opjose

I have to disagree.

I love the O.S.'s as they ARE a bit easier to tune than others.. BUT...

But there is absolutely nothing wrong with the T.H. nor GMS engines.

You are making the mistake of equating inexpensive ( or cheap ) with inferior... those do NOT necessarily go hand in hand.

O.S. seems to purposely keep their profit margins high, both on their engine and parts pricing.

Just try purchasing a replacement ABC liner from them... you might as well throw away the engine. With the GMS, or T.H. I can buy the same thing for FAR FAR less...

I've flown MANY GMS & TH engines right out of the box after running ONE TANK of fuel through them and needing to adjust nothing but a click or two.

They "come into their own" after a gallon or so, but you're assuming that this is not flying time...


One of the mistakes frequently made by newbies is to immediately try to set the needles.

They are set purposely rich to permit the newbie to start the engine and run some fuel through the ABC engine.

Even with the O.S. ABC's this is needed.

It would also be foolish not to run a tank or two through an O.S. engine prior to flying a plane.

Pricewise there is a significant savings to be had.

You'll be running about equal amounts of fuel getting the O.S.'s tuned up as you would the T.H.'s and the GMS engines.

Performance wise the GMS and T.H.'s are known to be extremely powerful engines, typically producing far higher BHP ratings than their O.S. equivalents... this is one reason O.S. released their new set of engines, to keep up with their less expensive competition.

The "reliablity" issue threads are unfortunately repleat with many people who are relatively new to all of this. To them I WOULD recommend an O.S. due to it's slightly more forgiving tuning range and less sensitive needles... but once you know how to tune an engine properly, there is no problem using these engines. You can also find many similiar threads about problems with O.S. engines as well.

Unless there are other fuel line problems I spend all of 10-15 minutes getting a T.H. or GMS tuned up. If I spend more time, something else is wrong... that same "something else" would likewise affect an O.S. engine too.

You've spent more money and time on your AX55 and and pipes trying to get the RPM's up.

I got the same RPM's swinging larger props, from the start...

Littlepiston commented about the 14000 RPM on the GMS, but that is approaching the engines design limitations, not it's power output...

e.g. you can increase the pitch with an 11x8 or a three blade prop, as I did, and still get the same RPM's out of the GMS... all at a good bit of savings and w/o needing to pipe the engine.

Indeed the TH and GMS mufflers are often used on O.S. engines to increase their RPM's as well...

I also have NO need to run anything more than 10% nitro on these engines making them more cost effective over the long haul than the AX.

30% is useful to make an engine a bit LESS sensitive to tuning. I'd imagine the T.H. and GMS engines would be easier to tune with 30%, but for me it's a waste of money.

Moving up to the GMS .76 only had a minor impact on the weight, as I did not have to worry about things like pipes and fittings
But moving up also increased power dramatically.

I've used the T.H. .75 on other planes to swing much larger props than what the AX .55 can begin to handle... including 16x8's on my Passion 3D plane.

My .52 Bobcat - SCREAMS - through the air now with the GMS.

Unfortunately I do not have a radar gun, but the local "jet guys" seem to be impressed with the performance of my little plane.

I have my flight timer set for 12 minutes on the Bobcat and I seem to have a good bit of fuel left over, after a flight...

A ground test at WOT resulted in well over 18 minutes on the stock tank.


Marktur, it's great that you've gotten such good performance out of your plane...

But it doesn't sound like you've tried the GMS or T.H. engines.

I've flown many different brands. I like the O.S.'s a lot... but I feel no compunction to spend more money on them when I can substitute a GMS or TH engine, as I know I can get similiar or better performance for less money with NO added hassles.

littlepiston

My Feedback: (12)

the os is running at its top most rpm because of the small displacment
my GMS 76 needs the EXauhst rasied and it will turne higher. for people that dont know how to modify engines the os is there best bet (i race boats and im vary handy with tunign and modifing engines) ill problably get about 16000 to 16500 out of the gms 76 with the 11x7

marktur

I'm at Sea Level....geez, I didn't even think about that before I started ranting...sorry....but still, I'm making comparisons to the Tower running HERE, too, so I guess it still counts, eh? I got a buddy running a well broken in Tower 75 here, using a 12x8 I believe, but he's only turning around 10k if I remember correctly.

Now my question is: will the 2 motors perform equally at 3300', since they are pretty darn close at sea level?

This is an interested topic - a bit off-topic, but maybe not because we're sort of limited when dealing with pushers... I think everyone has good points. There is definitely a money savings by going with the Tower. The GMS...I dunno....a few people have them and they all want big props. The GMS has the most horsies according to Tower, but it doesn't want to spin as fast as the OS.

I will argue one point strongly: The OS is simply a better motor. Yes the piston and sleeve costs more, but we're not talking about cars and replacing them every 2 gallons of fuel. I think OS is also better at marketing, and yes, they have found a way to get more cash out of us for the motor than Tower does....HOWEVER, I am getting the same performance out of my .55 that you guys are getting from a bigger, heavier motor, and that ABSOLUTELY counts in the way our planes fly. Nobody can argue that the same plane, wing, airfoil, etc. flown at a lighter weight will fly better than a heavier plane. This translates into both fast and slow flying... In summary, I feel like I'm getting a better VALUE with the OS motor on this particular plane. In contrast, if I were flying a warbird, I would put the Tower motor on, and swing a bigger prop with higher pitch and it would be great I'm sure - because of the power, and also because most warbirds need more weight in the nose anyway.

I also wanted to say something about using 30%....it does not necessarily mean it will make it easier to tune, nor does it mean that every engine will perform better. In the case of the OS, it WILL. The AX is designed to be a high-perf. motor. It has better machining and tolerances, and I'm sure it's also a better and more expensive alloy - which can possibly also explain the higher prices. Anyway, the deal with the nitro percentage is this: Nitro is a cooling agent. It will cool off the motor, and provide more power. It will require adjusting as well (you need to lean it out more) but I don't think it makes it any easier or harder to tune.

Jose - if you're turning an 11x8 at the same speed I'm turning the 11x7, theoretically, you should be moving faster...but you're also carrying more weight, so how does that play in? Maybe not at all once we acheive a certain speed and get it "on step" so to speak. Luckily, there's a new Bobcat at the field with the Tower motor using the 11x8, so hopefully I'll get into the air with him tomorrow so we can see the performance. I will go as far as to say that if he is faster than me I will get the Tower motor and bolt it on right away (how can I not for $90???). And I'll say "you were right!"

I'm located in Palm Beach, Florida, and if anyone lives in the area, I would be happy to bring you out to our field as a guest. We have a killer club.

Cya!
Mark

opjose

Pretty good. That translates into a Prop Stall Speed of 127 MPH.

You could push it to 141 MPH with the Graupner 11x8 prop.

Ken LaPointe

My Feedback: (4)

Thanks opjose.

I have set up my plane to be between whats shown on the middle and right picture.

Fingers crossed

Ken

opjose

But by the same token we are not talking about replacing them on the T.H. or GMS engines with that frequency either.

I accept that OS seems to have a bit better tolerances and are more consistent in their manufacturing.

T.H./GMS offset this advantage to a degree in being good at sending you out a new replacement if you have problems and inexpensive replacements.

I can't speak to the materials used in the motor casings of the OS... but I've a couple of O.S. FX engines where the built in remote needle mount has fractured.Likewise the remote needle mount on the GMS/Tower could be stronger, but it's replaceable.

So I dunno...

Yes, but remember that you are running your engine at it's limit while the TH/GMS .75 still has more room to go... In a way you are running yours more "efficiently" than I am.

The problem I have is obtaining the right prop!

I cut down a 14x7 to 13x7 and that works fine, giving me plenty of thrust... but what I'd really like is more pitch for this engine... something that I've had a hard time finding in a pusher configuration...


Yes, but the difference here is minimal.

Switching planes... I threw a big motor and battery into my Super Sporster EP bringing it up well over a pound and 1/3.

This on a much smaller plane.

Now it does indeed fly "better" than the lighter versions. It is less prone to being buffetted around by winds and there was no appreciable effect on stalls and landing. If anything others have adopted my configuration after seeing it fly this way.

So there is indeed SOME effect, but the degree varies as does the performance difference.

I'd imagine that I'm coming in hotter than your plane does, but by how much? Most calculators seem to cite about a 2-3 MPH stall speed difference.

I'm not going to notice that.



Heh. I can't speak to the alloy, but good marketing can also explain the higher prices.

The O.S. RACING engines look fantastic, but Ouch the cost!

Correct me if I'm wrong, but as I understand it, nitro is not a cooling agent at all. It actually aids the combustion at low RPM's.

Cooling is obtained by mixing in more non-burning materials such as higher oil/castor mix.

I cite a reference:

"NITROMETHANE. Also commonly referred to as NITRO for short. Nitromethane is derived from propane. In itself, nitro is not even extremely flammable as one might think. The real power producing potential of nitromethane is that it carries additional oxygen with it into the combustion process. It acts as a fuel too, but the molecule carries oxygen along with it."

Oxygen is not a cooling agent... quite the opposite.


Eh, With all due respect, that's exactly the opposite of what everyone ( except for a handful ) tend to state here and what CoolPower/Morgan Fuels state on the matter, at least that I've read.

Hmm... it will be fun to see the results.

In your shoes however, why change a known "good" configuration! "If it ain't broke... ".

Instead try the 11x8 prop too.

Let us know.

littlepiston

My Feedback: (12)

the prop and niro content is not everything , the timing for the ports is were it counts . my gms with the 11x7 is running 14000 but i can hear the engine go passed the tuned muffler i need to raise the ex port and posibly the transfer ports i like to thinker with engines so dont go post "well if you would have got an os 55 ,you wouldnt have to be tinkering with your engine".[:-]

marktur

Man, everyone's got great arguments here (friendly ones). I hope I'm not pissing anyone off...I'm just not in the mood to work today.

I believe nitro cools things, as well as aid in the combustion. (You always end up leaning the motor out when switching to higher nitro). Maybe it's not the "nitro" but the combination of ingredients that they put together to make a fuel they call "30%". Of course the heli fuel has more oil in it...does oil help to cool, or just lubricate?

Pipes, plugs, props, and timing all add into the mix. From my RC car days (not too long ago) I know that each item I listed is a BIG variable in the actual performance. I have done about 3-4 hardcore years of on-road 1/8 scale racing, and the average price for the engines at the level I was racing at (state series) was around $500-600 (for a .21!!!). Boats I'm sure are just as bad. The motors are absolutely killer - highly modified, which also makes them harder to tune, but holy cow - what power and rpms!

The Sportster is a good example to prove me wrong, too...they have a semi-symmetrical wing, and they float...but they are also great, fun sport planes. More weight on it, especially in wind is a good thing. Gliders, too (for slope, not thermal). More weight on the Bobcat is noticable on takeoff and landings, I'd guess, but I cannot see more weight making the flight characteristics "better" in any conditions for THIS plane.

Well, with all this Bobcat talk, I think I'm going to close the office early, and go fly! I just checked Surfline.com to see when the swell will arrive, and I found that it will arrive on Sunday, along with 25 knot winds (ugh), so I better get it today and tomorrow.

I'll try to get some video...

Hope you all have a great weekend.

I'll say it again, so we can all agree on something today: I love my Bobcat!

C'ya!
Mark

littlepiston

My Feedback: (12)

BOBCAT('S) i have two!!!

opjose

Anything which lowers the amount of combustable materials in the fuel or changes the ratio of combustable to non-combustable would lower the temp.

Oil is sort of in the middle as it CAN be burned, but usually ends up in the exhaust.

As such it acts to cool the cylinder and head.

Better, probably not.

I can't see any effect of the weight on landing or takeoff, but my Bobcat takes off like a bat out of hell...

The .76 produces so much thrust that I have to watch that there are no planes 20+ feet behind the Bobcat, least they be blown over.

I really don't notice the weight on takeoff. My only issue is speed. I want MORE!! ( NOT THAT IT'S BAD AS IS! ).

Three of them here...

The .25, the .52 and the Falcon 1.20.

The .25 is starting to crank well too. It has a 3 blade 10x7 MA prop on it... I'm going to try a 9x9 2 blade too.

marktur

HI guys,
Didn't get to fly today, and with 25-30 knot winds, I don't think I"m "in" for tomorrow either. I *will* be surfing, however, as it's supposed to be well overhead in the mornining!

Have fun!
Mark

AGR413

My Feedback: (1)

I'm probably wrong but the whole OS .55 vs TH.75 Sounds a little pretentious. This is some research I did in an earlier post on this thread. Granted this was for the OS .75 but I think it puts things in perspective. Bottom line an equivalent OS is about 10% more efficient the OS .55 is not going to make up the difference in displacement even if it is 10% more efficient than the same size motor of another brand.

O.S. .75 AX ABL Engine

SPECS: Displacement: 0.75 cu in (12.3cc)
Bore: 1.02" (25.8mm)
Stroke: 0.93" (23.5mm)
Practical RPM: 2,000-16,000 r.p.m.
Power Output: 2.4ps/15,000 r.p.m.
Weight: 20.39 onuces (578g) w/o muffler
26.46 ounces (750g) w/ muffler
Recommended Props: 14x6, 14x8, 15x7
1 ps = 735.499 watts, 2.4px*735.499 = 1765.1976 watts output at 15,000rpm




Tower Hobbies .75 Ball Bearing ABC Schnuerle & Muffler

SPECS: Displacement: 12.21cc (.75 cu in)
Bore: 26mm (1.02"
Stoke: 23mm (.91"
Horsepower: 2.2 BHP at 15,600 RPM
Weight: 652g (23.0 oz) (with Muffler) (jl10/00)
Weight: 535g (18.9 oz) (without Muffler)
Recommended Props: Break In: 12x6 or 13x6
1 BHP =745.7watts, 2.2BHP*745.7=1640.54 watts @ 15,600 rpm


So if you look at pure watt per rpm efficiency the OS is 10.64% more efficient than the Tower motor.

@ 13,000 rpm that is a difference of 162 watts. The OS will spin a larger propellor which may be where you will see the advantage of the 10% greater efficiency. Not to mention those large props won't fit between the booms of the Bobcat. In this case I seriously doubt you will see the benefit of your extra $100 bones by buying the OS, the OS is also heavier than the Tower motor which means you will need more nose weight and make wing loading that much higher.

I would like to see what the Tower .75 would do with a tuned pipe on this plane.

Nitrodan73

My Feedback: (12)

I have a question regarding mounting the .75 engines. Is there any modifications that any of you had to make to get a .75 to fit? engine mounts changing thrust angles, etc... I am going to have someone build this plane for me since we had our baby 2 days ago. Now I'm on diaper duty so no time to build right now. I just wanna have some tips for him if there are any regarding the engine mount.

Ken LaPointe

My Feedback: (4)

Big thanks to OPJOSE.

Test flew this morning went ok. CG is at 10.3 Elevator needs to be more like the right hand picture than center.

Pitches down a bit when power turned off.

HW-730 fan is very quiet and it looked great in the air

Lands quite fast, need to add in some spoileron

Hope it lasts the rest of the fall

Thanks again for the help

Ken

knighttwister

My Feedback: (59)

the tower 75 has a somewhat larger crankcase than some and so the bottom of the mout needs to be relieved or the mounts need to be shimed to bring them higher. i did both to try to retain the original datum line. good luck with your bobcat. when they are setup correctly they fly fast and land like a dream..

opjose

I cut into the base with a dremel sanding drum and widened the "stance" of the included block.

The Fiberglass covering is quite thick there. I sanded down until I hit the hardwood ( good hard stock used here ) in the shape of the motor botom.

I then epoxied in the mounting blocks and finally heavily fuelproofed the whole area.

I drilled through the blocks and wood for the engine mounting holes, and put in blind nuts at the bottom.

I use the blind nuts in conjunction with regular nuts to make sure that there is no chance that the bolts will loosen.

This has worked very well, as the engine sits exactly on the original designed thrust line.

AGR413

My Feedback: (1)

I went the other route on mounting the .75. I cut spacer blocks and put them under the blocks included with the kit.

marktur

I created spacers using 1/4" ply. Gave it a good coating of epoxy to fuel-proof it, too.

Nitrodan73

My Feedback: (12)

Any pictures of your mods?

marktur

Yes, I posted some pix previously - you can probably see it in the last few pages where I took pictures of the Jett Pipe.

If you can't find it, let me know and I'll take more pix for you...basically, I used 1/4x3/8 ply. Match the length of the blocks...strip off just under 3/8 of the covering on the hardwood blocks (so that the covering is held slightly under the new blocks - no fuel can be allowed in or it will rot), and use 5 minute epoxy to hold it on. When it's set, mix up a bit more, and "paint" it on all the bare wood, and you're done.

Hope that helps!
Mark

Nitrodan73

My Feedback: (12)

Mark,

Abvout what post? I looked a few times. Still looking.

Nitrodan73

My Feedback: (12)

Found it Mark! Thanks!

Crazy4Flight

My Feedback: (540)

NOW ON SALE $99.00 + shipping at NitroPlanes

marktur

That's a deal on the Bobcat. Just make sure you all get the retract version - That's the best $10 spent ever!



Surf's up - I haven't been flying at all....except down the line on these nice tropical swells rolling in....

TUMBLER

My Feedback: (13)

Man yiou got that right! I just finished up the retracts in the .25 size. Wow, what a project! I would not do that again! I wish they had an option with retracts like the .50 size!!!

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