Four Stroke Torque!

25-04-2004

I'm now a total convert to the four stroke engine for aerobatics, especially 3D. I've had two very graphic demonstarations which have really surprised me. The first was a few years ago when the whole fun fly thing was just begining. Aircraft hadn't developed into what we now recognise as Fun Fly, but still looked a lot like Wot Four's. I designed my own and christened it Hanky Panky. (See Pix) It was powered by a Merco 61 and, boy, was it a disappointment. Despite having a low wing loading the thing was a dog to fly. I just couldn't understand it. So, after much fiddling for no improvement, I retired it.

Later I bought a broken HP 61 drum vavle four stroke, got it repaired and then needed something to try it out in. Enter Hanky Panky again. Well, with the HP stroker installed it suddenly flew brilliantly! Everything I had hoped it would do when I designed it, it could now do. Eventually I stuffed this much loved aircraft while flying a four point roll at waist height - how stupid was that? But that was the kind of confidence the model inspired in me - such was the difference.

More recently I went down exactly the same road with a Funtana. First I had a Thunder Tiger 61 Pro two stroke installed, and once again was sooo disappointed. On advice from RCU members I fitted an ASP 91 four stoke and once again the difference was incredible. With the four stroke this aircraft is fantastic despite the fact that the TT 61 two stroke produces about 1 bhp more!

It's all down to torque, apparently. The broader power band of the four stroke providing much faster spin up and thrust much more quickly than a two stroke, so throttle variations get a more instant response. Bigger, more efficient propellors help too. Below is a graph that someone sent me which shows the truth of it.

**wonwinglo** · 29-04-2004

Could not agree with you more Wowbagger,I have had similar experiences when trying a two stroke and then converting to four,the sheer torque produced is fantastic and they appear to be getting better all the time,some of the early four strokes suffered from bad workmanship,sloppy engineering and finicky tappets ( remember that Magnum lump that kept throwing tappets ? ) my favourites are the Saito,what motors these are ! they just ooze quality,I miss my Saito 53 which I just wore out the cam,my model engineering brother in law built the cam up with weld and did a super grinding job it gave me two more seasons until she finally died on me.
Problem is they need a second mortgage to buy one and I never replaced it,then there was that 120 flat twin,bad luck struck when I hit a metal pole stuck in the daftest place on the approach,knocked the cylinder head clean off,Aitree wanted too much to repair it so I just left it with him in lieu of the stripdown cost of £90,it is probably rebuilt and giving good service somewhere to someone ? with my flapped Quarter Flamingo it would pull the model up at 45 degrees and climb to about 800 feet before it reached the end of the strip,the MDS 90 currently fitted is a super motor but not a patch on that twin which I mourned the passing.
So yes the four stroke revolution changed the face of radio modellings power sources and made the impossible fly really well.

**wonwinglo** · 15-01-2005

For anyone wishing to try a four-stroke motor then give the smaller OS.26 series a go,not sure whether these are still in production but for sheer reliability,economy and fun for small models they take some beating,provided that you keep the tappets in trim and at the proper settings then these little motors go on and on,a days flying will only use a thimble full of fuel.
The best models for use with these motors are lightly loaded scale or semi-scale types,the sound is just right and they are relatively quiet and can be operated from small fields.
I understand that Enya produced a small four-stroke as well but have never seen one ? the only drawback is the initial cost of buying fourstroke engies which is double that of a Two-stroke,however the operating costs soon recoup the initial expense.

Greyhead · 15-01-2005

I quite agree, you can’t beat a 4 stroke for reliability and low running costs soon mean you’re into profit over an equivalent 2 stroke. If you’re into scale then the sound alone is worth the extra initial payout and if you decide to go down the 4 stroke route this season you can’t do better than a Laser ; British designed and built, runs on “straight” fuel and with excellent after sales care from Neil Tidey.

I’ve used these engines now for many years and I’m never disappointed, first time starting (even after the winter lay up), super reliable tick over and instant pick-up. With its very realistic sound, what more can you ask from an engine?

Just in case you’re wondering; no I don’t own shares in Laser Engines!!!

**wonwinglo** · 15-01-2005

It is interesting to know how he manages to get his engines to run on straight fuel ? most OS/Saito really need the extra dope of the nitro content to get them running smoothly,this converts into acid once it has run through the engine and does the damage to the bearings we know so well.

Greyhead · 15-01-2005

Straight fuel is a definite advantage; I had an OS 40 FS and had to replace 2 sets of bearings, this in spite of the fact that I “flushed” the engine after each outing.

I may be a bit paranoid but I still flush the Lasers although Neil Tidey says it’s not necessary. I also use an air filter as well as fuel filters in the fuel line, crankcase breather pipe and tank overflow. Better safe than sorry I say!!

Grahame