Helicopters B

[chalker7]

As usual, I agree with basically everything Jeff says above. In particular, his advice regarding gauges is spot-on and is very essentially identical to how I run events. One point that new people often get hung up on, especially when moving from AMA/FAI or previous SO aviation events into helicopters, is the round circle gauge. In regards to that, I would note that the rules explicitly cite a "diameter" for the dimensional limit on helicopters while a "span" is often the dimension cited in other competitions. A diameter is always a circular measurement while a span is almost always a linear one, hence the round hole for helicopters.

From a "running the event" perspective, if you have any say in the matter, racquetball courts are definitely my favorite place to run Helicopters. Racquetball courts often are not available for a variety of reasons (sometimes as simple as they don't exist,) but when they are, they offer a great mix of easy access for spectators, low risk for competitors and clear visibility for officials. If you have any choice and can advocate to the tournament director, the primary consideration for running a helicopter event should be how few obstructions exist on the ceiling. The fewer the obstructions, the less likely any single helicopter will be hang on the ceiling. This is significantly different than in other aviation events (and can be an adjustment coming from the AMA,) where ceiling height and floor area are much more important due to the general ability to control altitude.

As far as timing goes, Jeff's description is spot-on, I have nothing to add to his detailed explanation.

[TrueshotBarrage]

Do you think I should use the Freedom Flight kits or homemade helicopter?
Also, do you think the kit will come in time for me to use it? The regionals is this Saturday.

[jander14indoor]

If you have never done anything like this before, get the kit.
On time, perhaps with premium shipping paid.
In time to build it for Saturday, hope you are out of school next week.
Sorry, but this is NOT (as if any SO events are) an event to pull off at the last minute.

Good luck,
Jeff Anderson
Livonia, MI

[chalker7]

A new FAQ discussing rule 3.f and some creative interpretations of it has been posted. You might want to check it out at http://www.soinc.org/node/828

[jgrischow1]

A few questions, and yes, I have read the entire thread, by which I mean every thread back to 2010. I also copied and pasted relevant posts from Messrs. Anderson and Chalker into a single document for ease of reference. I call it "Jander14indoor & chalker7's greatest hits". I am willing to sell this unedited masterpiece as a 21 page .pdf. I also am working on a dramatic reading with narration by Morgan Freeman and James Earl Jones in .mp3 format. Expect your royalty checks in the mail, gentlemen.

Our design is a standard two prop counter-rotating with two blades each. It's based on the Freedom Flight kit. We are keeping our weight down and don't have the time to adjust pitch, chord, or other designs by our next competition so are mainly working on rubber. Yes we are lubing our motors. No, we don't have a torque meter (hangs head in shame). As I said, I have read everything multiple times, but wanted some clarification.

-We should be winding just before breaking every time, even if we usually have unused winds left, correct? For example, if we were using 1/8 rubber and winding to 600 and only using 450 consistently, would there be any purpose in winding to 750 if we could do it?

-Kinda related, if we feel we have maxed out and have winds left, do we just shorten the motor to reduce mass?

-It has been said multiple times that motor stick length isn't that important as long as it's long enough to be stable and to leave room for the air to push down from the upper rotor. So if we reduced from 12 to 8 inches to conserve mass, would this be a big deal?

-Kinda going in the opposite direction, I read earlier that motor length isn't related to motor stick length and the motor should probably be longer than the motor stick as it will shorten on winding anyway. We have always used a motor the same length as the stick or shorter, and we always have winds left. So if we were to use a longer motor and therefore have more winds, would there be any benefit? What if we reduced the width to 3/32 and went with a longer motor (assuming we had enough power to get up to the ceiling)?

-In general, the goal, once you have built to minimum weight, is to make the most efficient helicopter possible by using the thinnest rubber that still provides enough power to lift the copter, with a byproduct of efficiency being the blades won't have to spin as fast, leaving more winds for the longest flight time, is that somewhat on the right track?

Any advice would be appreciated.

[bjt4888]

jgrischow1,
Congratulations on your extensive research. Another website you will want to reference related to rubber powered indoor free flight is Hip Pocket Aeronautics. There are a significant number of world and national record holders posting on this forum. Here is a link to a thread that includes a formula by John Barker of the UK for calculating maximum turns that can be packed into Tan II rubber strip (similar to the current Tan Super Sport).

http://www.hippocketaeronautics.com/hpa ... opic=983.0

Sample calculation for an 18" loop of 1/8" rubber: 11.35 times 18 divided by the square root of two times 1 over the square root of the product of .125 and .04 = 2,043 projected maximum turns.

I use this version of his formula that uses the 11.35 constant for stretch and it works quite well. If you are going to increase the length of your rubber motor to increase the number of turns it will store you will also need to increase the cross section (thickness) in order to maintain adequate torque.

It is common in some indoor free flight classes to use rubber loop lengths from 1.5 to 2 times the hook-to-hook distance. This would mean that if your hook-to-hook distance is 12" you could be using a motor 18" to 24" long, if it is the correct thickness. It is determining this correct thickness and length that you will want to experiment with and yes, for maximum duration you will want to wind your motors close to maximum. As an extreme example, a Federation ROG (old competition class not flown anymore) designed by Jim Clem has an 18" motor stick and uses a 40" rubber motor that is wound to almost 7,000 turns and flies around ten minutes. This aircraft weighs about 3 grams, includes nichrome wire bracing on the backside of the motor stick and uses a modified off the shelf plastic propeller.

Rubber motors that are much longer than hook-to-hook distance and that are wound to maximum usually require special setup including: heat shrink tubing "anti bunch" protectors at each end, plus o-rings at each end, and use of the "reverse s-hook". Again, see Hip Pocket Aeronautics for pictures and descriptions of these techniques. There is also an excellent youtube video on bending the reverse s-hook.

If you do not have time to experiment with different rotors/blades to determine maximum efficiency, it will be matching the profile of the torque curve of a particular rubber motor to your propeller and aircraft that is the key to very long duration. I believe that I am repeating advice previously posted to this forum by Jeff Anderson.

Bjt4888

[jgrischow1]

Thanks for your detailed reply, bjt. We will keep working.

[SOCoach]

My builders have been using the Freedom Flight kit but ran out of the straw used in the center of the rotor to house the music wire axis. Since we were out, we ended up using a straight pin and poked a hole in top and bottom of the wooden rotor and ran the wire through it. On both versions we used a small glass bead to reduce friction.

Flying our original copters with the straw we were getting around 1:40 minutes. Without the straw "hub", using strictly the wire running through the wood our times were around 1:20. Now since these are different helicopters we realize there could have been several factors contributing to the shorter time. Our question is does the straw tube hub make that big of a difference? Does it generate that much more friction?

Also, does anyone have a good source for purchasing rubber motors? Our copters are use .125 rubber . . . should we use smaller?

Thanks!

[hogger]

My builders have been using the Freedom Flight kit but ran out of the straw used in the center of the rotor to house the music wire axis. Since we were out, we ended up using a straight pin and poked a hole in top and bottom of the wooden rotor and ran the wire through it. On both versions we used a small glass bead to reduce friction.

Flying our original copters with the straw we were getting around 1:40 minutes. Without the straw "hub", using strictly the wire running through the wood our times were around 1:20. Now since these are different helicopters we realize there could have been several factors contributing to the shorter time. Our question is does the straw tube hub make that big of a difference? Does it generate that much more friction?

Also, does anyone have a good source for purchasing rubber motors? Our copters are use .125 rubber . . . should we use smaller?

Thanks!

I don't know whether not having the straw makes your time worse but certainly you have less weight without the straw (0.1 gram less), but I suspect the rotor should not be as stable and flex a bit at the beginning when the rubber is fully wound at the highest tension. Does it fly more wobbly without the straw? BTW, I have been using the straw tube that comes with a can of compressed air or silicone spray as replacement.

I have used the following places for rubber before, you can also get extra rubber from Freedom Flight also. .125 and .140 are the sizes I am using.
http://www.faimodelsupply.com
http://www.a2zcorp.us/store

[bjt4888]

I would like to express my thanks to Jeff Anderson, Matt Chalker and John Schubert for answering my questions about judging the helicopter event. All comments and suggestions were very helpful. The invitational event that I judged went very smoothly thanks to your assistance and thanks to a super set of public schools science teachers that were all in attendance volunteering their time and encouraging the students. The students that I judged demonstrated varying levels of preparation for the event, but all had their brains working either trying to solve real science problems or carrying out well planned and practiced science procedures in the heat of competition. I enjoyed interacting with each team, regardless of the level of preparation, but particularly enjoyed watching an entire school team of about 20 students (and almost as many parents) show up to watch their star helicopter team. A crowd cheering for the school science team is a joy to watch.

Sorry for the belated thanks,

Bjt4888