What are some good mods to try on the FFM 26 Heli?

[sonicboom]

Hi, I’m currently building the FFM 26 heli, and I just finished making the rotor “templates.” Other than that, I haven’t worked on any of the other parts yet. I’m trying to decide whether I should build both helicopters completely stock or try modifying one of them. I’d appreciate it if anyone could suggest some good modifications that might improve performance. Also, I’d like to know what kind of flight times I could realistically expect from the 26 heli if it were built and optimized perfectly, assuming a 10-foot ceiling flying environment. And assuming it is perfectly built, is it able to weigh under 4 grams without ballast? I just want to know what bar I should try and set my goal for.

[bjt4888]

Hi, I’m currently building the FFM 26 heli, and I just finished making the rotor “templates.” Other than that, I haven’t worked on any of the other parts yet. I’m trying to decide whether I should build both helicopters completely stock or try modifying one of them. I’d appreciate it if anyone could suggest some good modifications that might improve performance. Also, I’d like to know what kind of flight times I could realistically expect from the 26 heli if it were built and optimized perfectly, assuming a 10-foot ceiling flying environment. And assuming it is perfectly built, is it able to weigh under 4 grams without ballast? I just want to know what bar I should try and set my goal for.

Sonic,

I am coaching 12 students from three schools this year and we have 12 FFM helicopters built that all fly pretty well. We use a custom fuselage stick (no carbon) and attach the lower rotor to the fuselage to make building quicker and loading the motor a little easier and we use the Harlan pigtail bearing for its thrustline adjustable feature (thrustline set perfectly in line with fuselage). We also are using rotor spars that are selected density balsa (higher density than kit) to skip the carbon tow spar steps. Carbon tow is a fine idea though if you use the good quality lower density wood in the kit.

As for performance, perfect rubber density and motor weight and winding to max is the primary method to get longer flights. When my students wind to 90+% of max turns and 2.2-2.3 in oz max torque, they fly consistently 2:41-2:46 with best flights of 2:48-2:49 in a 7.5 ft ceiling (10 ft would be another 2.5 seconds approx). Testing altered rotor blade tip pitch is a good way to try customization. We definitely find that the 7-8 degree kit recommended setting on the lower rotor is good (so far) and the 13 degree tip on the upper rotor is pretty good. We have had some success by reducing tip pitch to 7-8 degrees on the trailing blade of one of the upper rotor blade pairs. I'm thinking of the upper rotor blades as "paired blades" with each set of two blades that are close together possibly acting as a pair. Think of the concept of the slotted leading edge on full scale aircraft and also the aero effect of the jib sail on the mainsail in sailing.

We are finding that adjusting tip pitch on rotors this year with the balsa spars to be much more of a challenge that last year with the carbon spars. Last year, we could just bend hard on the rotor blade TE gusset, using fingernails as a fulcrum, to bend the tip TE up or down. This took 30-60 seconds of hold on the bend to adjust tips about 2-4 degrees up or down. This year, bending in this way seems to just crush the balsa spars thereby weakening the joint excessively. The kit instructions suggest that this gusset joint can be loosened with acetone to allow adjustment. I agree that this would work, but I find that most students struggle to control application of acetone in the correct small amount to loosen a joint like this. If too much acetone gets applied, you will loosen all of the adjacent joints (center rib, inner TE) and also loosen or detach the covering (if the rotor is already covered).

We are making a small diagonal cut in the outer TE spar near the gusset and adjusting tip pitch angle while measuring with the FFM protractor and then regluing with a tiny dot of CA. If you don't over apply glue, you can still adjust slightly to get the exact pitch desired before gluing more securely. Make the cut with a very sharp double edged carbon steel razor. If this cut procedure seems a little precarious and unsafe, it is allowable to have a coach perform it (per the SO build rules, coaches or mentors are allowed to perform dangerous tool steps for students; ex. bandsaw, etc.).

Good luck and keep asking questions!

Coach Brian

[bjt4888]

I added a picture to this folder of this outer TE spar cut for rotor tip pitch adjustment. Most of the pictures in this folder are from previous year's helicopters (2015-2020).

https://drive.google.com/drive/folders/ ... drive_link

Coach Brian

[sonicboom]

Hi Coach Brian,

Thanks for the response — I’ll try and experiment with modifying the pitch angles on one of the FFM helicopters.

I also wanted to ask about the free-spinning disk. Is it better to use the nail that comes with the kit as the axle, or would it make sense to substitute it with something like a thicker carbon fiber rod? I’ve heard that having more mass toward the top of the helicopter can be beneficial, so I was wondering whether switching to a lighter axle would actually help, or if the added mass from the nail is preferable?

[coachchuckaahs]

I do not think the weight difference is enough to worry about.

I have discussed CG location before, and have in the past advocated for considering a higher up CG. However, I am not sure this has been proven out. Others promote a lower CG as ideal.

Our current 2-bladed rotor heli has significant clay (over 1g), and we have ended up with it mounted low, but have not fully explored the stability issue (we had other things hurting our stability too, so it is not clear yet where the CG is best on our design)

Coach Chuck

[sonicboom]

Hi, this is a dumb idea but I wanted to ask if it would be a good idea to try and glue the two given motorsticks in the kit together using Duco to create one motorstick. Would that be a better option than using the CF tow + trusses? I'm not sure which one weighs more but I weighed both of my motorsticks and each one weighs 0.62g. Does anyone know how much a motor stick with the CF tow + trusses weighs? I just want to know how much heavier would two glued motorsticks be compared to the stock motorstick design. Would the added weight be significantly detrimental to times or is the tradeoff for the added strength and ease of handling the helicopter worth it?

[bjt4888]

Hi, this is a dumb idea but I wanted to ask if it would be a good idea to try and glue the two given motorsticks in the kit together using Duco to create one motorstick. Would that be a better option than using the CF tow + trusses? I'm not sure which one weighs more but I weighed both of my motorsticks and each one weighs 0.62g. Does anyone know how much a motor stick with the CF tow + trusses weighs? I just want to know how much heavier would two glued motorsticks be compared to the stock motorstick design. Would the added weight be significantly detrimental to times or is the tradeoff for the added strength and ease of handling the helicopter worth it?

While you could glue the two kit supplied motor sticks together and not use the carbon and probably have a strong enough motor stick, the result might lead to an overweight helicopter. The sticks in our kits are mostly between 0.6-0.62 grams and the carbon application, if done carefully, will add about 0.4-0.45 grams giving you a 1.0-1.07 gram motor stick.

Brian T