shakeelsid

Now here is something for all you gurus and pundits to ponder upon. I must confess it perplexes me more than anything else that I have encountered with when playing with engines, and I want to hear your opinions on it.

The story is about my weed whacker converted Tartan single glow, that I bought on ebay about three years ago. It is very similar to Tartan ST-7G, the only difference is a supertigre carb and stock muffler. I put it on a scratch built 120inch piper super cruiser. It flew fine and I put on around 30 flights before deciding to tinker with the engine to reduce idle Rpm. I got the engine to run a 17-6 airflow down to 500 rpm on a 9% castor 5% nitro 15% unleaded mix.
I flew it for another 15 or so flights after adjusting the idle. On its first troubled flight, I chopped the throttle to idle at threshold and was about 20 ft AGL on short final, when the engine sputtered and the plane suddenly started falling like a brick and banked right. I kept feeding in elevator and aileron till I ran out of authority, the airplane squatted hard on the grass and broke the landing gear and engine mount. The engine was still running when it hit the ground.
I examined the airplane, recreated the accident many times in my head but could not figure out what had happened.
I fixed the gear up and flew another five or so flights before it happened again. This time I was about 30 ft and a little before the threshold. I gave down elevator till I ran out of it. The airplane slowed down and fell like a brick, this time rolling sufficiently to total the right wing, its super-structure and the entire landing gear area. Obviously this time it was beyond repair.
I spent several sleepless nights, I thought and I thought and I couldn’t figure it out.
A few days ago, thinking about mounting this Tartan on a quarter scale Stampe bipe, I put it on a mount to check it out. Since I hadn’t run it in two years, I gave it a clean and put in a new plug. I made some of my Tartan brew (9% castor, 5% nitro, 15% unleaded) and started it up. It fired right up on the second flick and ticked over very gently – after a minute I advanced the throttle – voops, whats that, the engine sputtered and reversed its direction. I didn’t realise it until I had gone way past half throttle and the tachometer was showing 5000 rpm. I shut it down and tried to replicate it again – lo and behold, on my fourth attempt it was running backward again. Now I know Tartan being a reed valve engine with symmetrical crank/pin/piston layout can run efficiently either way, but to do it on its own, while still under load, I have never come across it before.

Having figured out what had happened, I slept like a baby that night. My reputation as a model flier is safe for now, but that of an engineer is in serious doubts.

Your comments.
Sid.

Zippi

My Feedback: (10)

shakeelsid,

That's a good head scratcher. I have heard of and seen and engine as it was running spit a little and start running backwards but that was at a low idle, not under much of a load at all. Have you checked the timing? You may try advancing or retarding the timing just a little to see if it will repeat the problem or cure it. Sure would like to hear the end of this story.

zx32tt

My Feedback: (9)

Blooddy hell lad, why in blazes are you trying to get this engine to idle so low? Most 2 stroke engines have an unreliable idle in this range. 750 rpm or better would be reasonable. If your ignition timing is adjustable, you may want to adjust that some.
Z

Diablo-RCU

My Feedback: (1)

If it's running a glow plug, it can stop and reverse at idle. What causes this behavior?
Too little flywheel (prop acts as a flywheel too) and too low rpm.
Solutions....use a heavier prop and/or hub and increase the idle setting to around 2200 rpm until it's on the ground.

Pelle Gris

I have head about a guy in Germany doing a trick like that. I believe he also used a Tartan of some sort. After taxi´ing in from his flight, he put the engine in very low idle, and it would change direction, making it possible for him to back the model into the parkingstand. It sounds like you were trying the same trick, but you started the "reverse gear" part of it a bit too soon...

F1race79

My Feedback: (6)

I have seen a Webra Bully taxi, chop throttle and go back up on throttle and taxi backwards.

tkg

My Feedback: (1)

Chip Hyde lost an Ultimate Bipe at the TOC one year when his engine did the same thing.

shakeelsid

So it does happen then, to other poeple. Thats a relief.
Now if I could only learn to master that, I could start flying 3D in reverse

mmattockx

There are some snowmobiles that use this effect to provide a reverse gear. The ignition simply idles the engine down to low rpm, then intentionally fires the spark with a lot of advance and kicks the engine over backwards. When you flip out of reverse, the ignition kicks it forward again. It used to be not unheard of for two stroke motorcycles to do this if the idle was too low, as well. There is no funnier sight than seeing your buddy start up his bike, drop it in gear and then nail it, only to go flying over the bars as the the bike shoots backwards...

So you are definitely not alone in this.


Mark

karolh

Sometimes the behaviour of glow engines are nothing short of magical. I have an old ringed ST 40 from the early 60's that on occasion would start if flipped while in a semi flodded condition without any battery power on the glow plug.

Karol

gboulton

My Feedback: (15)

I've heard stories (don't know how accurate) that he KNEW it would do it, and had mastered the little throttle blip that would cause it....and that time, it just didn't come back up for him.

Nogyro

My Feedback: (10)

A buddy of ours that used to fly had a Webra Bully on a big profile plane. We would go to fly-ins, and no matter how crappy his flight was, all he had to do was land, stop the plane, then taxi the plane back in reverse. The crowd would go nuts and give him a standing ovation! All it took was to have the low side a bit rich and a low idle. It would kick in reverse almost every time.....Next came all the lies about having a reversing prop, gear box, etc. LOL He always came clean with them though.

rmh

piston port engines - they will all do it
back when the world was young the OR23 and others would also do it
All of my engines are rotary valve (front or rear type) - and they never do it .)

davewallace

My Feedback: (29)

Sid,
Am I correct in assuming that this is a glow engine running running on a mixture of glow fuel and gasoline? "weed whacker converted Tartan single glow, that I bought on ebay about three years ago. It is very similar to Tartan ST-7G, the only difference is a supertigre carb and stock muffler". The ignition timing usually prevents this from happing, but with a low idle and glow plug it could happenif you are not using a hot enough plug. The conversion guys who run glow/gas mix usually use a hot four stroke plug or glow driver to keep thing going.

Dave

BillJ

My Feedback: (19)

Thought I might jump in here..... I have a tartan twin on glow...and its my understanding that an alcohol based engine would not only run under more compression but would fire under the combination of compression and glow....In reading ...you are running this engine ( glow) with gasoline ? Wouldn't the fumes of gasoline fire sooner (detonation) under a glow plug and probably before TDC causing your engine to rotate backward ?? Most gas engines I've heard of fire on an ignition which sparks the plug at a pre determined degrees BEFORE TDC (Top dead center) and keeping the engine from doing the behavior you have described..... Your situation, IMHO, would be like having a spark plug sparking Constantly and in theory COULD fire at any point when the gas /air mixture in the cylinder was concentrated enough to be ignited before fully compressed. Might want to retune for a simple alcohol / oil mixture and about no more than 5% nitro...

shakeelsid

Seems like we are getting into tangential debates here. My fuel mix is a result of years of experimentation with glow engines to run on gas. after many permutations and combinations of plugs, mixtures and glow tempratures - engines running on dyno under controlled conditions, I have found that this fuel mix would provide optimal performance on a Tartan as a glow. On my regular glow engines I use 10% gasoline, 5% nitro with about 1% ether and I get better fuel consumption, performance and life out of them than anybody else on the field. Gasoline has more than twice as much energy per volume than methanol, and a 15% gas component in a glow fuel provides about 25% fuel economy. This combination is stable and keeps well for over a year. The engines start with a flick, idle lower and provide a better peak and midrange than the commercially available fuels. I fly around a dozen airplanes with all sorts of glow and gas engines and put on around five hundred flights per year. There are several very long threads about these fuel experimentations on RCU and many of these details can be found there.
As for my engine, having tested it on the bench, I think I now know what causes a reversal - slightly richer mixture and too low an idle. sudden surge in chamber pressure (sudden opening of throttle, possibly with surge occuring just around BDC, but I have no way of confirming) causes a detonation, which coupled with the low inertia of a wood prop causes it to backfire and continue to run in the other direction. Fuel is not a factor here as I have tested it with 10% straight fuel as well. The only thing I need to figure out is that how to control it, as this could give you few pretty neat tricks.
And then because I have learnt so much after posting this thread, I must thank you all guys.
Slan

BillJ

My Feedback: (19)

THATS 15% GASOLINE ADDED TO WHAT ? I RECEIVED MY INFO FROM CLINE AND CO WHO IMPORT THE ENGINE TO THE USA AND WHO ALSO BUILT THESE TARTENS FOR THE TOC WHEN THEY WERE THE LARGEST ENGINE OF CHOICE.....THEY'VE NEVER MENTIONED USING GASOLINE IN ENGINE or NITRO BECAUSE OF THE ENGINES HIGH COMPRESSION STOCK.. fwiW

Antique

My Feedback: (4)

There were a few different Tartans made, some had different cylinder heads depending on the fuel to be used, some had schneurle porting and taller cylinder fins....Two different displacements...
I have an igniton Tartan twin here, factory made, CM6 spark plugs, definitely made for gasoline...Glow Tartans had adapters in the heads for glow plugs, and a different carb...There was even one model with a pull starter on the back....A few were made into flat fours and raced at Madera in the early '90s.....

shakeelsid

My Tartan is a gas engine converted to glow - it was supposed to run on gas all the time. RCIGN1 mentions the other types produced. Cline have a business to run, they would never recommend you to experiment wiht your engine, blows the warrenty and ruins company's reputation. As for fuel, it is 15% gas, 5% nitro, 8~9 % castor oil and the rest methanol. I wont recommend you to run it in your engine either, but there are thousands out there who benefit from such experimentation

the Wasp

My Feedback: (1)

""slightly richer mixture and too low an idle. sudden surge in chamber pressure (sudden opening of throttle, possibly with surge occuring just around BDC""

I agree with your findings,,

but let me add, the "Super" Tartan "plane" motor (as I have 1) has a compression of 11 to 1, if your motor is this high (and I'm betting it is) you should know with your experience, therefor if it is 11 to 1 I suggest you drop your nitro percentage to 3% and raise your idle a bit and lean the low a bit and or maybe a glow plug change..

Jim