? To use stiffer motor stick OR lighter motor stick plus ballast for helicopter stability

[RayBab]

Hi,

Our middle school is building the J&H Tornado. When weighing all of the parts (including the supplied motor stick) before final assembly, we estimate the final weight to be approximately 3.5 gms. The supplied motor stick weighs only .5 gms and feels very “fragile”. Knowing that we will need clay ballast to bring it up to 4 grams, we have a question.

Is there a benefit to the helicopter’s flight stability by going with the lighter weight motor stick build and strategically locating the clay ballast (maybe at the bottom) to alter the center of gravity?

Or, is using a heavier , and therefore stiffer motor stick(with no ballast required) a better option for meeting the 4 gram requirement?

Thanks
Ray

[bjt4888]

Hi,

Our middle school is building the J&H Tornado. When weighing all of the parts (including the supplied motor stick) before final assembly, we estimate the final weight to be approximately 3.5 gms. The supplied motor stick weighs only .5 gms and feels very “fragile”. Knowing that we will need clay ballast to bring it up to 4 grams, we have a question.

Is there a benefit to the helicopter’s flight stability by going with the lighter weight motor stick build and strategically locating the clay ballast (maybe at the bottom) to alter the center of gravity?

Or, is using a heavier , and therefore stiffer motor stick(with no ballast required) a better option for meeting the 4 gram requirement?

Thanks
Ray

The teams I coach will build light to low ballast added near the tail for added stability. Our custom fuselage Heli builds to between 3.2 and 3.3 grams. Fuselage is a 14.75”x.3125”x.1875” stick of good quality 5.5 lb cu ft balsa. Lateral and dorsal trussing with Kevlar thread.

Brian T

Brian T

[coachchuckaahs]

I agree with coach Brian. There are precious few parameters that can be adjusted on helicopter, with CG location being one of them. CG does seem to affect stability in climb, so maintaining that adjustability is an important factor.

Coach Chuck

[RayBab]

Hi,

Thanks for the quick responses. We will build light and add ballast.. I am not sure if I understand LATERAL AND DORSAL TRUSSING WITH KEVLAR THREAD. Is there a quick explanation, picture or link for us to look at? Thanks for your help...this is our first helicopter.

Thanks,
Ray

[coachchuckaahs]

Ray:

The trussing helps prevent bending and buckling of the motor stick. A truss is basically a string or wire that goes to each end of the motor stick, and has a post in the middle holding it about 1-1.5" from the motor stick, triangulating the loads.

The dorsal truss is the most common for lightweight airplanes, and is on the side opposite of the rubber motor. On an airplane, this is along the top of the motor stick, and counters the fully-wound tension of the rubber motor.

For very lightweight heli motor sticks, there can also be a danger of the MS buckling to either side. SO the lateral trusses are added to each side of the motor stick. Generally a single stick or post goes all the way across the center of the motor stick, about 2-3" long, and holds the center of each truss out from the sides of the MS. This delays buckling to the sides. In 2018, we used all three trusses, using Kevlar thread, and were able to use 1/8" square 6# wood for our motor sticks, with little to no deflection when the motor was added. This of course to very careful selection of the wood for the motor stick, but compared to a much larger cross section without trusses, this approach was substantially lighter.

There is a picture in the FFM kit of the complete set of trusses. Unfortunately cannot post photos here.


Coach Chuck

[RayBab]

Thanks Coach Chuck,

Great explanation of trussing. I did find the FF helicopter picture with the trusses...it makes sense now. Thanks again for your help.

Ray

[bjt4888]

Hi,

Thanks for the quick responses. We will build light and add ballast.. I am not sure if I understand LATERAL AND DORSAL TRUSSING WITH KEVLAR THREAD. Is there a quick explanation, picture or link for us to look at? Thanks for your help...this is our first helicopter.

Thanks,
Ray

Ray,

This directory shows several pictures of previous season's helicopters with truss threading: https://drive.google.com/drive/folders/ ... sp=sharing

These previous season helis are from 2017 and are not typical of this year's successful designs as that year's rules were 2.5 gram minimum weight and 20cm maximum rotor diameter.

Brian T