Twin 2 strokes or twin 4 strokes

[David Hayward.]

I am interested in building a twin i.c engine model and intend to build Peter Miller's Grumpy Tiger Cub as a starting point, then perhaps go on to build a Mosquito at some point in the future.

I would be interested to hear people's views on the various merits and or advantages/disadvantages of using either 2x stroke or 2x 4 stroke engines.

[Nigel R]

2 strokes are cheaper.

4 strokes sound brilliant.

Little difference in power to weight, end of the day.

An engine's reliability has got to be top factor when choosing for a twin. Perhaps worth making the extra spend for a brand known to make good carbs and their reliable running (OS, Laser?).

If it helps, my bucket list of projects has, very near the top spot, a sport twin running on 2 x OS 40 FPs. Those particularly, because they are extremely easy to start and run like clockwork (and there are loads on Ebay).

Take my post with a pinch of salt, I am not an experienced twin flyer.

Edited By Nigel R on 26/09/2018 11:24:22

[Peter G Simpson]

I agree with everything in Nigel's post above.

I'd add that whilst a single 4 stroke sounds much better than a single 2 stroke engine, a pair of 2 strokes running together do give a really nice clean sound and sound great for a twin R/C model.

As Nigel says, OS FP's are excellent, cost effective, simple to operate. I had a pair of OS 15 FP's in a DH Hornet, the model shown in the pictures here they were ultra reliable and sounded great. They also had bags of power so would be spot on for the Grumpy Tiger Cub.

Edited By Peter G Simpson on 26/09/2018 11:40:44

[Jon H]

I recommend 4 strokes for everything! but if the objective is to learn to fly a twin then two strokes are perfectly satisfactory and keep the cost down especially if you are building a smaller model. That said, if you are intending to get into a mossie at a later date you could select engines that will do both models. In which case a pair of 4 strokes would probably be nicer in the long term.

I recommended this to Chris Walby (see his threads on the subject on the forum) as he wanted a seagull mossie. I suggested he have at a seagull dual ace first and he went for it as a stepping stone to the mosquito.

I think he would agree it was a worthwhile experience. Chris, if you are here your input would be handy!

[Peter Miller]

I think that you might find problems with installing even 30FS engines in Grumpy TigerCub.

I used SC 15s whichc work very well.

You will also find that Grumpy Tigercub is very easy to fly on one engine. Obviously once you are on one engine a bitmore care is needed but it has been shown to be able to turn into the dead engine with care but without biting.

Infact when one enggine dies it takes quite a bit of time to work out which engine still has power and that is not just my opinion. Several club members have flown it and found the same

[Ron Gray]

I've scaled up the GTC and am going to use 2 x ASP30s in it. Can't tell you what it's like as I haven't started building it yet but on first look, the engines will fit into the (revised) nacelles no problem.

Note to self, get on and build it!!!!!!!

Edited By Ron Gray on 26/09/2018 13:30:36

[The Wright Stuff]

It's traditional at about this point in the thread for someone to suggest going electric instead!

[Chris Walby]

Hi,

I just love DH mosquitos and have a couple of electric versions (BH & Parkzone), they are nice although there is one missing thing on a nice low scale pass as it almost silently flies by! The sound of a four stroke.

So I hatched a plan to convert my BH Speed Air to IC and get some experience, then obtain a pair of 70's that would fit the Seagull Mossie and fit them in the Seagull Dual Ace (cheap airframe) if I get it wrong!

Prevention is always better than cure in my book so reliability over cost (not that Laser are expensive compared with other 4 stokes, but Jon is on the end of a phone/email) was a consideration.

Once I had a few hours flying the Dual Ace is was time move across to the Mosquito.

So IMHO its best to have a plan and cut your teeth on something that does nit have sharp teeth!

[Nigel R]

What, and take all the fun out of a twin?

[Charles Smitheman]

My only twin so far is a Ripmax Harmony.

I have really enjoyed the challenge, and have found it a lot more complicated than the usual singles. It has two OS52FS which I chose for reliability. I used one from another model, having replaced the bearings etc, and purchased another as new. I had a lot of trouble getting the two to run similarly until I discovered that the rubber seals on the bearings I had purchased were tighter than the original ones. After replacing all of the bearings they run well together. The point of my story: Buy two new engines, unless you are a member of the difficult club like me!

One thing that I dislike with the Harmony is the sidewinder engine mountings. It is quite spectacular sometimes if the speeds of the engines are not identical how much vibration occurs. I think an upright or inverted layout might be better, what do others think ?

Now that I have this model of course thoughts turn to building a twin with two strokes!

[Levanter]

My Grumpy Tigercub (standard size|) has twin OS FS 26s which Peter said will be fine. Part of this years Mass Build but distractions have slowed the build. Engines inverted.

Must get back on to it.

Levanter

[Bob Cotsford]

Posted by The Wright Stuff on 26/09/2018 13:40:43:

It's traditional at about this point in the thread for someone to suggest going electric instead!

Ok, if someone has too. I got a s/h model of Peter's American Nightmare a few years ago, converted it from brushed to brushless and LiPo. It actually sounded quite good in a nice subdued way. I now have a HK Skymule twin and even that sounds ok for a double deskfan. Much of what we hear from an ic model is prop noise so twin leccys aren't that bad.

Just saying. Is that my coat?

[David Hayward.]

Great feedback everyone, this has to be one of the best forums if not the best for good information and advice. My current thinking is to go for 2x 2 strokes for the Grumpy Tiger Cub and 2x 4 strokes for the Mosquito. Where can I hear 2x 4 strokes together?

I hear Jon's suggestion of using the same engine's for both models - did consider this but decided I did not want to 'disable' the Grumpy Tiger Cub.

As for 2x electric, hell will freeze over first.

[Jon H]

Posted by David Hayward. on 26/09/2018 15:23:45:

Where can I hear 2x 4 strokes together?

This is my pulse 125 modified to carry a pair of laser 70's. once the ASP120/briggs and stratton gets off the runway its easier to hear. not the best video ever but it gets the point across

[Peter Miller]

I must say that those two four strokes sound really good.

[Martin McIntosh]

If this is of any help, I always yearned for a TN 11ft Lanc so having a 6ft x 1ft wing plus a tailplane from an old model I cobbled together a simple fus and put a pair of OS 26`s in it to try my hand again at a multi engined model having previously flown a Mitchel on a pair of OS 40 two strokes. This flew fine but one day a motor cut. I was able to do a circuit and land on the other motor without much trouble. I was amazed because the model weighed 7.5lbs.

I used four strokes because they are generally more reliable. Could have run out of fuel on one.

The Lanc was built and did fly. The sound or four four strokes was great.

I decided to try the BT Mossie with YS 63`s but the build became too complex and was very flimsy so I abandoned it and did a leccy TN one. On the second flight an ESC died on take off so don`t assume that electric is better.

I now have an electric Lanc and it actually makes quite a nice sound.

[Nigel R]

That pulse does sound the business jon. Very nice.

[Jon H]

Cheers guys. I have rebuilt the engines since then as they were of different specs and ages so didnt always run together. They are now pretty much set and i rarely if ever touch the needles.

Charles, i had a RMX harmony with two 52 two strokes in it. I loved that model and it wend like the clappers! Anyway i found that even with a two stroke those laminated biscuits they use for nacelles were hopeless and i had to glue the strengthening braces back on several times. They really do not have much torsional rigidity so im not surprised your engines are falling about laughing.

[David Hayward.]

Thanks Jon, that's about the right sound for a Mosquito when I eventually get to it.

[David Hayward.]

Finally went for 2x Saito 40's for the Grumpy Tiger Cub because that's what I had, and tweaked the appearance to somewhat resemble a Mosquito. Had its maiden flight yesterday, flown by Jeff who has experience of flying twins. Very easy to fly and docile, fantastic job Peter on another great design. Image and link to maiden flight...

 

https://www.youtube.com/watch?v=fZCvNWVhozk

Edited May 29, 2022 by David Hayward.

[Ron Gray]

That looks really good, must build mine!

[Chris Walby]

Well done David and hope you have many happy hours flying your GTC. I went electric with my GTC as I couldn't find any suitable IC engines + with electric I wanted to try out flying with one cut, but have the ability to bring it back should I need to. I am still thrashing my laser dual ace and have the laser SG mossi for high days and holidays.

 

Peter Miller did mention that the GTC is tolerant of one motor quitting so you should be ok and I might have to poke mine with a stick at 3 mistakes high to get it to bite back.

 

Keep posting with your progress ?  

[Peter Miller]

That looks great!! Well done.

The problem with mine was that we could never work out which engine had cut until we made a low pass it was so forgiving.

[Andy J]

Slightly off topic but would you get away with a .12 2 stroke for the GTC?

[Nigel R]

might be undercooking it, the airframe is about a "40" size sort of thing, isn't it?