Chinook Chat!

[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?

[bjt4888]

Ash123,

If you want to know if longer rubber motors are better, try them to find out. A Chinook does not inherently require shorter rubber bands except that extra length is also extra weight and there are two motors so each extra inch of motor length is four inches of additional rubber. Each Heli design will have an optimal rubber motor thickness and length, so test as many variations as you can.

Yes, larger diameter and greater rotor pitch will increase lift and increases drag. Think in terms of discovering the best lift/drag ratio, not maximizing lift.

Brian T.

[Ash123]

Ash123,

If you want to know if longer rubber motors are better, try them to find out. A Chinook does not inherently require shorter rubber bands except that extra length is also extra weight and there are two motors so each extra inch of motor length is four inches of additional rubber. Each Heli design will have an optimal rubber motor thickness and length, so test as many variations as you can.

Yes, larger diameter and greater rotor pitch will increase lift and increases drag. Think in terms of discovering the best lift/drag ratio, not maximizing lift.

Brian T.

Brian
Thanks! That really helps a lot. For the props, do you know of a good way to construct them? I don’t understand how I could develop a pitch with my spars. Also, for the material in between the spars, what would be a good material to use? I thought about carbon fiber but I’m curious to see if there is a better idea out there.

Thanks Again!
Ash123

[Ash123]

Do the props affect flight time directly in any way (such as their pitch or diameter)? Or do they only affect lift and it’s really the rubber length and number of twists of the rubber that affects flight time directly?

[bjt4888]

Ash123,

Things to focus on in approximate order of importance (approximate because all are interrelated):

1. Build to the minimum weight of 3.0 grams, or as close as you can.
2. Rubber thickness matched to the rotor design (determined by testing)
3. Rubber length that allows maximum turns
4. Rotors should be to maximum diameter allowed
5. Rotor blade design as efficient as possible
6. Rotor blade incidence angle optimized (determined by testing)
7. Learn to wind motors to maximum number of turns (maybe 90% of breaking turns)
8. Get all of the bonuses

Brian T

[Ash123]

Ash123,

Things to focus on in approximate order of importance (approximate because all are interrelated):

1. Build to the minimum weight of 3.0 grams, or as close as you can.
2. Rubber thickness matched to the rotor design (determined by testing)
3. Rubber length that allows maximum turns
4. Rotors should be to maximum diameter allowed
5. Rotor blade design as efficient as possible
6. Rotor blade incidence angle optimized (determined by testing)
7. Learn to wind motors to maximum number of turns (maybe 90% of breaking turns)
8. Get all of the bonuses

Brian T

Thanks Brian! That really helps a lot! Could you clarify though- by rotor blade design as efficient as possible, what exactly could I change? The pitch and material of the rotors? And what’s an incidence angle and how could I change it?