stroke is 12% nitro

[Vince Findlay]

Thanks for the info Denis, It just felt strange I had to re tune the engine for so much for a different fuel.?

[Martin Harris - Moderator]

A significant change in nitro content will need large changes in the volume of fuel to achieve the correct mixture ratio as nitromethane has a lower calorific value than methanol. The more nitro, the richer the mixture needed but power will increase noticeably - partly due to the sheer volume of fuel being consumed.
 

A proper explanation is a little above my pay grade but I’m sure info is out there if you Google it. 
 

The increased fuel volume also gives some advantage in tuning as the crude needle valve becomes less sensitive due to the higher volume of fuel being metered - contrast this with running petrol where the opposite is the case…

[Vince Findlay]

Thanks for the info Martin, Both fuels have the same 5% nitro and I expected to have to adjust the needle a bit for the different oil content. I was just worried after running the engine with the needle half a turn leaner on the new fuel and it still sounding ritch I was doing something wrong. I will try a tacho the next time to see the difference. Thanks again for the info, I have used MT fuel over the years many times and it was spot on. Just the 1st time I’ve used it here and I wandering if anyone else had changed between the same fuels.

[Jon H]

Engines will always need a retune when changing fuels. I really dont recommend this tune and forget idea as things change day to day. It only takes a few seconds to tune the engine before the first flight of each flying session and even if it ends up exactly where it started you at least know its set exactly right

[Chris Walby]

Hi Jon and hope all is well at the home of Laser Engines.

 

As per ASM Tigercat thread I am now the owner of a pair of Laser 100's and would like some advice of what to do next.

 

History.

Laser 100’s have been sitting for about 5 years (no after run oil) and installed inverted in the Tigercat so the big question from me is should I take them out of the model and strip them to check for knackered bearings/rust or fuel them up and give them a test run?

 

Attached photos are of one engine and TBH it does not look like they have run much at all IMO.

 

 

I'll put them on the regular Laser 5 fuel recommended and go through the recommended tuning process on each.

 

PS The carbs barrels seem a little sticky so is it worth taking the needles out and giving it a flush or complete strip down?

 

Cheers Jon and keep motors running ? 

 

 

[Jon H]

What i would do is whip the front housings off and check the bearings. Only 4 screws per engine and it wont touch the timing. If they look ok then rebuild and crack on. Carbs should be fine with a bit of fuel through them but if you get tuning problems then take them apart. 

 

I would give each engine a 5 minute blast on a 14x7 to get the revs up, and fly on 15x8's. 

 

My biggest concern is the usual...tank position. Its not looking good from what i can see and it will need to be fixed. 

[Chris Walby]

Thanks Jon,

 

Will do with the inspection on both and so as suggested with the carbs if the engines look ok.

 

The engines have come with 14x7 APC's on them so I was planning with sticking with that, do you think a 20lbs model will need 15x8's or stick with the 14x7's?

 

I'll look into the tank position....

[Jon H]

i would use the 15x8's. Nice big prop disk, good for the engines in terms of load, should be quieter than the 14x7's and generally the balance of thrust/speed should be good. 

 

At that weight though it will be absolutely vital you maintain airspeed if an engine stops. Gliding will not be an option and i doubt it will climb and accelerate on one engine. You will probably get climb or acceleration, but you wont get both. If an engine quits on climb out after takeoff and landing ahead is not an option then step climbing (accelerate flat, zoom climb with the momentum, accelerate flat, zoom climb etc) is likely your safest way out.  

[Martin Harris - Moderator]

Best to make any turns toward the live engine when using any appreciable power. 

[Gary Manuel]

3 hours ago, Martin Harris - Moderator said:

Best to make any turns toward the live engine when using any appreciable power. 

Why is that Martin?

[Ron Gray]

Presumably to counter the adverse yaw produced by the single engine. Mind you, when I had an engine out on my Tigercat I wasn’t even thinking of that, I was only bothered about getting it down safely!

[Martin Harris - Moderator]

Standard practice to reduce the chances of stall/spin if the airspeed (well angle of attack if we're being correct) gets too close to the limit.  Classic perfect storm - rudder doesn't have enough power to pickup the dropping wing (if the pilot is sufficiently aware) against the asymmetric thrust enhanced yaw, instinctive reaction to pick up the wing with aileron deepens the stall on the dropping wing and you're looking for the black bin liner.

[Chris Walby]

Guys, you are off topic, this is the Laser Engines - technical questions thread. Either a new thread or the ASM Tigercat one.

 

Anyway back to the point.

 

Both stripped and there was some oil (still nice and fluid) no signs of sludge and a couple of areas of very light surface rust. Photos speak more words than I care to type.

 

Stripped and before a wipe down

 

Post wipe down, cloth indicates mostly discoloured oil and surface rust. Port engine bearings don't feel as smooth as the starboard however there is no play.

 

I'll let Jon have the casting vote, but if I have to rotate the engines to get the correct tank position (carb is level with bottom of the tank), they are coming out anyway.

 

 

 

[Gary Manuel]

Hmm. This is going to be one of those questions I regret asking. I'm sure that you are both right but my few remaining brain cells are struggling with it.

 

1 hour ago, Ron Gray said:

Presumably to counter the adverse yaw produced by the single engine. Mind you, when I had an engine out on my Tigercat I wasn’t even thinking of that, I was only bothered about getting it down safely!

Why try to counter (or fight to use an alternative word) the adverse yaw rather than using it to help the turn?

 

1 hour ago, Martin Harris - Moderator said:

Standard practice to reduce the chances of stall/spin if the airspeed (well angle of attack if we're being correct) gets too close to the limit.  Classic perfect storm - rudder doesn't have enough power to pickup the dropping wing (if the pilot is sufficiently aware) against the asymmetric thrust enhanced yaw, instinctive reaction to pick up the wing with aileron deepens the stall on the dropping wing and you're looking for the black bin liner.

Won't it be the wing with the failed engine that drops?

 

Sorry for being a bit thick.

[Gary Manuel]

Chris. Apologies, but your post came in as I was typing.

[Martin Harris - Moderator]

These were related to Jon’s advice regarding handling the Tigercat should an engine quit but I’ll happily split this subject onto a new thread if Jon would prefer…

[Jon H]

Chris, those bearings are marginal. Poke a finger in and give them a spin, if they feel gritty replace them. The dark colour of the oil is caused by the rust so it might be worth swapping them. The engines are already apart so for peace of mind i would go for it. 

 

On the twin handling thing if we assume our left engine is dead the right hand engine is trying to yaw the model left. So, we correct with the rudder. We may also need a smidge of right aileron as the engine with the running wing has greater lift from the section of wing being hit with propeller slipstream. This aileron increases the relative angle of attack of the left wing tip. If we then enter a left turn the model needs to yaw left. The secondary component of yaw is roll (left yaw, left roll). The outer wing sees an increase in its airspeed vs the inner wing which sees a decrease. The decrease in airspeed, increased yaw, and increased aileron deflection needed to keep it level keep increasing the relative angle of attack of the left wing tip. This also increases the drag on that side as well as the overall drag as you are turning. The pitch increase in the turn will also increase the AOA and increase drag

 

So you have a choice between running out of rudder authority and suffer uncontrollable left yaw leading to a spin, or having the left wing stall due to high AOA and then a spin. Neither are good for the health of the model. 

 

Ideally, you keep the turns towards the running engine but the key is speed. The faster you fly, the less impact all the other factors have. banks more than 20degrees are also not recommended. 

 

If it does spin all is not lost. Kill the throttle, fix the spin, then recover...assuming the ground didnt grab it already 

 

 

 

No need to move the twin chat. If we get into a longer discussion then sure, but if these few posts satisfy the question i am fine with it

Edited July 13, 2021 by Jon - Laser Engines

[Gary Manuel]

That makes sense Jon (I think). Thanks all. Something to bear in mind when I next fly my Durafly DH88 (Not I/C powered unfortunately) or even my HK Lancaster I suppose.

[Chris Walby]

Decision made and visit to Laser engines booked in so that's that sorted and one less thing to think about.

 

Now on to the tank position as Jon pointed out...its not good and on further investigation is worse than that initially thought.

 Really not sure what the thought process was when the tank was positioned here as the engine is inverted and there is room to move the tank down.

 

 

I could just move the tank to the bottom of the nacelle which would put the top of tank at end of the main needle valve or rotate the engine 90 degrees.

 

Good news I think the cowl can just be rotated and new holes drilled, so ok until I noticed the engine mount sits in a recess. This will necessitate cutting 20mm out of the side of the nacelle and moving the throttle servo box (no sweat!).

 

 

I think I know what the advice will be and the thought of doing a big loop and both/one quitting will not end well with even the best plan B!

 

 

 

[Chris Walby]

I am clearly making this more complicated than it needs to be, but searching the Laser web site and looking at the engine instructions I can only find this:

 

"The tank should be as near the engine as possible and slightly below the centerline of the carburettor." I probably wrongly assume that means the top of the tank?

 

I can get my head around the logic and issues of having a tank in a less optimal location as its likely to lean the engine out during fuel usage/aerobatics especially inverter flight which in my book is about the worse time to be engaging plan B (low inverted pass). Unless someone knows better!

[Jon H]

Yep, time to send the tank south! Top of the tank, centre of the carb. Easy to remember and always going to work. Those tanks look pretty big so if space is an issue use smaller tanks. 10oz on a 100 would be fine, especially with the diet fuel. 

 

As for why tank placement is neglected there are 2 reasons. The first is simply forgetting. The kit came with a place for the tank, so that is where the tank will go. Were the engine an OS or saito it would not matter, but ours are different and i think this is simply forgotten. 

 

The 2nd reason.. People are lazy. The sad truth is that many will simply ignore any advice i give them on tank placement as they cant be bothered to do the work. Most of us a pretty blasé when it comes to instructions so i suspect most just assume they will get away with it and not have to make the effort. The snag is, they then expect me to sort it out when the engine protests. I had this with a customer some years ago and even though i told him it would be a requirement before he bought the engine, he proceeded to ignore me and then complain when the engine didnt work. I kept telling him to move the tank, he kept telling me it wasnt possible. I pointed out that it was possible (other customers had done it in the same model without issues) and then the truth came out when he said he didnt want to do all the work and wanted me to invent some other solution. At that point we didnt really have much to talk about as i will only ever recommend moving the tank.

 

When someone calls to buy an engine i always ask what model they are powering so i can look it up and see if the engine is both the right size, and if its likely to actually fit with the tank and all that. I do this to save me a great deal of time on the phone trying to fix problems. In some cases i end up not selling an engine as it simply wont work in the model the customer is building. Tucano's and PC9's are a nightmare as the nose retract sits where the tank should be. Side mounting is an option, but if thats too ugly then no worries, buy something else. I would rather that than wasting hours and hours on the phone flogging an install that will never work. 

 

To be clear, there is margin with this tank placement lark and its less critical in a tiger moth than in an extra. Its also possible to have pressure isolated header tanks and all sorts of other solutions, but the performance of these systems is dependant on the quality of the build. As a result, and to make life easy, i will always recommend top of tank, middle of carb. Its guaranteed to work on every model and i also know that its guaranteed to keep the engine happy.

 

If i get a call about a misbehaving engine the things to check are:

 

Prop, size and RPM
Plug, type, does it work?
Fuel

Tank placement

Valve clearance. 

 

I do this as i know that an engine will do X RPM on Y prop with Z fuel etc etc. If there is deviation from the benchmark installation and recommended prop/plug/fuel, how can i help troubleshoot? Is it the engine or the setup? 

 

There are similar issues with cooling recommendations as well with a number of toasted engines coming back for repair each month. 

 

Install it right the first time, use the recommended bits and bobs and life will be simpler in the long run. 

 

 

 

[Chris Walby]

Thanks Jon, super clear.

 

I couldn't possibly comment about engine location and looks, but my Aircobra looks ok and engine/tank position is ok ?

 

 

Oh I see what you mean...some don't like the look from the starboard side, but it suits me and the engine and that's all that counts!