Which Engine?
 

Hey all, I scratch built a bi-plane that weighs 8 pounds. It has a wingspan of 70 inches. The wings have flat bottoms, so it is nothing fast. I just want it to have enough power to have nice, relaxing flights. I would also like to stay with 2-stroke power because of price. All of my other planes are .40 sized or smaller, so after this plane is gone the engine might not get used again. Also, if it doesn't fly very well, I won't be breaking an expensive 4-stroke. I have heard that 65 la's and 65 Sportsters turn large props, but are they powerful enough for the weight? Or do I need to go to a 90 size engine? I'm open to any brand, what would you pick? Ross

RE: Which Engine?
 

Ross,


The K&B .65 Sportster is not a particularly powerful engine... The OS.65LA is more like it.

For a 70" span bipe, the recommended engines are [link=http://www2.towerhobbies.com/cgi-bin/WTI0095P?FVSEARCH=biplane&FVPCODE=RF&search3=Go]larger in general[/link]. From a .90 to a whooping 1.60.

I guess it depends on the way you intend to fly this plane.

A .60 size engine, perhaps even a bit smaller, will carry this model for basic flight, but for aerobatics and strong vertical performance, neither engine; certainly not the Sportster, is among those I would recommend.

RE: Which Engine?
 

I believe the man said that he wanted sedate performance so he could enjoy putting around with it. Guess you never ran a sportster up to it's full potential. It's not an MVVS obviously but properly set up and run you'd be suprised at what comes out of that motor.
Dennis

RE: Which Engine?
 

Dennis,


I guess I will have to take your word for it, as I don't intend to own such an engine.

The two I encountered locally were weaker than a .40, even with a 13x6 on them (~9,000-9,500 RPM).
Had they also been expensive, it would really have been a waste.

RE: Which Engine?
 

Get a Supert Tiger 75. Inexpensive and reliable

RE: Which Engine?
 

Hey guys, thanks for the replies... Dennis, Are you saying the Sportster would do the job? Cyclic Hardover, I see the .90 Supertigre is the same weight as the .75. Are these high RPM engines, better used on faster planes? Ross

RE: Which Engine?
 

The Super Tiger G90 is not a high rpm engine

RE: Which Engine?
 

Ross,


Most current .91 engines, the ST-90 (and 75) included, are bored and stroked .61 engines.

This means they breathe through .61 size, or smaller passages, bypasses and ports.


This in turn means they will be biased toward lower RPM ranges than the .61, into its frame they were built.


Specifically to the ST-90, please read [link=http://www.supertigre.com/engines/supg0235-man.html]this engine review[/link], for the right info.

With a suitable tuned-pipe, it can make the open-exhaust HP figure, 3,000 RPM lower.
No real attempt was made to get more HP at the open-exhaust RPM, since the porting is pretty much up to flow capacity...

You can, however, get more flight speed through higher pitch props - not high RPM.

RE: Which Engine?
 

The G90 is what I usually recommend on a 60 size plane. I have a few of them but some are particular on the size weight thing. Whenever I say G90, they bring up a 75 which to me is pointless. Also i feel the G90 is as reliable and powerful as a 91fx for which their is a huge price difference.

RE: Which Engine?
 

On pipe,

What weight does your model need to balance it? Is a normal .60 sized engine sufficiently heavy to balance the model without lead in the front?

If you are thinking of an engine to just putt around the sky with in a relaxing manner at part throttle (1/3rd to 1/2) with a minimum of flame-outs, I recommend that you avoid the ST line entirely.

While the design of their carbs is sound, their execution is poor, with a very rich mid range being a feature, even when the high speed and low speed needles are set correctly. This manifests itself as a rich "stumble" or reluctance to spool up when throttling on, which, at its worst, can put the fire out entirely. The STs run fine at WOT and idle all day if you want, but run 'em at part throttle and a great majority of STs will get the stumbles.

We have lots of STs at our field and there is a degree of variation in the performance of their carbs (call it manufacturing tolerance), from absolutely horrible (our resident ST G4500/WM Ultimate and an ST G90/Dragon Lady) to almost OK (an ST 2300/H9 Funtana 90 and a trainer with a G51 in it). You may be lucky to get a goodish carb but, then again, you may not...

Even the most rabid ST fan will admit that a carb change or a carb re-engineering job is a valid option if one wants great throttling on an ST engine.

OS's engines have the best calibrated, most consistent carbs of any widely available, mass produced two-stroke sport RC glow engine. If you want a hassle free in flight experience, get an OS two-stroke of the weight required to balance your model.

If you really want to buy a cheaper Chinese-manufactured clone engine in the .60 size, consider either the GMS .76 or the Tower .75. Both engines have delivered good service at our field and are sold by large reputable retailers (Tower and others) with proven after-sales support. The ABC Tower engine is a more performance oriented package with an unbaffled, free flowing, somewhat louder muffler whereas most of the ringed GMS .76s we've seen so far have come with finned, baffled mufflers that are very quiet.

Although manufacturing tolerances are greater on these Chinese engines than engines made in Japan, the GMS and Tower carbs generally come pretty well calibrated, and if you are unlucky enough to get one that's not well calibrated (there are a couple of documented examples of this) a reputable retailer should be able to exchange the bad part.

RE: Which Engine?
 

A good engine for a 70" bipe would be an SK 90 from www.kangkeusa.com. Turns a 15-6 prop very nicely. $100. The Tower .75 is also excellent, but tends to like small props. $90.

I have 2 of each. I run my SK 90 on a 15-6 and it pulls strong. The Tower runs on an 11-8 or a 12-6 and is speedy.

There is 1 ST 90 at my field and after 4 years and as many owners, it is finally broken in and running well on a 13-6. I would go with the SK 90 and the big prop.

Another thing about the SK is the carb is excellent. It 1-flip starts, as does the Tower, and it idles all day without quitting. Great mid range, too. I have many OS engines and the SK carb is every bit as good. The SK does take a good while to break in as do many large, ringed engines.

RE: Which Engine?
 

How about a K&B .61. They are very powerfull and reliable, made in USA - good quality control.

RE: Which Engine?
 

Ross, I generally don't like to recommend motors to anyone since we all have different expectations. That said i will say that the k&B 65 Sportster is a somewhat different animal. It was made and introduced in the early 80's and was cursed by that timing as cheap low oil fuels were introduced at about that time. A couple of runs with the stuff and a lot of plain bearing motors were toast. You can buy a lot of them cheaply and they run badly as Dar has stated but he didn't give any history on the motors that he saw run.
With that out of the way a properly broken in 65 will not set the world on fire with it's high top end and to expect such is un realistic. It is made to not exceed 12K and contrary to it's running instruction of using an 11 inch prop , really does it's best work on 12 and 13 in lumber. Rpm numbers on 13 in props are usually in the high 9's to mid 10's but you are getting a fair amount of thrust without over heating as you would with a lot of other 60 sized motors. Fuel econemy is very good and running and idle charicteristics are good. Cavat is to make sure that you never run the motor lean, and there is no real reason to. That said you will have a very long lasting motor and it will pull your 8 pound model in a realistic manner. Remember the motor was for all purposes designed as a cheap beginners motor, much llike the OSLA series or the TT GP series. As is normal for most plain bearing motors there is a run in period, Do not try and force the motor to perform more then it can at the beginning. They do take a little time to break in but properly done they are a one or 2 flip hand starter and will reward you with years of service.
dennis

RE: Which Engine?
 

I can tell you that the .65 has no problem with a 13/6 on the bench.
Just recieved a Tachometer and will run an LA-65 on the bench with a 13/6 and 12/6, and provide #'s if it will help.
Years ago I installed a Fox .40 sch/bbng engine in a 52" bipe. It was heavy but flew fine. After awhile I replaced the .40 with a Fox .60 non-sch. bbng. Eagle I. (both engines claimed near the same hp figures though at very differant rpms.) The .60 brought that plane to life! One of the things I learned from that experience was that fast engines need to be mounted on fast airplanes, and for the others, more torque and larger props work better on the slower ones.
I also believe that bipes have 2 wings. This says to me that a 52" main wing bipe is about comperable to a 104" monoplane in drag, and about 25% more weight. (just an uneducated guess) This tells me that your 72" bipe is about comperable to a 144" momoplane and about 25% more weight.
I think the LA-.65 would have no problem with this plane, but also believe a .91 sport engine with considerabily more torque than the .65 would be a better choice.

RE: Which Engine?
 

The .65 LA is reported to be a good engine but I think it has been recently discontinued by OS. For about the same money, the ST .75 and .90 are available and make good power. The G-90 is one of my favorites, swings a good size prop and is a easy engine to get along with.

RE: Which Engine?
 

As of 2 min. ago the .65 was still in-stock @ Tower.

RE: Which Engine?
 

I checked the ..52-.75 twostroke catagory and it was there with the bold DISCONTINUED next to the picture.

I would guess they have it listed somewhere else on the site though.

My fault for not searching harder.

They have discontinued others, like the .40, so I am guessing that the .65 will not be much longer.

http://www2.towerhobbies.com/cgi-bin...?&I=LXLL61&P=0

Yes, here it is in silver only

http://www2.towerhobbies.com/cgi-bin...?&I=LXPUE1&P=0

RE: Which Engine?
 

I'm with Ed Moorman. I'd go with the SK .90. Just what a huge bipe needs swinging that huge prop.


Ed Cregger

RE: Which Engine?
 

Pipe, from experience I can vouch for the LA .65, a friend of mine went through two K&B .65s (sent one back) trying to get one that would draw fuel long enough to finish a flight, he finally gave up and settled for a MECOA .46. The LA will start on the first flip and run as hard or as easy as you want and never deadstick.

RE: Which Engine?
 

I do not believe that a .91 swinging a 13/6 @ 10 grand has anymore thrust than a .65 would on the same prop at the same rpm. The differance on a 13/6 could only be due to the ability of the .91 to turn that 13/6 FASTER, or the .91's ability to turn a bigger prop ( which would be good on a slow airframe) at the same rpm. I vote for a slower, higher torque engine turning a large diameter prop, over some high rpm engine that takes 3 prior owners, and a couple gallons of fuel to get to run right in this 72" bipe. 4-strokes come to mind here, but are not your choice, nor would be mine either. There are plenty of really good 2-strokes that will do what you want, and the LA-.65 might be one of them. The OS .91 FX would probably last you the rest of yer life, and fly the wings off of that bipe.:D

RE: Which Engine?
 

Thanks again for the replies, and keep them coming! I'm learning a lot about choosing the right engine; things like whether it's a "stroker" or a "buzzer". I like the sound of the 65 LA for ease of operation, short break-in, and cheap price. I like the price of the SK's, and have heard good things about them. I thought about the 91FX, but the price is almost in 4-stroke territory... I actually originally bought a Supertigre 90 for this plane, but it was one with the slot machined incorrectly in the carb. I couldn't get it to transition and got rid of it. (rich midrange) A couple months later I got a Supertigre .40 and it's been super right out of the box. I thought about getting another .90, thinking it was just a one-time lemon, but it seemed to vibrate a lot also. I have a good K&B .61 but don't think it would have quite enough oomph. Anyway, keep the ideas coming, I'm still trying to decide... Ross

p.s. I see the 91FX and the 90 SK are ringed. Would they idle/transition well after a few tanks on the bench?

RE: Which Engine?
 

QuePasa,


You can say that again and again and again.

It is pure truth, but some find it hard to swallow...or to understand.

RE: Which Engine?
 

I opined once that a 14x6 and 12x6 turning the same rpm should fly a given airplane at the same speed. The larger prop would just reach the speed quicker. I was corrected but not given the reason why. I didn't know for sure, but it just seemed logical.

RE: Which Engine?
 

Tim,


The 14x6 will reach a slightly higher speed, since it slips less.
The drag of the model will cause the prop blades of the 12x6 to run at a slightly higher angle of attack (hence the slip), to pull the model forward so there is some speed loss.

The plane will also slow down less, if you go into a climb with the 14x6...

RE: Which Engine?
 

Think about what you wrote.

It's true of *any* engines.

In fact, an 0.049 "swinging a 13/6 @ 10 grand" will generate exactly the same amount of thrust as a 150cc gasser "turning the same prop at the same rpm"

The same prop will always generate the same thrust at a given RPM, regardless of what's turning it. It's *whether* the engine can turn the prop at those RPMs that matters.

In the case of a .90 like the ST, the 13x6 is really the *best* prop to harness it's useful power. Sure, you can run bigger props but they won't produce as much power.

What people seem to forget is that we generally grossly over-prop our engines because of noise and efficiency constraints.

Look at the manufacturer's figures -- they're often quoting the maximum HP at 15K-16K RPMs. To get those kind of revs out of a .90 you need to run an 11-inch prop.

My 175Kph "high wing trainer" has a TT61GP in the front. People can't believe how fast this thing flies or how much power I get out of that engine. How do I do it?

Well I only run 5% nitro but I don't load it down with a 12 or 13-inch prop. It turns a 11x7 at around 14KRPMs -- that puts it closer to the peak of its power curve but still allows the use of a prop that's efficient enough to make use of that power.

Yes, it's noisier than it would be on a 12x6 or bigger prop -- but I want power and prop it accordingly.