I am running a Saito 82 with a 15x4w apc prop on my UCD-46 turning 9900 rpm with 30% nitro heli fuel. Thrust calc is showing me a little over 14 lbs. of thrust on a 6 lb. plane (2.3:1 thrust ratio). My question is, how do I choose the best prop for vertical pull out? I watch videos of planes that in theory have less thrust to weight than my plane, but the pull out of a hover twice as fast as my current setup. The only thing I have noticed is that the other setups are running 6 pitch props and not the 4w or 4. For example, the EF yak-54 video from the EF web site shows his plane with a YS1.10 and a 17x6 prop. and a 9 lb. plane. This should be around 18 bls. of thrust on the calculator (2:1 thrust ratio). The yak pulls out vertical much faster than mine, I don't get it. Will the higher pitch help with the pull out, or am I just completely missing something here.

Thanks for any info.

It might be a spool up issue. Try the 14x4W as it will spool up faster and get you to max thrust quicker. Also, I have heard that when using moto calk to figure thrust, 75% of the figure it comes up with is a more realistic figure.

Thanks, I will try the 14x4w and see how it works out. I was just afraid of over reving the engine. It already turns 9900 on the ground with the 15x4w, the 14x4w would go up to around 10,500. I will have to keep that in mind while flying and not run the throttle high.

I was also under the impression that the larger diameter and less pitch would equal more vertical thrust and less diameter and greater pitch gives more speed, is this also true?

Thanks again.

The larger prop with less pitch will slow you down faster and is supposed to spool as fast as a smaller prop with more pitch. The larger prop puts a larger wash over the control surfaces for better control authority in 3D/stalled flight.

9900 on 15x4W produces more like 9~10 lb static thrust.

Also, YS 110 w/ 17x6 produces more like 15 lb static thrust.

I use Pro Zinger 15x4 on my Saito 82. It's on a 4.5 lb profile. It literally pulls out like a rocket, accelerating upwards faster that it free-falls. The spool up is instant too! This prop should be good for UCD 46.

Wood will spool faster than an APC. My YS .63 likes the APC 13x4W but not the APC 14x4W, but the Pro Zinger 14x4 works really well.

You have discovered the difference between static and dynamic thrust. The higher pitch but lower diameter props give lower static thrust, but higher dynamic thrust.

Dynamic thrust is the thrust generated when the prop is already moving though the air. As the plane accelerates, the dynamic thrust decreases, but decreases less on higher pitch props. Think about it - a prop with a pitch of 8 will try to fly the plane twice as fast as a prop with a pitch of 4 for a given rpm. Therefore at the max speed of the 4 inch pitch prop, the 8 inch pitch prop is still trying to accelerate, so is generating thrust.

So in the hover, as your plane accelerates upwards, the dynamic thrust drops very quickly with a 4 pitch prop, so quickly stops accelerating.

Try different props, I would go with a 14x6.

I've always used APC props, mostly the 4W series on my planes. This Pro Zinger sounds interesting. Where do you buy them? Are they wood? Wood props can be very fragile and sometimes, and I'm sure I'm the only person in the world, I have a little "squish" on the landing gear when I land and OPPS there goes another prop. But I would like to try a Pro Zinger. If seanychen says they're good. They're good. He really makes those planes sing. I've seen his video of a UCD60. Very impressive.
Thanks,
Barry

Really? Wow, I guess that program I was using isn't very accurate. (http://www.lcrcc.net/thrust_calc.htm) Are there any other programs that I could use for comparison purposes?

Thanks for all the input guys.

Question: How are you calculating the thrust? That would be very interesting to me. Please give me an equation or website or?
Thanks,
Barry

Thrust calc assumes perfect conditions, and estimates efficiencies of props. Real world is different. EG big cowls can reduce thrust substantially. Only way to really measure it is to hook it up to some fish scales.

Best thing is to try different props and see which you like best.

You know Bam, Sean is right, just try dropping to a 15x4 instead of the 15x4W. It'll make a world of difference in the spool up.

Sprink is 100% correct in his assessment of pitch vs. thrust. One of the main reasons I use 4 pitch props on my profiles is pull vs. speed. Large control surfaces + speed = a fluttering death!

Ken

Sean,
You quote some thust values. Where did you get them? measured? estimated? calculated? how?
I have used
http://www.lcrcc.net/thrust_calc.htm
and what little measured data I have found says the link calcs are way high. I would like to see more measured data so I can calibrate the calcs. Did you find the link shows thrust is independent of pitch for a given dia and rpm? That doesn't seem right to me.
I will try to rig up a thrust measuring setup and gather some info at my club.
I would like to hear if anyone has a better thrust calc method.

I got a chance to fly today, used the 15x4w first flight and then switched to an APC 14x6. The pull out of a hover was much quicker, but the lack of prop wash on the control surfaces made it more difficult to hold the hover. It took a lot more surface throw to accomplish the same amount of response in the air. I did not get a chane to try the 14x4w, I think I will try the 15x4 (not w) also to see if I get the best of both worlds. I know I want the pull out of the 14x6 with the wash of the 15x4w, finding that will be difficult.

Try the Pro Zinger 15x4 I think that will do it for you