Helicopters C

[bjt4888]

Daydreamer,

The FF kit rotors have 7/8" vertical separation between the leading and trailing edges (I am doing this from memory; so be sure to double check). As the center rib length is 2.125" (approximately) this pitch angle (as measured radially) is 24.3 degrees (arcsin 7/8 divided by 2.125). A 1/16" increase in the LE/TE vertical separation would change the pitch (or incidence) angle to arcsin 15/16" divided by 2.125", or 26.18 degrees; an increase of 1.88 degrees.

I see that I did the math wrong in my earlier post. So, we are not seeing significant flight time differences with a variance of 1.88 (or so) degrees of rotor pitch (or incidence) angle.

Sorry for the earlier error.

Brian T.

[retired1]

Has anyone tried to modify the Chinook kit so the rotors are on top instead? :0 Would the stabilizers still be necessary in this case? And do you think having two disks would be okay because technically they’re both still at the highest point of the helicopter? I’m not really sure how that would work so I just wanted to check if anyone has tried already.

At our regional on Sat I saw 6 FF choppers (from the same school) that all had the rotors facing up instead of down and they had very poor flights. (height and stability)
I talked to Dave and he says that is the nature of this model.
I mentioned it to one student and he had no clue what I was talking about, so he must not have read the instructions.

[Ten086]

Has anyone tried to modify the Chinook kit so the rotors are on top instead? :0 Would the stabilizers still be necessary in this case? And do you think having two disks would be okay because technically they’re both still at the highest point of the helicopter? I’m not really sure how that would work so I just wanted to check if anyone has tried already.

At our regional on Sat I saw 6 FF choppers (from the same school) that all had the rotors facing up instead of down and they had very poor flights. (height and stability)
I talked to Dave and he says that is the nature of this model.
I mentioned it to one student and he had no clue what I was talking about, so he must not have read the instructions.

Oh... :/ Why would they be more unstable though? And did you happen to see any original designs with the rotors on top?

[daydreamer0023]

Daydreamer,

The FF kit rotors have 7/8" vertical separation between the leading and trailing edges (I am doing this from memory; so be sure to double check). As the center rib length is 2.125" (approximately) this pitch angle (as measured radially) is 24.3 degrees (arcsin 7/8 divided by 2.125). A 1/16" increase in the LE/TE vertical separation would change the pitch (or incidence) angle to arcsin 15/16" divided by 2.125", or 26.18 degrees; an increase of 1.88 degrees.

I see that I did the math wrong in my earlier post. So, we are not seeing significant flight time differences with a variance of 1.88 (or so) degrees of rotor pitch (or incidence) angle.

Sorry for the earlier error.

Brian T.

I understand now. Thank you so much for your help!

[bjt4888]

Daydreamer,

One last thing. I am thinking that exploring less pitch might be more productive than more pitch.

Brian T

[Ten086]

I'm sure it's somewhere in the forums, but could someone remind me how increasing vs decreasing pitch affects lift?

[bjt4888]

Ten086,

Good question; For a particular free stream flow rate, increased rotor pitch (which likely produces increased angle of attack) increases lift and increases drag. Related to this aspect of the Helicopter event, what you would want to determine by experimentation is the rotor blade angle/pitch (area, shape, pitch distribution, etc.) that produces the best Lift/Drag ratio for your particular helicopter.

More important than this rotor aerodynamics analysis, would be to find the best possible motor length and thickness for a particular rotor set, minimizing overall weight and learning to get the greatest number of turns into your motors.

Good luck and keep building and testing.

Brian T

[izzanom]

Hey guys, I'm using the FFM chinook kit this year and I was wondering what others with same model were using for rubber bands. Dave supplied .094" and .1024"(or something similar I can't remember) but they seem rather thick. He says in his instructions that each rubber band should be about 1 gram but that requires each band to be rather short which means I can't do many winds without having a really high torque. I've been trying .072" since I can do more winds but I was wondering what other people were doing. I'm not sure what the "optimal" length and width would be.

[bjt4888]

Izz,

I would suggest that Dave Ziegler has given teams a major start in the right direction with the rubber supplied in the kit. The answer is to test, and test, and test, etc. As this is a major piece of the problem solution, I'm thinking that teams would be reluctant to share very much detail.

Knowing the average rotor revolutions per second (by testing and measuring turns wound and turns remaining and duration) and knowing the duration of the best flights so far at Invitationals and having knowledge of one of the "turns calculation formulas", you can back into the likely rubber motor length and "thicknesses" used by these best flights.

I posted a .pdf of the turns formula that my teams use (John Barker's formula: famous U.K. propeller engineer) in this wiki under a previous year's Wright Stuff thread. If someone wants to find it, I'll cheer them on from the sidelines. Or, you can do what I did as I prepared for my first year coaching Science Olympiad and read every post on every forum for every year (this is a tongue-in-cheek, sort of, challenge for the teams want to accept it).

Brian T.

[glouthan]

Hello all,

I've recently purchased some carbon fiber tow (3K) so that we can use it to reinforce our motor sticks, but our team is not quite sure the most effective and weight efficient way to attach the carbon fiber tow. We've read that whatever glue we use it needs to be applied so that the carbon fiber is fully immersed in the glue for the carbon fiber to be effective in strengthening the motor stick. However, after reading though the forums and other sources the most common method seems to be to use CA glue, but we can't help but imagine that it would add so much weight to the structure, so does anyone know of any other glues we can use to attach the carbon fiber that would be less heavy? We are looking for a solution that would be pretty readily available from stores as we have a competition coming up soon so we probably do not have time to order something online such as a specific epoxy, etc.

Thanks for the help!!