Thunder tiger 46

"George Thomas " wrote in news:Ub6%c.753$ snipped-for-privacy@fe61.usenetserver.com:

Based on my 2 .46's (one Magnum Pro, which is now TT, the other an OS FX), I'd say you're overpropping it a bit with that 11x7.

The "more air speed and less RPMs" doesn't quite work that way. If you want to go fast, you generally want a relatively small-diameter prop with high pitch and big RPMs. For static thrust, big low-pitched props win. If you want to hover your plane, go with an 11x4 or 12.25x3.75 (they need a shorter name for that prop).

The brand makes a difference, too, especially if it's APC.

I disagree that the 11x7 is overpropping the TT as the TT 46 is a good running 46 and I believe the TT tends to produce more RPMS than the others.

Depends on your plane. A plane with a lot of drag, like a biplane or Cub, will not fly well on a small diameter/high pitch prop. However, a sleek, sport-type plane will fly well on this prop.

GIVEN THE SAME RPM: Small diameter/high pitch gives more speed (high gear in a car). Large diameter/ low pitch gives more pull (low gear in a car).

Too much pitch or diameter can overload your engine. It won't make the RPM to generate either thrust or speed.

Dr.1 Driver "There's a Hun in the sun!"

Thunder Tiger know their engines better than supposed "winnable known as an expert" flyers who advise without knowledge of your plane and the drag factors etc. TT operating instructions recommend: Pattern Aerobatic 10 X 6.5, 10.5 X 6 and 11 X 5. Sport Scale 11 X 6 & 10.5 X 6. These have all been well tested with several TT Pro.46 and except for a 3D model which has a 12.25 X 3.75 [and one must be very careful not to over rev with this prop], I believe the recommendations should be adhered to. Buy one of each to suit your type of model and try it. An 11 X 7 is a Pro.61 and .61 FX standard size and will lead to short engine life and in all probability rear bearing failure on any brand of .46 especially if less than 5% castor content in the oil. There is a wealth of information to be gained under engines, props and fuel on my web page. regards Alan T. Alan's Hobby, Model & RC Web Links

I have this engine on a Worlds Model T-34. For speed I use a 10x8. I like this prop because it lets me generate enough RPMs to be in the power zone yet will not overspeed in a shallow dive (WOT in shallow dive). For unlimited climbs and short take offs I use a 11x6. This prop generates more static thrust. The 12.25x3.75 take the concept a step ferther. The best way to determine which is the best prop for you is to first understand why and how different props affect performance and then go out and buy a bunch and try them out. Also get this program

It will help you better understand prop selection. Like someone said earlier props are like gears on a car. If you have a 3D plane you may want to fly in 1st gear. If you have a pattern plane you may want to fly in 5th gear. Something in between and you may want 3rd or you may mke your prop selection on the type of field you're flying at. No matter which prop you select you will be making a compromise. Remember you can only fly in one gear at a time (alas a variable pich prop would be nice) but you can always land and "shift gears" if needed. If you want to experiment get this muffler It will allow the engine to generate more power than the stock muffler.

Try an APC 12.5 x 3.75 It is awesome on that engine!

oops, that should be 12.25 x 3.75!

For speed I alway's used a 10-6 or 7 on the WM T-34. This engine runs real well at 14K+. My favorite sport prop was the 11-4 or 5.

For speed I alway's used a 10-6 or 7 on the WM T-34. This engine runs real well at 14K+. My favorite sport prop was the 11-4 or 5.

An 11X7 is on the high end of the range for just about any sport .46. The TT does not turn props any faster than the OS FX and is a good mid pack performer. For most sport applications where all out vertical isn't important, I would use an 11X6. Keeps the RPMs down a bit and also the noise.

I'm new to this, and trying to sort things out. Barry's program shows a simple linear relationship between RPM and speed. The thrust curve is a simple parabolic. (He doesn't graph these values; I printed them to a file and imported into Excel to plot a graph.)

Can that be right? Is it as simple as that? Without crunching any numbers, I expect speed/RPM can be linear only if we treat the propeller as an air screw. One turn moves it forward some fixed fraction of its pitch distance.

Also without crunching numbers, I can only guess where the thrust figure comes from. I suspect he's looking at the mass of air a simple airscrew would move at the given RPM, and expressing that as thrust.

Help me out here. I'm new to fixed wing, but not new to flight or aerodynamics. My background is rotary wing; I recognize the airscrew model as a simplified version of a helis' actuator disk, the simplest of the simple, without the niceties of spillage and vortex loss.

Starting with the basics, it doesn't seem reasonable to expect the propeller to act as an air screw. I would think you would need the Cd, wetted surface area, and model weight to reasonably predict how the plane will fly. So, how would I use these two numbers, one representing (apparently) the no load speed, and one representing (apparently) static thrust? Are they enough for the knowledgable to select a good prop for a particular airframe?

No stupid questions, but if you are really keen to lean, buy a book on aerodynamics so that you can properly come to understand the inter-relationships which exist between engine characteristics, propeller choice and airframe type. Undoubtedly still one of the best and yet an easy read for the layman is Air Vice Marshall AC Kermode's "Mechanics of Flight". Been in (re)-print for many, many years.

A concise no brainer recommendation without the why?

Sport_Pilot's sizing suggestions for your engine are spot on, and as close to no brainer perfect as you're gonna' get from the "Dummies Guide to Propeller Selection" for the TT46Pro.

You might also *pay close attention* to the erudite A.T.'s comments.

Optimum propeller choice for any (& that) engine is dependent upon the airframe and intended flight envelope. Noise restrictions notwithstanding, you want to choose a propeller diameter or pitch combo (you can't have both) which will keep that engine turning within its optimum power curve.

A problem with marketing vs sport flyer need or reality, is that engines today are marketed as "most powerful" to outsell the opposition, but which is diametrically opposed to most R/C sport flyer mid-range power and noise restriction requirements. To achieve anything near these manufacturer hp claims, you have to operate these hotly timed Schnuerled two strokes at the top end of their RPM range which means high RPM and more noise. If you don't, then most of the time, you're only achieving about 65% of their potential or claimed power.

So to come back to your question, your best propeller choice is entirely dependent upon what you want to do, and into what model (type) you installed your TT, and how much potential excess power you have available and are willing to sacrifice in that compromise for lowering noise.

Further, an 11x7 is a big ask for the TT46Pro IME&O. It certainly won't operate anywhere near in the band, and will be a shitty performer indeed unless married within a quite narrow choice of airframe. If you have an aerodynamically clean model with a moderate to high wing loading and want speed as well as great vertical, then go with an aerodynamically efficient 10x7 (APC). If it's vertical you want but at a slower pace and have more wing area and lower loading, choose an APC 11x5 or Bolly 11x5.5. For pure 3D with big surfaces, slow speed flight, you want more diameter again and even less pitch.

The key to all contemporary Schnuerled 46's is that they are virtually all highly timed and oversquare, designed to develop their claimed potential hp outputs over 15k. If you aren't at 14k or >, then you're losing heaps of hp as that hp curve isn't anywhere near linear.

Even more importantly, power be it potential or developed is only proportional to thrust, and depends upon propeller efficiency for the constant (x). Choose intelligently and wisely, because with the limitations of a fixed pitch, you can't have it all over a wide performance envelope.

So, if you want more speed, you need to choose the maximum pitch and minimum diameter prop which will still offer you efficient *thrust* with your particular airframe. With such a fixed pitch combo, acceleration will of course usually be quite poor unless you have a very light *and* low drag model. If it's a large draggy airframe, form drag will be so high at speed, propping for speed becomes a pointless exercise. And of course, propping for speed unavoidably means more NOISE, both from engine and propeller RPM.

There is an excellent article in the October RCM that addresses just this type of question. Also the September MA has a similar article as well. However, both are really saying exactly what has been pointed out by the kind folks here in the NG.

Jim W

All these prograns will do is get you close. You cannot rely on them for absolutes. The key here is to experiment with different combinations to find what works best for YOUR engine and airframe.

Propellers suffer vortex losses, and short props lose proportionately more thrust to vortices, which is why they're poor in takeoff and climb.

Dan

They also lose to MUCH less air being moved. The area changes by the square. A prop with twice the diameter of another has about 4 times the area of air moved. Dr.1 Driver "There's a Hun in the sun!"

That also takes more horsepower. The larger prop gains efficiency through a smaller tip loss percentage, and also (as I should have mentioned) through a lower drag. It's more efficient to accelerate a larger column of air to a low speed than a small column to a high speed, if the net thrusts are the same. The faster, smaller prop loses much more to drag, as drag also increases by the square of the increase in speed.

Dan

have tried them all and find that these people make a 10.5 x 8 that is perfect for this engine. So much so that I have sold over 8 people at the flying field on this including our biggest die hard. They are also a cheap prop.