Engine equivalents

[Piers Acland]

Dear all,

 

Is there a rough and ready way of converting from a given size of two stroke glow engine so as to get the equivalent power from (i) a four stroke glow engine (ii) a two stroke petrol engine and (iii) a four stroke petrol engine?  So if the plans recommend say a .50 two stroke glow, what size of engine would be appropriate if I wanted to use a 4 stroke glow, a 2 stroke petrol or 4 stroke petrol instead? 

 

Mant thanks

 

Piers

[Andy Stephenson]

Piers,

 

I can answer the 4stroke glow question... Saito 82 as for the others I would say any petrol engine of equivalent power would have a lot of overhead in weight due to the ignition unit and diaphragm carb.

[Denis Watkins]

Is the .50 glow significant Piers, as the combination would help decide, as this ballpark is 6lb 60 inch wingspan model.

Looking at 10cc petrol, to 15cc FS petrol, and a 61 FS Glow,

Horses for courses

And if it was a Bi-plane, We need to start again.

So there is no rough and ready list, but more model size, and weight considerations too

[Piers Acland]

Thanks Denis.  The plane I have in mind is Jan Hermkens' Short Sunderland - 4 engines, 136" wingspan, 18 kg.  So quite big. He used four 0.45 two stroke glows but I would prefer to use 4 strokes.  As Andy says, petrol engines might add too much extra weight so probably looking at 4 stroke glows.   

[Nigel R]

Add 30% to two stroke to get four stroke equaled. 

 

from a 7.5cc two stroke that gets to a 10cc ish four stroke glow, say os 65v or fsa 56.

 

Same size of petrol two stroke, 10cc, roughly equal to glow four stroke.

 

Petrol four stroke add another 30%, in so 13cc or thereabouts.

 

Saito 82 would be about equal to a 60 two stroke 

 

Edited July 22, 2021 by Nigel R

[Denis Watkins]

What a great project Piers.

Motors have come on a little in 30 years, so what was OS45 power, you can get from a good modern 40.

To go to 4 stroke, the natural jump is the ever faithful .52 FS, but would be in the ball park of 4 2 stroke OS45.

Acquiring 4 motors may be a task in itself.

Lads on this site will have more to say about 4 motor set up.

On the side of " money is no object ", have a look at Saito fourstrokes.

[Nigel R]

Quite a project.

 

Some quick calculations...

 

18kg equals approx 40lbs.

 

Each motor is thus being asked to pull 10lbs of model.

 

You could expect approx 1hp from a 0.45 at sensible RPM. In metric terms, that is about 750W.

 

On the suggested motors, this gives a total power / weight of about 70W / lb.

 

As a performance indicator, a vintage model will fly quite happily on 50W / lb. A sport model with aerobatic capability will want more like 100W / lb.

 

A Sunderland is not aerobatic, so the 4 x 0.45 suggestion seems quite valid.

 

Money no object, perhaps 4 x Laser 70s might fit the bill. Or the newly available Force 46 could work as a more budget option.

 

A though occurs - you could use two different pairs on this, a larger pair in the inside positions, a smaller pair on the outside. Acquiring 4 motors may be slightly easier if thus. 

 

 

[Jon H]

It is often difficult to convert as there is a difference in the way 2 and 4 stroke engines give their 'power'. Power is such an awful way to measure performance anyway as its thrust which makes our models move and here propeller choice is almost more important than the engine its bolted to. 

 

I flew a 100 inch ASM Hercules on 4 30 4 strokes using 10x5 props and it was very scale indeed. 

 

As Nigel points out, rules of thumb vary from model type to model type. 

 

In the case of a sunderland, i would probably go slightly larger than the barest minimum as water is very draggy and a little reserve may be needed. Also, it still needs to fly if one or more engines get fed up and stop so his suggestion of 70 4 strokes is probably where i would pitch it. Jan himself used 4 of our 70's in his B17 which is a smidge larger but, i suspect, a little more aerodynamic. 

 

Personally i dont recommend the mismatched pairs idea. The two larger engines then end up doing all the work and if you loose one of those its a greater impact on overall performance. I would always suggest using 4 of the same irrespective of what they are. 

[Nigel R]

22 minutes ago, Jon - Laser Engines said:

propeller choice is almost more important than the engine its bolted to. 

 

Bears repeating.

 

[Matt Carlton]

I suspect that there is also a gain in that a multi engined model has airflow from the props across virtually the whole span.

 

Perhaps that is why a model can be adequately powered by seemingly low power levels. Much less than an equivalent single engine anyway. 

 

To be frank though, given availability and reliability over the water, I would be very tempted to look at an electric conversion. No embarrassing engine cuts needing a boat trip to retrieve the model for one thing!  Lowish Kv outrunners would let you turn a decent sized prop at sensible rpm, so as Jon says, efficient thrust without needing huge 'power'. 

Edited July 23, 2021 by Matt Carlton

[Chris Walby]

I have a high wing trainer Nova 40 with a Laser 62 in it and that's nice, had a low wing Speed Air 40 with a a Laser 70 in it, nice again.

SG Dual Ace with Laser 70's in it that the spec says use 46 2 strokes and no lead to get C of G.

 

Have a look at the VQ range as they spec 2 and 4 stroke sizes + I have a FW190 and Aircobra with Laser 80's in them + an a ARTF Acrowot  (40 to 55 2 stroke) with a Laser 70 in it which is very nice.

 

If size wise Laser 70's would fit I would go for them, less weight than petrol equivalent, less complex, sound better and very reliable.

 

As for power isn't that just a product of torque times RPM? Just make it rev another thousand or two to get the power looking more impressive! Great, as its screams its head off and does not perform well overall. The most impressive engines I have come across racing motorcycles and flying model planes are those that perform over a wide RPM range as they are easier to live with.

 

Whatever you chose to do, if you can then keep it simple as it makes finding problems so much easier. Oh and build blog please!

 

PS - Matt, I have had more twins electric engine cuts than I have IC, sorry but I think that's an old school thought and not my experience.

Edited July 23, 2021 by Chris Walby

[Nigel R]

16 minutes ago, Matt Carlton said:

I suspect that there is also a gain in that a multi engined model has airflow from the props across virtually the whole span.

 

NASA seems to think it is worth investigating.

 

https://en.wikipedia.org/wiki/NASA_X-57_Maxwell

 

I forget the name of the scheme where you actively blow air over the top of the airfoil, vs. dragging the whole wing through still air with a central main propulsion.

 

The downside is, if you rely on that, when you lose a motor, there is a greater drop in lift than you might be able to compensate for in terms of the reserve power of the remaining motors.

[Nigel R]

12 minutes ago, Chris Walby said:

flying model planes are those that perform over a wide RPM range as they are easier to live with.

 

Point of note

 

Aircraft are not direct coupled like ground vehicles, so this 'flat torque curve' isn't relevant. Of course with bikes it is, wide torque curve gets greater acceleration in single gear etc etc.

 

If your engine makes good power with the appropriate prop, the specific nature of the power curve either side makes next to no difference to us. The only requirement being that there is sufficient power to actually get the prop up to peak rpm.

 

The only time I can think of that we might see a problematic situation, where the motor cannot get to peak rpm, is e.g. with a very peaky tuned pipe setup on a two stroke where the mixture might not be controllable enough to get to peak, or the airspeed needs to be above a certain figure to allow the engine to speed up beyond some critical rpm. See C/L speed models for instance.

Edited July 23, 2021 by Nigel R

[Chris Walby]

Nigel,

 

That's exactly my point as I live in the real world of meeting noise limits and as with all of the engines be it 2 or 4 stroke they have to come in at 82dB. The only current recognised way of achieving this is to over prop the engine and drag it way off its peak power RPM hence its needs to have a wide torque range to perform well.

 

I have a 150 size engine that at WOT the RPM is about 4500 RPM (to achieve 82dB) and is so far off its reliable tune point its unpleasant to fly. 

 

Point of note 

All of my IC engines are over propped to some extent, that's my real world.

[Nigel R]

Most of us are in that very same boat Chris, propped and silenced for 82db we're usually a long way off maximum power. C'est la vie. 

 

Getting 4x two strokes through 82db might be a challenge.

 

Out of interest, what is the 150?

 

[Denis Watkins]

I fly almost exclusively now with fourstrokes, and these models and motors range from .30 - .70 and due to those all being 

Soft Mounted, yes, Soft Mounted,

All come in well under 82db, and can fit adequate sized props, and get a good range of power.

It must be 15 years ago, I watched lads sailing rc I/C powered boats, relatively quietly, and was shown glow motors, mounted in a cradle on soft mounts.

I now machine an assortment of motor mounts, to accept rubber bobbins, where the mount meets the firewall, and motor bolted to mount.

I know that the motor damages the crank and the propeller flys off and the bearings collapse,

But this has never happened to mine in 15 years and the lads still boating have experienced no adverse crank imbalance or motor implosions.

[J D 8]

11 minutes ago, Nigel R said:

 

 

Out of interest, what is the 150?

 

  Classic Laser engine, very popular with scale modelers over many years.

[Jon H]

On the engine failure point, as Chris suggests electric is not a silver bullet and reliability is an issue for either system. The reliability of engines these days is good and dont think i have had more than 5 engines stop in the last decade. All but one were fuel starvation and the other was a thrown prop. None resulted in any damage to the model. 

 

It is however less critical on a 4 engine model than a twin should one stop as you do still have some power on that side. Also building a scale model can help as they were designed with large tail surfaces to cope with this sort of situation. 

 

But, correct power train installation, and then correct out procedure is going to be the thing that really saves the day. 

 

If you fly mostly i/c as it is then stick with it, if you have suitable batteries in stock then maybe electric is for you. But i wouldnt go electric just because its a multi engine. The cost could be pretty high, especially if you need to buy 8 batteries just to guarantee 2 flights a day!

[Chris Walby]

My Laser 150 has a few more wings than JD8's ?

[Jon H]

18 minutes ago, Denis Watkins said:

I fly almost exclusively now with fourstrokes, and these models and motors range from .30 - .70 and due to those all being 

Soft Mounted, yes, Soft Mounted,

All come in well under 82db, and can fit adequate sized props, and get a good range of power.

It must be 15 years ago, I watched lads sailing rc I/C powered boats, relatively quietly, and was shown glow motors, mounted in a cradle on soft mounts.

I now machine an assortment of motor mounts, to accept rubber bobbins, where the mount meets the firewall, and motor bolted to mount.

I know that the motor damages the crank and the propeller flys off and the bearings collapse,

But this has never happened to mine in 15 years and the lads still boating have experienced no adverse crank imbalance or motor implosions.

 

I dont recommend soft mounts as more often than not they arent set up correctly and the engine flails around. This causes all sorts of problems with exhausts falling off and needles wandering. Its not that they cant be made to work, its just that their performance is related directly to the skill of the chap installing it and, crucially, maintaining it. As a result i recommend glass nylon mounts as they will always work almost irrespective of the skill of the person installing the engine. 

 

I know it is not wholly representative, but i always use this video to illustrate my point about badly setup soft mounts

 

 

[Piers Acland]

Thanks all,

 

Really useful information.  I think I'm headed for the Laser 70s.

 

Piers

[Jon H]

55 minutes ago, Piers Acland said:

Thanks all,

 

Really useful information.  I think I'm headed for the Laser 70s.

 

Piers

 

I best get building then ? 

[Piers Acland]

Thanks Jon!  I have a couple of Laser 70s already so only two to go ...

Piers

[Jon H]

how old are the current pair? I only ask as they have been upgraded a bit over the years and may not all run together. 

 

If the two you have run together then i can send you another matched pair. You could then fit one pair to the outer engines and another pair to the inners so each engine is matched across the model. Its better than putting a matched pair on each wing as the pairs may run differently to eachother. Depending on their condition the pair you have might also enjoy a service to bring them as close to new as possible. 

 

I would start with 13x6 2 blade props as well when the time comes. 

Edited July 23, 2021 by Jon - Laser Engines