Scioly.org

Let me add a counter view to those recommending rubber strippers.
Not that they are totally wrong, if you really get into this sport/hobby (a whole other debate!), you will want a rubber stripper. I certainly have one.

My counter view is that a stripper is NOT necessary to a beginner nor an intermediate flyer to achieve a reasonable level of success. Reasonable level, having a plane that will fly 80% plus of nationals winning time, which will be competitive in most regionals and some state tournaments.

Keep in mind, the key to success in this event remains matching prop and rubber. As Coach Chuck already mentioned, you can vary EITHER to match the other to near optimal performance, so long as the rubber is in a reasonable range. Prop may be harder to vary than changing rubber size, but it can be done, and is far cheaper.

Two things I'd consider FAR more important than a stripper (especially for beginner to intermediate teams).
- First and foremost, an adequate amount (Enough for at least say 20 motors, preferably 100. Checking www.faimodelsupply.com, 1 lb of rubber is only $32. That should make easily 100 motors.) of approximately correct rubber size to allow LOTS of testing. Its painful to watch teams come to tournaments with an adequate plane, but one or two broken down motors to compete with!
- Second, a torque meter (its cheap to make your own, many designs on the internet) to know what torque you are winding that motor to. This is a key tool to ensuring consistent winding, consistent power storage, and data that can be correctly interpreted.

My point is, you do NOT need to spend a fortune on a rubber stripper to get started or to have success with this event.

Slightly separate topic, testing props. You don't have to move the prop/motor combination. You can move the air instead, wind tunnel anyone? And you don't need a lot of speed, keep in mind, these things fly relatively slow. Look around on the net for old wind tunnel designs, instrumentation to measure drag. We're all used to fancy electronics, but this stuff was mostly worked out 100+ years ago by a couple of bike mechanics. Stuff that would be easy to reproduce today. Thrust is just negative drag after all...

Jeff Anderson
Livonia, MI

We're gonna stick with 1/16" rubber and tweak the motor length and props, no stripper.

The school gym is empty on Sundays and there's usually something going on elsewhere in the building so that solves our gym access problem. We flew the plane pictured on the previous page all day yesterday and we were able to do 1.5 laps either way using leftover 3/32" rubber and the four bladed prop, spectacular climbs to the rafters in 1/2 lap but the power was overcoming our ability to nicely trim the plane, with climbs banking akin to a normal plane, and the motor spent after 15 seconds (no knots remaining).

We have 1/16" rubber now that we can try next Sunday, and we'll use two bladed props to start. In order to facilitate prop changes, I made a new bearing assembly that uses a tiny threaded rod from a micro RC helicopter that the prop threads onto:

The hubs are bits of plastic and carbon fiber from our drawers full of old RC plane stuff, and the blades are SOHO cups, yogurt pots, anything with a curve we like. The blades are reinforced with thin fiberglass cloth. A complete prop weighs 1.2 grams.

We also built a third prototype, 8.1 grams:

In order to reduce banking, we raised the upper wing and added upper fences (we don't want the complexity of dihedral). The tail moment is now 4 times the wing chord, so hopefully the stab trim will hold over a wide power range. The rudder is forward to reduce its effectiveness and ease balancing. Preliminary flights in the house are encouraging with a 1/16" x 17" motor (1.5 hook distance) at 1800 turns (85%), nice gentle climb at full power, slow enough to intercept and catch, gentle descent at 45 seconds.

If this plane flies well, the quickest way I can think of to increase flight times without going past 85% on the motors will be to see if 2x hook distance will fly, to try more winds. If anyone is willing to let me know any other tricks, I'm all ears. I promise we won't win, lol.

One thing I notice on your prop is the blades are behind the spar. If you put them ahead of the spar, you will get some flaring, which will help control climb (though the effect may be limited this year). Also, we have used "beer cups" for props with success, and did not need any reinforcement, even at a larger size.

The changes to your plane are all in Positive direction. Good job so far. Keep observing, testing, and improving.Keep a good log book.

Coach Chuck

Hello everyone!

I'm excited for a whole new year of flying and testing. The rules seem hard but I'm sure my team will have a lot of fun figuring things out. Since there has been a lot of discussion on propeller design and I couldn't find any 8cm propellers, can someone point me in the direction of/walk me through the process of making these propellers? What materials do I need/where I can get them? What are some principles of prop-design (you know, the physics stuff)?

Also please explain flaring, been trying to figure that out since last year but still don't have a good grasp on it (or how to make it).

Thanks for all the help last year, everyone! Hopefully, I can contribute a bit this year as well.

Xiangyu

xiangyu wrote: ↑September 16th, 2019, 5:57 pm Hello everyone!

I'm excited for a whole new year of flying and testing. The rules seem hard but I'm sure my team will have a lot of fun figuring things out. Since there has been a lot of discussion on propeller design and I couldn't find any 8cm propellers, can someone point me in the direction of/walk me through the process of making these propellers? What materials do I need/where I can get them? What are some principles of prop-design (you know, the physics stuff)?

Also please explain flaring, been trying to figure that out since last year but still don't have a good grasp on it (or how to make it).

Thanks for all the help last year, everyone! Hopefully, I can contribute a bit this year as well.

Xiangyu

Xiangyu:

There are a large number of posts in prior years, especially last year, on props, and flaring. Please read those.

For an ideal prop, you would want a helical pitch distribution (or nearly so with some modifications, perhaps). However, this year the blade will be so short, I don't believe that helical is as critical as with a longer prop, especially if you have a wide chord. We have successfully simulated a helical pitch distribution in the past using the "bucket prop" approach, cutting the blade form a Solo "Beer Cup" or other recycled container. The Bucket Prop Spreadsheet will help you determine the correct angle to cut the cup to get close to the right pitch distribution. It is made by Fred Rash, and is available on the Hip Pocket forum Builders Plan area for download.

You will need a method to glue the blades to a spar at the correct angle. We modified a FFM pitch gage to do that. It will take some practice. Again, older posts on HipPocket show some options for such a fixture. Your hub can be part of your spar, or a short piece of Basswood. You then need to bend your own prop shaft from music wire, and be sure to get it square to your hub.Finally, you will need a prop hanger, such as the Ikara (available form FFM and others) or the Ray Harlan pigtail hanger. He makes one specifically for SO. I believe Josh Finn makes one as well.

A flaring prop is a poor-man's (and rule-abiding) variable pitch prop. Basically, with more of the blade ahead of the spar, at higher loads (start of flight), the blades flex and increase in pitch, slowing down the motor and reducing the climb rate. The flex may be in the blades or the spar (or both), but cannot include an active mechanism such as used on F1D. Adding flaring may give you better control of the rubber/prop optimization. However, it also gives you multiple new variables, including flexural stiffness, percent flaring (percent of blade ahead of spar), and initial (static) pitch. Making your own prop also opens up the possibilities of setting your pitch, blade shape, pitch progression, and other aspects. You will need to build a number of props (we typically will test at least 25 in a year, some years as many as 50). Building your own prop can be rewarding, and is an excellent skill to learn.

With this year's very small prop, it is not clear yet how much help home-made props will be. You can start with the 6" Ikara prop, and cut it down to fit diameter. I believe some kits are using this approach. Be sure to cut the DIAMETER, and not the LENGTH. The prop should fit through a 8cm diameter circle template. This is likely a better stating point than cutting down a molded plastic SIG or Flite Streak prop.A big advantage of home-made props is weight, but with the small size this year I don;t think that will be a substantial concern.

As always, whatever approach you take, be sure to figure out what your variables are, and record them for every flight. This is the Science part! As you change ONE VARIABLE at a time, you will be able to look for trends in your data and improve your performance.

Coach Chuck

coachchuckaahs wrote: ↑September 16th, 2019, 4:49 pm One thing I notice on your prop is the blades are behind the spar. If you put them ahead of the spar, you will get some flaring, which will help control climb (though the effect may be limited this year). Also, we have used "beer cups" for props with success, and did not need any reinforcement, even at a larger size.

The prop on the first plane had the blades in front but with the high rpms, any impact contact broke them free. I decided to put the blades behind the spar and reinforced the glue joint with fiberglass cloth so we could fly more and not worry about repairs. Hopefully we can revisit the four blade flaring prop as we increase the plane's stability and master height control.

coachchuckaahs wrote: ↑September 16th, 2019, 6:36 pm
With this year's very small prop, it is not clear yet how much help home-made props will be. You can start with the 6" Ikara prop, and cut it down to fit diameter. I believe some kits are using this approach. Be sure to cut the DIAMETER, and not the LENGTH. The prop should fit through a 8cm diameter circle template. This is likely a better stating point than cutting down a molded plastic SIG or Flite Streak prop.A big advantage of home-made props is weight, but with the small size this year I don;t think that will be a substantial concern.

Thanks for your help! Do you have any suggestions on where to start with cutting the propellers? Like is there a specific shape I should cut to or just do a straight cut etc? Also how to I make sure bother propeller blades are cut the same?

Xiangyu

does anyone have any tips for determining the airfoil of the ribs or the length of the motor stick/tb?

xiangyu wrote: ↑September 17th, 2019, 4:40 am

Thanks for your help! Do you have any suggestions on where to start with cutting the propellers? Like is there a specific shape I should cut to or just do a straight cut etc? Also how to I make sure bother propeller blades are cut the same?

Xiangyu

There are resources online on prop shape. However, with this year's very short prop, I am not sure it means a lot. Getting blade area without compromising efficiency will be hard, and I don't think there is good guidance on this. However, if it makes you feel better, top F1D flyers have told my kids not to spend too much time on prop shape, as it has not made a huge difference. Pitch, pitch progression, and flare (if any) will matter. The pitch progression on a bucket prop is determined by the angle the template is placed on the cylinder or cone, see spreadsheet. I suspect the blades will be rather square. Chord of the blade will have to be determined by experiment.

Make a simple template (we used index cards) and draw your blade on the cup. Cut with scissors. Lay two blades on top of each other to ensure they are identical. you can sand the edges to true them up.

Coach Chuck

klastyioer wrote: ↑September 17th, 2019, 8:47 am does anyone have any tips for determining the airfoil of the ribs or the length of the motor stick/tb?

For airfoil, most just use circular. One could try Simplex, but that has not shown to be significant on SO planes. I would probably look at 6% for the wings.

MS length will depend on rubber length, and flexibility (or stiffness). TB length to get stability, but also limited by CG concerns. Get teh length so that you can balance without going over 8g.

Coach Chuck