Chinook Chat!

[Unome]

Finally, would it be wise to try and include a third prop for extra lift?

Several competitive designs in 2012 were three or four prop chinooks. The highest-scoring chinook at 2012 Nationals was just over 3 minutes raw time, and most top chinooks were over 2 minutes. The rotor size was 5 cm longer back then, but the minimum mass was 3.5 grams. Consider that the FFM kit already has proven times over 2 minutes by students, and according to Dave can fly for 2:30. I wouldn't recommend trying three or four props unless you really know what you're doing.

[Ash123]

Anything is possible, but is it practical. It takes a large amount of energy from the rubber band(s) to achieve a competitive time. I doubt if a lite chopper could withstand the force of a single band. Turning the corner to vertical will drain a large amount of the energy, requiring a stiffer chopper and thus heavier and thus probably not competitive.

Oh ok, thanks! That really helps. As far as the body of the chinook goes, what type of wood do you think is best? I’ve heard a lot of people say they are using balsa wood but I’m wondering if there is something better. Could carbon fiber work well? I’ve heard some people are using that.

[coachchuckaahs]

A "single rubber band" Chinook would be difficult, as you need counter-rotating. Some have used a horizontal rubber, but it is pinned at the center and each half wound opposite, so no advantage over 2 pieces.

Light balsa is needed, 5-6pcf range, with judicious use of carbon reinforcement such as that noted by Brian in other posts in the heli forum. You may be able to use pure carbon for some parts, but I suspect the motor stick best strength to weight will be either balsa/carbon or balsa with rigging.

3.5g should be fairly reachable. 3,0g is possible, but takes careful wood selection, very careful gluing, and a good design.

Coach Chuck

[Ash123]

A "single rubber band" Chinook would be difficult, as you need counter-rotating. Some have used a horizontal rubber, but it is pinned at the center and each half wound opposite, so no advantage over 2 pieces.

Light balsa is needed, 5-6pcf range, with judicious use of carbon reinforcement such as that noted by Brian in other posts in the heli forum. You may be able to use pure carbon for some parts, but I suspect the motor stick best strength to weight will be either balsa/carbon or balsa with rigging.

3.5g should be fairly reachable. 3,0g is possible, but takes careful wood selection, very careful
gluing, and a good design.

Coach Chuck

Thanks! That really helps a lot. As far as the rubber band goes, how would you recommend picking a thickness for them? I’m a little confused by that.

[Ash123]

Also does anyone know how to actually build the perpedicular motor stocks that hold the rubber bands? I thought about just doing two motor sticks that are used on axial helicopter designs, but I think that would weight too much. Also how would I reinforce it with carbon fiber? And what could I use as hooks for the rubber bands?

[SluffAndRuff]

Finally, would it be wise to try and include a third prop for extra lift?

Several competitive designs in 2012 were three or four prop chinooks. The highest-scoring chinook at 2012 Nationals was just over 3 minutes raw time, and most top chinooks were over 2 minutes. The rotor size was 5 cm longer back then, but the minimum mass was 3.5 grams. Consider that the FFM kit already has proven times over 2 minutes by students, and according to Dave can fly for 2:30. I wouldn't recommend trying three or four props unless you really know what you're doing.

Does "three or four prop chinooks" imply having to wind as many motors? Or was there a different method used to supply power to multiple rotors?

[coachchuckaahs]

Ash123:

See Brian's forum entries on carbon reinforcement.

If you use light balsa (we used 5.5lb 1/8" square), you can reinforce with either carbon, or with rigging (like older axial designs). Ours are rigged, which is more complicated than carbon reinforcement, but are virtually the same as we used last year on our axial design.

We used 0.020 music wire (hobby store) for the rubber hook.

As far as rubber width, you will need to match the rubber width (and torque) to your rotor design (pitch, chord), your heli weight, and your rubber weight. So far we have seen good flights with anywhere from 3/32" to 1/8" wide, though 3/32 is probably too thin especially if you are overweight, and 1/8" gets a bit thick. If you are overweight, you may need 1/8", but it does increase the loads on everything. Selection of the rubber width is a big part of the data collection and analysis on heli, as you do not have much room to adjust the rotors after they are built. Obtain a range of widths, make various loop lengths, and test, take data, and follow the results.

Chuck

[Ash123]

Ash123:

See Brian's forum entries on carbon reinforcement.

If you use light balsa (we used 5.5lb 1/8" square), you can reinforce with either carbon, or with rigging (like older axial designs). Ours are rigged, which is more complicated than carbon reinforcement, but are virtually the same as we used last year on our axial design.

We used 0.020 music wire (hobby store) for the rubber hook.

As far as rubber width, you will need to match the rubber width (and torque) to your rotor design (pitch, chord), your heli weight, and your rubber weight. So far we have seen good flights with anywhere from 3/32" to 1/8" wide, though 3/32 is probably too thin especially if you are overweight, and 1/8" gets a bit thick. If you are overweight, you may need 1/8", but it does increase the loads on everything. Selection of the rubber width is a big part of the data collection and analysis on heli, as you do not have much room to adjust the rotors after they are built. Obtain a range of widths, make various loop lengths, and test, take data, and follow the results.

Chuck

Thanks! That really helps a lot. Do you know where I could obtain a variety of rubber bands of such a large length? I can only find uniform sizes online, so getting get a variety pack or buying individual bands to save money would help a lot.

[coachchuckaahs]

Check with Dave at Freedom Flight Models. I believe he custom cuts, or at least has a variety of widths. A long shot might be to check with local AMA flying clubs to find someone with a rubber stripper. We did that successfully a few years ago, and I eventually bought one (about $180)

Chuck

[Ash123]

Check with Dave at Freedom Flight Models. I believe he custom cuts, or at least has a variety of widths. A long shot might be to check with local AMA flying clubs to find someone with a rubber stripper. We did that successfully a few years ago, and I eventually bought one (about $180)

Chuck

Oh ok sweet! Will do! Thanks!

Also, I was wondering, is there any hazard to using long rubber bands for the chinook. if I used two that were mounted on the perpendicular motor sticks that were the same length as a long axial helicopter, would that negatively affect it? Would a chinook inherently need shorter rubber bands? I thought longer rabbier bands aren’t better cause they can provide more time.

Oh and I just wanted to clarify on the props: a larger diameter prop with a higher pitch generates more lift, right?