Custom Helicopter Question

[bridgeisbest]

Hello,

I am planning on building a custom helicopter, based off of JH's Hurricane and Dave Aronstein's NFFS plan, where the helicopter is placed differently in the box making the rotor larger and the motorstick shorter.

I've looked at Coach Chuck's replies about how helicopter stability works with rotor pitch angle (lower rotor provides power from lower pitch angle, upper rotor provides stability from higher pitch angle). What I don't know is what pitch angle JH's Hurricane is at, so I am not sure what sort of pitch would be good for this type of custom helicopter.

My current thinking is to have a upper and lower rotor, each rotor having 2 blades that are rectangular (like JH). With the information about rotor pitch angle and stability, I am thinking that the upper rotor would have a higher pitch angle (10 degrees maybe?), and the lower rotor to have a lower pitch angle (5 degrees maybe?). Would this theoretically give me a decent helicopter, if I also used a vane?

I am also planning on a way to make the rotor blade's pitch angle adjustable, so I could be able to experiment a bit.

Another question I have is how important a vane is. With a shorter motorstick, it would be harder to make a stable helicopter, so is that why JH uses a vane and also rotor dihedral? With this custom helicopter with some stability with the rotor pitch angles, and also a vane, how important would dihedral be? Without dihedral, making the helicopter should be simpler.

That's all of the questions I can think of for now. Thank you in advance for the help!

[coachchuckaahs]

All good questions. Josh has obviously done some testing with that heli design, I am sure. I have not so I am only speculating.

We made a heli in 2016 that was same pitch on both rotors. I am not sure, Josh may do the same.

One key difference is that Josh believes that a constant PITCH ANGLE is beneficial (see the Hurricane), and it is certainly easy to build each blade like a wing. Dave strongly believes a starting point is constant PITCH, so a helical pitch angle, which is afforded with an X-rotor. It is not clear to me which approach is better.

The X-rotor inherently has area that is near vertical in the middle, and this acts like the vane and the theory is that this is why a vane is not needed. On the constant [pitch angle helis, the vane is absolutely needed for stability, and yes, that takes away from the MS length. If you could figure a way to put the vane below the top rotor, that might work, but the vane MUST be free to rotate on its own.

Dihedral, I have not tried. It is certainly easier with the non-x-rotor, and is supposed to help with stability. HOWEVER, on unique part of our rules is that the dime-sized disk of balsa must be position so as to be the first part that touches the ceiling. If you add dihedral to the top rotor, you need to make the disk tower or vane taller to meet this rule.

The X-rotor advantages include no vane needed, strong construction, and helical pitch progression. The flat blade approach advantages include pitch adjustability (of such facility is built in), pitch progression is independent of the spar arrangement (to the point that a flat progression is possible.

The advantage of a larger rotor will be lower "wing" loading, but that MAY require wider rubber (Josh reports that initial testing worked on 1/8" rubber as well, see his video).

I think it is wise to build facility for pitch adjustment, in order to answer your questions. IMHO, 5 degrees may be a bit low. If the helicopter climbs quickly, your apparent angle of attack can go negative. But give it a try. Keep a careful log and try a bunch of pitch combinations to see what works!

Coach Chuck

[bridgeisbest]

That's really interesting, thank you for answering. With your answers, I might start with ~7-8 degrees of pitch angle on the rotors, and adjust if needed. As for the vane, making it slightly larger should help with stability if I don't do dihedral, but I also might end up using dihedral.

If this design ends up failing, would this sort of big rotor short motorstick design work well with an x-rotor? That might be a bad question but I think it would be fine.

I do have a few more questions though. What do you mean by "pitch progression is independent of the spar arrangement"? I interpret this as you don't have to rebuild the spars to change the pitch (like you have to do with FFM), since you change the pitch angle by adjusting the hub, but I'm not 100% sure that's what you meant.

Also, why doesn't this design require much wider/denser rubber compared to standard coaxial helicopters? With much larger rotor surface area, shouldn't it take greater force from the motor to turn the rotors?

What is the difference between having 2 freespinning rotors and 1 rotor being mounting on the motorstick with the other free spinning? Specifically, does the fixed rotor being on the top or bottom affect anything? I am asking this for [maybe] a future custom (and just general knowledge) where it's just a traditional longer motorstick with 2 rotors like FFM.

Thank you so much for reading my yap!!

[coachchuckaahs]

I am answering with my phone so I'll be brief.

Will it work with x-rotor? I don't see why not. But testing will tell. Downside is less flexibility in adjusting to perfect your build.

Pitch progression. Yes, you can adjust the overall pitch at the hub if so designed. But the pitch progression is the change in pitch angle along the radius. Josh's designs appear to have none, for a constant pitch angle. The x-rotor naturally has helical progression, with constant pitch instead of pitch angle. You can modify it, like FFM does, but it is still driven by the spar layout. The hub approach allows you to build in any pitch progression without regards to the spars, at least if you use a primary center spar.

Rubber. One might expect it to use wider rubber dye to larger surfaces. Not sure what Josh used for pitch. Perhaps he pitched it down somewhat. What you really need is a given power level to support 4g. Power is torque x rpm. So if rubber is the same, torque is the same, so to get same power you need same rpm. You can get this with lower pitch.

Free rotors. Except for a very small rotating drag on the MS, it should not matter one free rotor or two. But being able to single person handle the rubber loading and launch is, IMHO,a huge advantage of living in one rotor. From what I have seen, locking the bottom one appears to be easier to launch straight. Locking the top one may have a VERY small performance boost, allowing the bottom to work without the rotational drag. But I think it would be to small to measure, and with a 10-minute flight window safe handling is critical.

Coach Chuck

[NEonGhOsT]

About The X rotor I plan to make one with around 4 degrees dihedral and wide rotors (5 cm at max) with washout of around 2-4 degrees, but I am more interested in the actual pitches, right now I am going with a 48 cm rotor with around 8 inch pitch on the top and bottom which is a p/d value of 0.41 but I don't know if a lower pd is good or bad, with the wider rotors. what should I expect and what can I do.

[coachchuckaahs]

TBH, I have not looked at actual pitch, especially the way FFM makes the rotors, as it is not a constant. The tip pitch is critical to prevent fluttering, and so that is what we usually look at. I do not know of any rule of thumb for P/D on heli rotors. If you look at J&H heli, they have a constant pitch angle, so pitch varies greatly over the blade.

Coach Chuck