I've always subscribed to the school of thought that running the lowest pitch and largest diameter prop that your engine can handle is the best for 3D flight. I have a 65" OMP Yak, YS 91AC, and APC 16x4W. Weighs 7lbs. 4oz. dry. I couldn't be happier with the performance. It has unlimited vertical, hovers at less than 1/2 throttle, and has enough thrust to save it from any attitude. However, some of the better 3ders at my field are telling me to try a wood 15x8 for 3d. Their reasoning is that the higher pitch will pull it out of a hover faster, and the spool-up will be quicker, since the prop is half the weight, and smaller in diameter. I used a Zinger 15x8 on the same engine in an Ultra Stick 60, and though the plane was heavier, it also had unlimited vertical. With my current setup, the engine turns at approx. 9000-9100 rpm. With a smaller diameter and steeper pitch, will it present the same load to the engine? Since the YS will turn 16" APC in it's ideal rpm range, should I try a Zinger 16x6? 16x8? I realize that I will have to be more aware of airspeed with a faster prop. By the way, the guys that are telling me this are running gassers on 80" Aeroworks Profiles. Is their theory limited to gassers, and completely false on glow engines? Just want to get a larger sampling of theories. Thanks.

You have to realize the less pitch there is the more thrust there is, simple as that. Then of course that limits your top end (hence the need for adjustable pitch on full scales) but that isn't what our 3D stuff needs anyway.

If you have a big heavy or uneffecient prop (like Zingers) then it will load the engine down more, and it will obviously not spool up as fast or have as much thrust (bad airfoil). For my large gassers I like the Fuchs props (27x10) because they have narrow blades, are very light, and have a nice effecient airfoil. All the best characteristics for 3D flight.

If you want to try different props, try and keep the ratio of diameter and pitch the same. As in a 19x4 to a 18x5 or thereabouts.

Try to stay in the 4 or 5" pitch range. You have already found that you needed a wide blade to get the performance you want, and you don't have the available torque in your 91 to turn a 19" prop without putting a huge load on your engine. I don't care how you look at it, yout rpm will go waaayy down with an 18 or 19" prop. You will think that you are trying to fly scale.

The Zinger Pro props are ridiculously inefficient, well below the performance of the Zinger standards. The MSC props give a lot of thrust not found in Zingers at low rpm, and have a "predictable" feel as the rpm increases. The blases on an MSC wood props are wide all the way to the hub. Carbon props are really heavy, are hard on the engine if a blade strikes the ground, are expensive, and are don't give as great a return to the smaller engines as they do to larger gassers.

Keeping the prop light reduces the engines "spool up" time. something you want for 3D. I would stay with the wood props, and do some experimenting. You may already have the best you can get for your plane and engine combination, but I don't believe so.

Just my opinions.

I say "Try it". Why? Because most theories get flushed away when put to the test. Also, correct me if I am wrong but the YAK is not a funflyer. Ultra-slow flight is not the only regime for this airplane.

If it does not work well then you have lost about $2.00 and gained knowledge through empirical evidence, if it works well...then it works well.

I know a few people that use 13x6 on a YS63 for 3d and swear by them! My personal preference is low pitch props or 4" pitch

If you couldn't be happier with the performance, don't change anything!

If you do change, I wouldn't go wood with this motor. Less efficient and you don't need to be that light to spool up fast -- but the APC 16x4 is a bit heavy
for fast spoolup. I've used the APC 16x4 and 15x6 on a saito 100 and strongly preferred the 15x6. The 15x6 spins up much faster and I couldn't tell the
difference in thrust on a 7.2 lb airframe.

this is the way i see props:

diameter equals the power, bigger dia it swings the more power

pitch is the speed, like gears in a car. lo pitch, low gear, accelerate quick dont reach very high speed. hi gear, high speed, cant accelerate at any good sort of speed

A few factors to consider when choosing a prop:

1) Max static thrust
2) Spool up response
3) Air Velocity VS Static Thrust for "hovering" RPM

1) For the same peak RPM, usually a lower pitch, larger diameter will yield greater static thrust than a higher pitch prop. This is only true to an extent. Because when you go to too low of a pitch, the tips have next to zero angle of attack, which generage next to zero lift, although RPM is impressive. Also, max. static thrust is a priority only if your power is marginal (1.2 ~ 1.4 thrust-to-weight ratio). If you are already at 2:1 thrust-to-weight ratio, you should use the power in other aspects, like speed.

2) If you learn to fly w/o having to gun throttle constantly, maybe you don't need fast spool up. Also, higher nitro (in 4-strokes) help spool up.

3) This is what your "advanced 3D'ers" may be referring to. Consider a plane at hovering throttle, at which the prop @ XXX rpm is generating the static thrust equal to the weight of the plane. A higher pitch prop will yield a higher velicity air flow in a smaller disk, as compared to a lower pitch prop. This will make a difference in tail control response. So for example, if you massively over-power a plane and use a low-pitch prop (for your case if you use YS 120 for a 7.5 Yak profile, swinging APC 17x6), the plane will hover at just above idle, maybe 3000~3500 rpm. This is enough thrust to counter weight, but I doubt it has enough airflow past the tail for good tail response. I have a 5-lb Funtana w/ Saito 91. When I use 15x4W prop, it hovers @ 1/4 throttle, tail response is simply not there. When I switch to 14x6, it hovers @ more like 1/3 throttle, and tail resonse is a little better. Of course, if my Funtana is 6 lb, or if the rudder is larger, it's a different story. But it proves the point. A higher pitch may not be a bad thing for Fun Fly, especially if you already well surpassed the min. thrust-to-weight ratio.

edible_engine.....What if you run gears that are too low for a powerful engine? You don't get traction....all you do is torch the tires. What I need to do is find the lowest "gear" that will still let me hook up.

barryb-RCU.....check out what it says under my name. No matter what I do, there's always something telling me that it can be a little bit better.

seanychen.....Come to think of it, I don't need the throttle response, what I need is more air moving over the surfaces. Thanks for the explanation of velocity vs. static thrust. It makes perfect sense now.

I had a chance to try the 15x8 today. With the less-than-ideal conditions, it's kind of hard to determine whether or not I like it yet. There was a 10-15mph wind straight down the runway. Coupled with this plane's tendency to float and a steep pitch prop, I didn't think the plane would ever come down. The vertical performance is awesome. The plane is like a rocket, straight up forever. Couldn't tell a real difference in thrust, only that the plane had much more momentum to get through manuevers. One example is that on a fast vertical upline, I could chop the throttle, glide vertical for 20 feet, outside snap, tailslide, then catch it with a hover. The engine was idleing through the entire sequence, that is until the hover, of course. There is no way the 16x4 would give me the vertical airspeed to get through all those manuevers. No, I'm not trying to fly pattern, but I wanted to see what increased airspeed could do in terms of 3D. Didn't have a chance to get low and slow, as my exhaust pipe fell out mid-flight (dang weak YS heads!)

If your wing loading is less than 15 oz per sq ft, you need 5" pitch or less to slow down enough to land. Good rule is to divide this figure by 3 to get your prop pitch. An exception to this is if you can get idle down really low, then you don't need a low pitch prop to land.

16x4 @ 9000 gives a 36mph pitch speed, not enough of a speed range to fly much of anything. 15x8 at the same rpm or higher gives you 70+ which is probably a little too much. Get it propped for around 50 and you get the best of both worlds.

Mike