Propeller

waffletree
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Re: Propeller

Post by waffletree » April 19th, 2018, 5:27 pm

Almandine wrote:Does anyone know a reliable place to by an Ikara Propeller - 8 7/8"? Or anything of similar quality?
the ikara website would probably work. it's ikara.eu
builder cult vp // #treegang

Crtomir
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Re: Propeller

Post by Crtomir » April 20th, 2018, 4:47 am

coachchuckaahs wrote:Crtomir:

How has your testing gone on home-brew flaring props?

We have continued to test and build. Variables include the initial pitch, the softness of flex, the width of the blade, shape of the blade (planform) and the extent of washout (imposed by the angle of cut on a conical source like a cup). Careful data taking is critical.

Interestingly, we have our two best two props that are opposite ends of the spectrum that perform about equally well. One is 20-degree pitch, narrow blade without washout, and the other is 35-degree pitch with a wider blade. And, of course, these optimize with far different rubber widths!

Fortunately, our plane has been stable, durable, and repeatable, and so we have been able to focus on props and rubber. We do virtually all of our flights with half rubber so that we can explore prop adjustments without risking tangling in the rafters. We record all prop parameters, and then the altitude for each lap. Early altitude is determined by prop flex and launch torque. Mid flight altitude gain has more to do with rubber width, and flare stiffness. Letdown speed has more to do with initial pitch, maybe some on flex, and rubber width. Having many items in the data chart, with many flights (probably about 400 so far), starts to provide a picture as to what to adjust and change, and what new prop ideas to build. Piecing all of this together to maximize the performance in each phase of the flight will be the differentiating step to do well at Nationals. I think we passed 30 props this week. At least they are cheap when home made, probably less than $1 in each prop when cut from cups and using home-build hubs.

The kids are loving seeing the data, seeing what to try next, and then seeing it pay off in an improved flight portion.

Coach Chuck
Flared props seem to be the way to go. What we are not sure yet is what pitch angle to use. We still need some experimentation on that. We are just now starting to use half motors. The best flight times on half motors have been 1:12 - 1:24, so presumably we could get twice that with full motors. However, we haven't really optimized the pitch angle. As you said, changing the pitch will change the optimal rubber thickness.

I have a question though: Is it better to go with a higher pitch and thicker rubber or lower pitch and thinner rubber?

Thanks!

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Re: Propeller

Post by drcubbin » April 20th, 2018, 5:19 am

When you use "half-rubber/motor" are you substituting half of the motor with a 1/4 or 1/2 motor spacer, or are you just cutting down on the mass of the rubber with 7.5 mass (which I am guessing will not give the same desired results)?

coachchuckaahs
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Re: Propeller

Post by coachchuckaahs » April 20th, 2018, 6:05 am

Indeed the half motor is 0.75g, and is supplemented with a stick that weighs 0.75g and is half the hook-to-hook length.

It is important also that the altitude be limited to half the final gym height, or a full motor will put you in the rafters.

As far as pitch and rubber thickness, it will depend on the stiffness of your flare, the shape of your blade, etc. As I noted, we have one thin prop low pitch which uses thinner rubber, and one that is wider, higher pitch, and uses heavier rubber. Both give similar results. So, unfortunately, the answer is "depends". I have seen in the past people claimed that higher pitch, thicker rubber would be more efficient, and that seemed to be the case with larger props. On these small props I have not seen that pan out as much.

Chuck
Coach, Albuquerque Area Home Schoolers Flying Events
Nationals Results:
2016 C WS 8th place
2018 B WS 2nd place
2018 C Heli Champion
2019 B ELG 3rd place
2019 C WS Champion
AMA Results: 3 AAHS members qualify for US Jr Team in F1D, 4 new youth senior records

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