My Tower 75's on sticks. One of our club members saw my sticks tear up the air and decided to replace his 61 thunder tiger on his 4*60 His comment was WOOOOW He says his backup 4*60 with a OS91 will get the TOWER 75 this spring.

I'm using this engine on my first plane, GP Big Stik .40 it has a lot of power but not reliable, my problem is the engine keeps dying on me. Not very confident with it, I'm sure it's just my needle setting. I like doing low fly bys and usually the engine dies when transitioning from idle to 1/2 throttle. Luckily I'm right above the runway when it dies and I can save it. But my last dead stick, the wind was blowing hard had to turn the plane around didn't reach the runway and it went to the trees. Just a little tear on the wing. On the bench there's a good transition and everything seems fine. I'm using a 12 x 6 APC, tach it around 11,800 RPM WOT and idles around 2,600. Vertical is insane. I stop using the Tower plugs and switched to OS 8. (got free plugs with this engine). Since then I had about 3 flights now with no dead stick. Keeping my fingers crossed. I almost gave up on this engine and was going to put a 4S in. But I'll give it another chance....

I run my Tower .75 for every RPM and ounce of power it will produce. I run
it on 15% PowerMaster with a dash of extra castor, and a K&B 1L plug.
The best speed I can get out of it is with a MAS 11-7 1/2, at 14,000 RPM's.

FBD.

Me too on my pattern plane. On my Top Flite Spit I usea 13-6 -- that works too.

Well it finaly got into the 50's yesterday, and I broke in the .75. I ran four 14 oz tanks of omega 10% w/2 oz of caster added through it on a APC 12-6. Then I started to prop it. I first ran it up with a 12-7Apc and it peaked at 12,100and I packed it off to 11,800. Then I ran it up with a APC 13-6 and it peaked at 12,000 so i backed it off 11,700. The engine idles fantastic wiht a OS #8 and bison pitts muffler. The transition might be a just a little on the rich side, absoulutly no flame outs while running it.

I`ve got the 75 on a hawk fun fly plane turning a 11x7.75 prop with 15% fuel 14.200-14.500 k and its awesome. I have tired several props and if you want instint response and turn like a banchee I`m very impressed and there is several others that have the same setup in the Texas panhandle.

Tim,


Your engine is under-propped!

In the 05/03 MAN magazine 'shootout', Dave Gierke found the Tower Hobbies .61 peaked with 1.59 actual HP @ 14,150 RPM.

Your .75 is breathing through the same passages as the .61 with similar timing numbers, and as such, peaks lower in the RPM band. It peaks around 13,500 (don't read what Tower claims...), with slightly greater HP numbers than the .61; perhaps 1.80.

Your numbers (on an MA prop) suggest this is right, but the RPM is too high.

You are running your engine over peak power RPM and as your plane speeds up and the prop unloads, the higher RPM would bring it further off peak; off the meatiest part of the HP curve.

The engine will, of-course, sustain no damage, but your model will fly much better, even faster, if you load it down into the fat part of the band (~12,500 RPM), with a larger, higher-pitch prop.

I have been flying it with all different props this summer, and come to the conclusion that the PRO Zinger 12-8 provides the best performance. I have tried like 15 different props from TF, APC, MAS and Zinger. The performace between the APC 12-7 and the Pro zinger 12-8 seem to be real close, with the APC holding a little advantage in the climb performance. But it seams like the straight line performance with Pro 12-8 seams to be better.

David,


Your observations are correct!

The APC is a more efficient prop, so even at the same RPM it would produce a higher thrust level, in a climb.

The Pro Zinger has more pitch, so a higher level speed results.

I have thought about clipping a APC12-8 down to 11-1/2-8 and trying that, as the APC 12-8 does not fly it anywhere near as good as the Pro Zinger 12-8 or APC 12-7.

David,


That would be counterproductive.
The APC tip design makes up a good part of its efficiency.

Maybe if you accurately 'remodel' the tip, you could get away with it.

The APC props have washout in their tips which means that the tips don't have as much pitch even though they are out futher from the center. Some speed flyers clip the tips of a larger diameter prop to gaine more speed which seems to work in some cases. I have clipped several props along the way and did seem to gaine some benefit from it which seems to be productive. It only cost a few dollars to give it a try.

Zinger props have an airfoil shape at the tips that produces thrust, and
pulls the plane forward. The tips of the prop are moving at the highest
speed. The APC props have little or no thrust at the tips. They are designed
this way to be a "quiet" prop.

A "quiet" prop is great for quiet, but not for speed. This is why the Zinger
whoops the APC for speed.

FBD.

Look at this picture again....look at the shadow under the leading edge
of this prop comparison. See how the APC has an exaggerated angle
from the hub....out to to aprox 2/3 of the blade ? This is where they
produce their thrust....this is also where the exaggerated angle robs
the engine or RPM's.

Now look at the silly tips on the APC. It is made this way to reduce noise
certainly not to increase performance. Good speed props, like the
Zinger's not only give more RPM due to the design, but give more thrust
and speed to the plane because the tips are turning fast and producing
thrust.

Why people think an APC prop is the best....I have no clue. An Internet myth
I suspect.

Dave,


I do not argue about the actual speed result achievements of the straight-tip Zinger, like the one in your photo.

But I do suggest you take a close look at the profile at the tip of the blade...

I attached a photo I just took of the blade tip of such a 12x9 Zinger.


The top is convex and the bottom is flat, but the leading edge is as round as the trailing edge and the thickest part of the blade is at about the 45% station. Not really resembling a Clark-Y...

I would say it is not a very good airfoil shape...

It doesn't have to be a perfect Clark-Y....the convex side opposed to the flat
side will offer the desired effect in any case. If you look at the pic I posted
again you will see that it is clearly airfoil shaped....

....designed for maximum speed/thrust at the tips.

Look at this Master Airscrew tip....same story, airfoiled shaped....more speed.

FBD. [8D]