Propeller

[Almandine]

Does anyone know a reliable place to by an Ikara Propeller - 8 7/8"? Or anything of similar quality?

[retired1]

Freedom flight or Ray Harlan would be my choices.

[jander14indoor]

An alternative if you want more control over prop design is the prop kit available at www.RetroRC.us.com

Jeff Anderson
Livonia, MI

[Froggie]

Some of the kits come with propellers I think.

[Crtomir]

Anyone know where to buy flared props, or is that something only a "master" can build? Yes, you can cut some ;arger Ikara props down to SO length restrictions, but the weight is still to high to be effective.

[coachchuckaahs]

The Ikara is the only commercial flaring prop of more or less suitable size available. While it may be heavier than some other options, it is plenty effective and used to great effect by many SO teams. Even with Ikara, we are adding 2+g of ballast to get up to weight.

Building your own may give you access to more variables, such as the stiffness of flaring. You may want to Google for a spreadsheet for cutting props from plastic cups ("Bucket Props"). That spreadsheet specifically indicates it may not be suitable for flaring props, but we have used it. The hub can be 1/16" square basswood and 0.020" carbon rods. The number of rods and unsupported length (length between the basswood and the blade material) impact the flaring stiffness. Too stiff and she'll climb too much early. Too soft and she'll over-flare, killing your motive force.

You will need to make a fixture to set the angle of the blades when installing. We used a modified FFM pitch gage. Variables will include flaring stiffness, initial pitch angle, blade chord, blade length, blade planform shape, camber of the blade (set by cup diameter), twist of the blade (set by angle when cutting from the cup). Be sure to keep good notes and measurements! It opens a whole new world of variables, but if you like scientific method, this should be fun! Schedule plenty of gym time!

With a suitable fixture and control of the variables, it is not hard to build props. My kids built 3 in less than an hour last week to try out some new ideas in the gym, so, no, its not terribly hard once set up. None panned out, but they opened thoughts to what to try next!

You'll also need prop wire (0.020 music wire), bearings (teflon tubing, or beads from Ikara), prop hangers (Ikara or Ray Harlan).

Coach Chuck

[CMS AC]

Does anyone know what kind of propeller would work best? I want to starting making my own propellers/buy kits but I'm not sure how to start.

[coachchuckaahs]

The real heart of this event is matching the propeller to the rubber. This may take significant experimentation. The small diameter means it is challenging to control the power at all phases of the flight. In addition, you may find that you need a different prop for different venues. A high ceiling may accommodate a fixed prop, while a low ceiling may do better with a flaring prop. there are many variables in building a flaring prop.

Please see my prior post for a description of how we build our props. We have built 27 so far for this year's diameter limitation. Our Region, tomorrow, has a nice 30' ceiling, while State in 3 weeks as a 19' ceiling with obstructions.

If you are not wanting that level of pursuit, then I suggest buying some of the Ikara, both the small fixed pitch and the flaring, cut them down to diameter, and try them out. This limits the variables, as there is no adjustment for flare stiffness. You can adjust pitch (use a pitch gauge, see FFM).

Our home-built props are indeed lighter than Ikara, but we are able to accommodate the Ikara and still need to ballast up to minimum weight. This adds weight at the nose, which with the small tail this year is a good thing.

Coach Chuck

[Crtomir]

We have been experimenting with different thickness rubber with the standard 15cm Ikara props cut down to 14cm. We did indeed find an optimum thickness, but that wasn't enough to give us the flight times some other teams are getting with their flared props. So now, we are making our own flared props. The first method is to use a plastic cup to cut the blades from and attach them to the plastic hub/spar assembly of previously broken Ikara props. That definitely slowed down our planes. Haven't done enough testing to see what the max times will be, but it looks promising. Now we are moving on to the build-your-own-prop kits from Freedom Flight Models. That's a very good kit. It takes maybe an hour to build one prop, but you can cut your own prop blade profile which is really nice. You have to be careful to cut both blade profiles exactly the same and then balance the prop after assembly. Finally, you have to make sure the prop pitch angle is the same for both blades. We are not sure the optimal pitch angle with the flared props. That's something else to experiment with in addition to rubber width. I'll try to post the results of flight testing with them. Unfortunately, we won't be able to fly again till April 1. We are trying to approach this problem step-by-step, doing one experiment at a time in order to find out which variables affect flight time the most. I keep telling out kids, if other teams can get those 3+ minute flight times, you can too!

[coachchuckaahs]

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