Continuing of Discord Thread on Heli Design

[AdithA]

With the Science Olympiad season building up to its halfway point, I've started testing my helicopter and have been having struggles with the hurricane(2026). If possible, coach chuck and brian, could you please provide some advice/answers to the following questions.

The model has only flown for 45 seconds before coming down with more than 70 turns(with a 2.9g 0.09in rubber motor). This may be due to my winding but I believe that my winding cannot be so bad as to end up with such a large amount of turns left.

The model came out 0.5g under so I attached 0.5g ballast onto the bottom of the motorstick. Rather than keeping it at the bottom, is there a specific place that it should be placed depending on the center of gravity of the model?

The model does struggle to center itself and stay still during the flight like a freedom flight model would. Are there any trim adjustments I could do to improve this stability?

I am considering a custom build based on the hurricane but with the following adjustments, a wider chord(like that of a ffm), maintaining the 2-blade rotor for the bonus, having the upper rotor spin rather than the bottom rotor. Do you see any potential con's with this that you could point out early-on to avoid wasted time?

Edit:
thanks for your help guys. I got it to 2:16, with low-mid build quality.

Probably, instead of doing a full-on custom, I'll lean on pumptato's advice and rebuild the hurricane but with a few changes. I'm thinking of making it all balsa, except for the bottom rotor, like this years freedom flight, making a better wind vane with a free spinning top disk, @Coach Chuck and @bjt4888 could you provide your thoughts on this?
For me, the trick to get it from 45s to 2:16 was the rubber. I put a 0.125", 15" loop length, 2.0g motor that was getting around 105-110 turns max with 4-5 backoff to a 1.1oz launch torque from a peak 2.5oz wind torque.
Invincible — Yesterday at 5:24 PM
Still having issues with the helicopter moving around a lot after the torque lessons, but that's almost certainly due to poor build quality

[bjt4888]

Interesting questions. I can reply tomorrow afternoon.

Coach Brian

[coachchuckaahs]

Adith:

I am glad you figured some of this out. I was certain a big part of it was rubber selection and winding technique.

A few keys for helping those that would help you:

Rubber varies greatly in thickness (though the cut width is fairly consistent). Reporting the g/in linear density is a great help, rather than just reporting cut width (you indicate 0.090", or I presume inches. Always include units.

Second, you are reporting "turns" on the winder rather than the motor. This is meaningless as your winder could be10:1, 15:1, or 20:1. We can only guess, and usually guess wrong. Best to report turns ON THE MOTOR.

Question 1: That motor sounds very narrow. I think the turns sound low as well, as does the torque (I think on Discord you reported winding up to 1.1 and no dewinds). Many turns left is a classic case of rubber too narrow or pitch too high. The WHOLE EVENT centers around matching the rubber and rotor. You do not appear close, and your later testing with 1/8" rubber confirms that. As you refine the match, you need to track linear density as it will vary substantially, even in the same bag of rubber.

Q2: Ballast placement. I believe FFM indicates to put the ballast at the bottom. I have seen some evidence to teh opposite, but minimal testing this year seems to agree with FFM. This is certainly something I would have as a variable in my log (CG location on MS), and try various positions and observe results.

Q3: Wobble. I would look at a spinable disk like FFM, though Josh indicates it is not a big effect. More importantly, I would be absolutely certain that the tip pitch angle is the same for all blades on a given rotor. Very small differences in tip pitch will cause significant wobble. Sloppy building will do similar and is harder to measure/account for. Build accurately in the jigs the kit provides, don't over-tighten the covering, and then measure the tip pitch, and possible pitch at several intermediate positions. Check the tracking of the blades as well.

Q4: We had upper rotor spin last year (ffm kit, lower rotor fixed). It proved to be difficult to handle when attaching rubber. We are going to lower rotor spinning this year, and two-bladed props (custom design). I do not think there is a time advantage either way, just a potential handling improvement. Also, if you are running a lower pitch angle on teh bottom, then allowing it to spin up 1 second before launch may help avoid issues with blade flex in early high torque flight (theory of mine, not proven).

Coach Chuck

[bjt4888]

With the Science Olympiad season building up to its halfway point, I've started testing my helicopter and have been having struggles with the hurricane(2026). If possible, coach chuck and brian, could you please provide some advice/answers to the following questions.

The model has only flown for 45 seconds before coming down with more than 70 turns(with a 2.9g 0.09in rubber motor). This may be due to my winding but I believe that my winding cannot be so bad as to end up with such a large amount of turns left.

The model came out 0.5g under so I attached 0.5g ballast onto the bottom of the motorstick. Rather than keeping it at the bottom, is there a specific place that it should be placed depending on the center of gravity of the model?

The model does struggle to center itself and stay still during the flight like a freedom flight model would. Are there any trim adjustments I could do to improve this stability?

I am considering a custom build based on the hurricane but with the following adjustments, a wider chord(like that of a ffm), maintaining the 2-blade rotor for the bonus, having the upper rotor spin rather than the bottom rotor. Do you see any potential con's with this that you could point out early-on to avoid wasted time?

Edit:
thanks for your help guys. I got it to 2:16, with low-mid build quality.

Probably, instead of doing a full-on custom, I'll lean on pumptato's advice and rebuild the hurricane but with a few changes. I'm thinking of making it all balsa, except for the bottom rotor, like this years freedom flight, making a better wind vane with a free spinning top disk, @Coach Chuck and @bjt4888 could you provide your thoughts on this?
For me, the trick to get it from 45s to 2:16 was the rubber. I put a 0.125", 15" loop length, 2.0g motor that was getting around 105-110 turns max with 4-5 backoff to a 1.1oz launch torque from a peak 2.5oz wind torque.
Invincible — Yesterday at 5:24 PM
Still having issues with the helicopter moving around a lot after the torque lessons, but that's almost certainly due to poor build quality

I agree with all that Coach Chuck posted, of course. I'll add that the slight customizing of the hurricane using balsa spars is a good idea. Balsa at same weight of carbon rod is typically stiffer and stiffer spars has been shown to increase duration in other designs. I'm guessing that the carbon spars are flexing and possibly hurting the rotor efficiency. I like the free spinning top disc and think that it helps. The amount it helps depends upon the smoothness/roughness of the ceiling.

A 15" loop that weighs 2.0g and has two black rubber o-rings is only .0626 g/in, which is very, very low density for .125" rubber stock. The lowest I have ever seen (cutting about 5,000 ft of rubber from 10 years of different batches) is .075 g/in. Are you including the knot in the loop length? Typically, don't include the knot and definitely don't include the knot if using the NFFS turn calculator.

Are your winder turn counts for a 10:1 or 15:1 winder? If 10:1, then you are only winding to a little over 50% breaking turns, so I'm guessing that this is a 15:1. Also, you could supply info on which use of the motor this is and what lubricant you are using.

Good work so far!

Brian T

[AdithA]

Thank you both for responding @bjt4888 and @coachchuckaahs .

I didn't think to meausre the motor length until after our conversation on discord after which I had fully broken in the motor and used it multiple times so that's likely why it was so long. My winder is 15:1 and I am using Charlie's May 25' batch at the moment with a mix(i'm still deciding on whats better) of josh's rubber lubricant(the one in the small white toothpaste tube) and dave's motor mojo. I'm confused on what to use in terms of lube because I feel like dave's motor mojo is simply too thin to have any effect on the rubber motor while josh's lube comes in very minimal amounts so I have to use it sparingly. I have tried a mix of the two and have had somewhat decent results with it(i have gotten a nice compromise between sticky and slick on the lube coating of the rubber).

In my 2:15 flights, I started winding up to 2.0-2.5oz of torque which got me to around 1575 turns on the motor before dewinding down to 1.1oz torque and losing 75 turns. As for ballast placement, I emailed Josh and he said that the Hurricane benefits from having the ballast at the top by using the analogy of trying to balance a broomstick on your palm, and how the more weight you add to the top the easier it gets, but I will definitely experiment with it and test it out.

I am currently in the process of building a custom long rotor design using Aeromartin's openscad software. The key characteristics of the design include a longer chord and longer diameter(because i am opting for the long end of the box being the rotor diameter instead of motorstick. I am still plannning on using the fixed rotor design with the bottom rotor spinning but will incorporate a spinning disk at the top. I think the 3d printed jig will help tremendously with alignment especially because it comes with an alignment cap and hub and built in ridges on the edges for the spars to sit in.

I will update you once the build has progressed for that.

Thank you,
Adith Alagiri

[bjt4888]

Thank you both for responding @bjt4888 and @coachchuckaahs .

I didn't think to meausre the motor length until after our conversation on discord after which I had fully broken in the motor and used it multiple times so that's likely why it was so long. My winder is 15:1 and I am using Charlie's May 25' batch at the moment with a mix(i'm still deciding on whats better) of josh's rubber lubricant(the one in the small white toothpaste tube) and dave's motor mojo. I'm confused on what to use in terms of lube because I feel like dave's motor mojo is simply too thin to have any effect on the rubber motor while josh's lube comes in very minimal amounts so I have to use it sparingly. I have tried a mix of the two and have had somewhat decent results with it(i have gotten a nice compromise between sticky and slick on the lube coating of the rubber).

In my 2:15 flights, I started winding up to 2.0-2.5oz of torque which got me to around 1575 turns on the motor before dewinding down to 1.1oz torque and losing 75 turns. As for ballast placement, I emailed Josh and he said that the Hurricane benefits from having the ballast at the top by using the analogy of trying to balance a broomstick on your palm, and how the more weight you add to the top the easier it gets, but I will definitely experiment with it and test it out.

I am currently in the process of building a custom long rotor design using Aeromartin's openscad software. The key characteristics of the design include a longer chord and longer diameter(because i am opting for the long end of the box being the rotor diameter instead of motorstick. I am still plannning on using the fixed rotor design with the bottom rotor spinning but will incorporate a spinning disk at the top. I think the 3d printed jig will help tremendously with alignment especially because it comes with an alignment cap and hub and built in ridges on the edges for the spars to sit in.

I will update you once the build has progressed for that.

Thank you,
Adith Alagiri

Adith,

Very good follow up info. I am finding that May 2025 Tan SS is softer than average, almost like June 2016 (a very good year), and takes pretty good turn count. That being said, we're finding that a loop of this batch stretches almost 0.875" after the first use (typical is more like 0.5" or 0.625"). Using the Aeromartin jigs and testing the long rotor heli idea is eactly what we are doing. We are flying a slightly modified version of the Freedom Flight heli now and building custom large rotor designs; so the track you are on is likely a good one.

For Lube, I have heard from the F1D people that 800-1200 CST (relatively low viscosity) silicone RC car transmission "grease" is what they use. For the heavier density Science Olympiad rubber motors, we have been using 5,000 CST with good results (I think that Josh's is 5,000-10,000, which is also probably good for SO). The IFFS lube is also very good, but we do find that the lower viscosity tends to splatter on the airplane/heli as the motor unwinds. This adds weight and is not that great for the wood in my opinion. I haven't used the IFFS lube and you will have to observe to see if splatter is an issue; I'm sure it's very good lube. Here's what we use: https://www.hobbytown.com/team-associat ... OQQAvD_BwE

Be sure to use average linear density to specify motors as even slight differences make large flight time differences. For example, last year we used 2.15g loops of .076-.077 g/in rubber and did pretty well (not as good as Solon or Clague, but 2:45 in a 7.5 ft ceiling). Even a change to .078 g/in cause a loss of about 10 seconds of flight time.

Keep up the good work!

Coach Brian

[AdithA]

I am piecing a design together for the long rotor. But I am struggling to make a strong but spinnable disk with a fixed rotor design. How did you guys incorporate this into your build? Should I just make the disk attach to a bearing and leave it(Imagine the top rotor of the ffm helicopter but the rotor itself removed and the disk is the rotor)? The motor would not be attached to this hook but a hook connected to the motorstick like in the Hurricane design.

Also what thickness balsa do you use to make your motorstick without a truss? I'd prefer to have a motorstick without a truss as that would make the handling process exponentially easier. I'd even be fine with a heavier motorstick since most of my builds are coming out to 0.75g to 0.5g lighter than the necessary weight.

[bjt4888]

I can send a link for a picture of our disc a little later today.

Brian T

[coachchuckaahs]

Adith:

Lube:

For years I used 27.5wt RC Car Shock Oil (not tranny oil). It works well (it was about 180CST I think), but it is very thin and gets all over the plane. It also needs recoating every wind. IFFS sells if I recall a 500 CST oil, and a 5000 CST lube. I have been currently using the 5000CST. The theory is that it does not squeeze out of the rubber as easily, and therefore you can wind a little harder. you have to be careful to get a consistent but thin layer, as it can add weight if you use too much. Some F1D flyers are using silicon grease, which has a different application process. For now I recommend the 5000 CST, with a minimum of 1000CST if you prefer thinner. Look on amazon under "RC Car Shock Oil" and you will find a large selection.

For the spinable disk, we are using a printed disk/bearing assembly that I designed, https://www.thingiverse.com/thing:6949126. A pin is inserted from the top, and pressed into the top end of your MS (assuming top rotor is fixed to the MS). Then a balsa disk (1/32 or 1/16") is added to teh top (glued). It is IMPORTANT to add the balsa disk, as the rules require the disk to be balsa. There should be a glass bead between the printed disk and the MS, on the pin.

With the very large rubber (especially with larger rotors), we are still trussing the MS. It would be difficult to find lightweight balsa strong enough for at least no backbone truss. We used 6# wood, 3/16" thick, and about 1/4" tall. Without a truss, I would be looking at 8-10# wood, 3/16 x 3/8 to start with. That is my guess only, I have not tried a truss-free heli.

Coach Chuck

[AdithA]

@coachchuckaahs and @bjt4888 My Aeromartin jig for my long rotor design has finally finished printing. I plan on building it from purely balsa with no carbon. Do you reccomend that I use carbon on certain parts(like how FFM has bottom rotor carbon but top is all balsa)? I am running a fixed rotor design, so I don't know if this applies. What type balsa do you reccomend I use for the spars, ribs, etc?