RE: TOWER HOBBIES .75 PROP RANGE
 

Britbrat,


11,400 RPM over 11,100 RPM represents 8.3% more HP.
It may not be a huge difference, but it is notable nonetheless.

...It is not clear which engine you are referring to in the second example (12K+)...

RE: TOWER HOBBIES .75 PROP RANGE
 

I was referring to the Tower 75 in the second example.

RE: TOWER HOBBIES .75 PROP RANGE
 

Britbrat,


Thank you.

I didn't know the Tower was so much stronger than the GMS...


I though they share the design...

RE: TOWER HOBBIES .75 PROP RANGE
 

Externally they look similar, but not identical. Internally, the GMS is ringed & the Tower is ABC. I haven't actually examined the porting comparatively, but whatever the differences are -- the Tower kicks the GMS butt -- very hard.

RE: TOWER HOBBIES .75 PROP RANGE
 

The TH .75 won't run with a 91. Cubic inches is cubic inches.

The 91FX should be turning quite a bit more prop than any .75 engine. The 91 is 20% MORE cubic inches.

Dar:
I don't really know one way or the other if the TH .75 will have any poop at 10,000RPM or not. Maybe the muffler will have a different effect on running at lower RPM, and maybe it won't. There's no baffle in it. Just a long extension in the middle to make the muffler longer.

I've never tried it at those RPM.

Like I said, running a 2-stroke at lower RPM is going to cost you HP--no matter what muffler you have on it. 2-strokes make HP with RPM. Less RPM=less HP. We all know this.

I'd be interested to see what kind of thrust and RPM you'd get with the TH .75 at 10,000RPM. But, I'm not buying a bunch of props to find out. I run mine at 12,500RPM on a 13-5 and they rip the prop at full throttle. I never use full throttle though. These engines are in little 40 size 3D planes, and it's GROSSLY overpowered. Stupid Power. I use full throttle in short bursts when the nose is pointed straight up. All horizontal flight is done under 1/2 throttle. Hovering is accomplished around 1/2 throttle--depending on the weight of the particular plane. Pull out is rediculous. Hammer the throttle and BOOM--it's gone!! Ripping the prop when it's going straight up.

I'm happy enough with my setup--that I'm not going to spend $30 or $40 in props just to see how it flies with a 14-5 or a 14-6 prop--or whatever it takes to make it turn 10,000RPM.

RE: TOWER HOBBIES .75 PROP RANGE
 

Not sure but I think the GMS is a refined OS FSR clone. The Tower is a refined clone of the earlier Super Tigre S engines. Except for the carb which is basically an OS clone.

RE: TOWER HOBBIES .75 PROP RANGE
 

I use one of my Tower 75's with a Top Flite in-cowl, muffler & it still pulls hard.

I haven't tried over-propping it down to 10,000 RPM, but I will try that when spring gets here. I fouled up my garage with exhaust smoke comparing the GMS & Tower with their respective mufflers -- I wasn't popular with the Dragon.

RE: TOWER HOBBIES .75 PROP RANGE
 

The TH .75 has a 26mm bore and a 23mm stroke.

The GMS has a 26.6mm bore and a 22.4mm stroke.

In theory, the TH should have more torque. Smaller bore and longer stroke.

The GMS should be a higher RPM engine. Big bore and small stroke on the GMS. Same as a Ford 302 engine. Big bore--small stroke. You could easily twist one of them up to 9000RPM on the dragstrip. We're talking built race motor here--not stock.

Take a small block 400 Chevy, and it's more of a stroker engine. Can't handle much over 7500RPM. But, you build it to "short shift". Rap it to 6800RPM and grab another gear. The torque will slam you back in the seat again.

The Ford 302 relies on lots of revs to do the same work. Shift your 302 drag motor at 6800RPM and it'll fall on it's face as soon as you let out on the clutch. No torque. Run it up to 8800RPM and grab another gear, and it'll slam you back in the seat too. Just like the 400 smallblock at 6800RPM.

These glow engines are the same. Short stroke--big bore--power comes from RPM. Long stroke--small bore--more low end torque.

I'd like to see side by side comparisons with large and small props.

I'll bet money that the GMS will turn more RPM with the small props. And, when it comes to over-propping, I'll bet the Tower will be the winner.

RE: TOWER HOBBIES .75 PROP RANGE
 

Britbrat,

Those figures you are quoting for the Tower .75 are outrageausly higher than anything I've seen that engine do. Your 12x8 figures are more like the 12x6 figures I've seen. Your engine is within 500 rpm of my Jett .90L and exceeds an OS .91 FX by a handsome margin with the 12x8 APC prop. The Tower .75s I've seen have done mid elevens with the 12x8, mid to high twelves with the 12x6 and mid thirteens with the 11x7 (all APC). In other words, a good margin more power than a good .60 but not a .90 beater.

What is your secret? Is it a standard engine? Is the prop a stock standard APC 12x8? Did you run this test in an Oxygen Cafe (just kidding! ;))

RE: TOWER HOBBIES .75 PROP RANGE
 

Come on, you must be kidding! This would have to make the garage and adjoining rooms smell wonderfull!

RE: TOWER HOBBIES .75 PROP RANGE
 

This has more to do with the available area for valves or ports than anything else. The differance in bore and stroke in GMS and Tower is rather insignificant in any case.

RE: TOWER HOBBIES .75 PROP RANGE
 

With an APC 13 x 8 propeller, we'd expect to see the Tower Hobbies .75 engine to turn in the high-10,000's. Any idle below about 2,600 RPM is considered 'good'.

We've also run them with an APC 12 x 7 prop and seen 12,000 RPM at peak.

We generally use the 13 x 8 propeller when we run the engine on our test stand. It tunes quite well when using that prop with a 10% nitro fuel.

RE: TOWER HOBBIES .75 PROP RANGE
 

Well that's a bummer! [&:] Maybe I have an enthusiastic tach -- it's an old GloBee. [&o] The problem there is that it makes my GMS a spectacular pig [:@]

Both the Tower 75 & GMS 76 are bone stock.

Air temp was -5C

Fuel --- 10% Omega,

Prop --- unmodified APC 12-8

3" Dubro spinner

Garage -- full of smoke [:'(]

Dragon -- pi$$ed [:@]

-- I forgot -- plug OS #8

RE: TOWER HOBBIES .75 PROP RANGE
 

Yes, the ports in the TH and the GMS could be slightly different. And that would account for fuel charge velocity inside the case.

But, I wouldn't dismiss the stroke/bore comparisons as insignificant.

Very minor compression changes make a huge difference. I would think that a .6mm difference in stroke or bore could have a measured effect in these relatively tiny internal combustion engines.

I guess I'm gonna have to buy a GMS .76 and a bunch of props to satisfy my curiousity and all this speculation. Darn the luck, another engine in the fleet.[&:]

RE: TOWER HOBBIES .75 PROP RANGE
 

Do you think some of us--okay ME--are over-revving these engines by running them at 12,500RPM constantly?

I could go to a 14-4 and load it down a bit more.

13-5 is a load of 31
14-4 is a load of 32
13-6 is a load of 32--but too much speed for my purposes. I want low pitch/big diameter and instant spool up.

It's been my experience with this size glow engine that for every increase in load factor--the engine looses about 400-500RPM. So with a load factor of 31 with the 13-5 prop and RPM of 12,500----I could probably get the RPM down to close to +/- 12,000 with a load of 32 on a 14-4 prop.

RE: TOWER HOBBIES .75 PROP RANGE
 

FWIW my tower 75 really likes the APC 12.5-6... I have also tested the 12-6,12-7,13-6, and 14.4 and all do well.. I wish I'd written down tach numbers, but for the sport plane it's in the 12.5-6 flew the best. (12-6 and 13-6 were not bad)

RE: TOWER HOBBIES .75 PROP RANGE
 

The difference between the GMS and Tower is surprising. There are several GMS .76s at our field - I must tach one or two of them when I get a chance. The one GMS .76 I did tach did 10,400 rpm on an APC 13x8.

The irony is, GMS claims 2.5hp to Tower's 2.2 - proving I guess that manufacturers' ratings are indeed not worth quoting.

RE: TOWER HOBBIES .75 PROP RANGE
 

Bore-stroke relationship is insignificant. Fact.

First of all, nearly all model engines are very close to square and besides, with a smaller bore, the valves are smaller even if it's a two-stroke engine.

A smaller force operating with a longer arm is EQUAL torque.
If there is any difference in torque, it will not be in its absolute value, but in the RPM it is maximized.

With a smaller bore, the breathing is more impaired and maximum torque will be achieved, at a slightly lower RPM, but it would likely be slightly lower in absolute value.

The torque paragons are in fact, big-bore engines.

long-stroke = buzz-word.

Nothing else.


The difference between these two engines results from other variables.

RE: TOWER HOBBIES .75 PROP RANGE
 

I'm afraid that this is incorrect. Relatively speaking, Larger Bore with shorter stroke engines are better at higher rpms and make more Horsepower. Smaller Bore with Longer stroke engines make more torque at a lower rpm. Horsepower is what overcomes drag. Torque is what accelerates the prop from a given rpm to a higher rpm.

A Horsepower engine. ie Large Bore short stroke will maintain a higher rpm with a given prop size, pitch and drag coefficient than will a Small bore, long stroke engine. But the Long stroke engine will accelerate to a given speed much quicker with the same prop size, pitch and drag coefficient. So, that is to say that the Smaller bore longer stroke engine will get up to speed quicker but not as high a speed. The Larger Bore shorter stroke will ultimately be faster, but take longer to get up to speed.

This has been proven time after time after time in every form of racing on the planet. Go Carts, 2 and 4 stroke boats, big planes, small planes, off road, R/C Cars, R/C Planes, If it has an engine and it has been raced, the above holds true in every instance.

If you really wish to debate engine design theory, I suggest that we create our own thread or forum. We could spend days discussing the relative merits of Rod Length, to bore ratios, port timing, swept volume, carb venturi size to air flow ratio, venturi shape to air flow characteristics, and every other variable in engine design. These principals apply to some effect in every two and four stroke engine ever created. Then, there is the whole can of worms concerning fuel type, fuel chemistry, flash point, compression ignition timing, cylinder pressures, piston dome shape, upper cylinder and head shape, flame propagation and literally hundreds more variables that control, shape and contribute to the ultimate quest for engine power.

It might be time to return you to the regularly scheduled broadcast and the original topic of this thread:

TOWER HOBBIES .75 PROP RANGE

RE: TOWER HOBBIES .75 PROP RANGE
 

Thanks everyone. I am going to break it in with a 12-6 and then slowly work my way to the 13-8 like Baxter from GP had mentioned. I hope the imformation keeps on comming because this is one hell of an engine for the price.:D:D:D

RE: TOWER HOBBIES .75 PROP RANGE
 

See if you can get tach data from both the Tower & the GMS. I'm in a quandry now. My tach data seems to be way out of sync with you guys. [&o] I don't particularly want to buy a new tach & mine is most likely good for comparitive purposes, but if it is way out, it is useless for discussions like this. It is pretty ancient, but it works well at a safe distance from the prop & it works in low light -- except that maybe it isn't actually "working" at all.

RE: TOWER HOBBIES .75 PROP RANGE
 

Check it against a flourescent light bulb. It should read 3600 rpm if set on 2 blade.

RE: TOWER HOBBIES .75 PROP RANGE
 

Doh -- of course. Except, that now it doesn't read anything at all. I changed the battery & it is still as dead as a rock. I tried adjusting its attitude with a couple of sharp raps -- it didn't even flinch! Crap -- now I do have to buy a new one.

RE: TOWER HOBBIES .75 PROP RANGE
 

Fromeco now makes the TNC tach. (http://www.fromeco.org) I have the old one and think it's great. You deserve the best!

RE: TOWER HOBBIES .75 PROP RANGE
 

Thanks Murray.