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Hello,

I'm in the midst of a Mil Mi-17 right now, and was wondering whether anybody had a good way of manipulating plastic rotor blades to give a realistic droop. This is the first whirlybird I've done in over 20 years and the first of this level of complexity, so any advice would be more than welcome.

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You could try building a jig, clamping the blade in position and pouring over boiling water. I've not tried this but it should work. I have bent blades by hand at room temperature but its possibly a bit risky.

Nigel

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I saw someone tape them to the outside of a mixing bowl, and douse them in boiling water from a kettle, which I thought was quite clever... dunno whether it worked though :shrug:

Don't forget that some modern rotors are made from composite, and don't droop like traditional rotors, so always check your references before embarking on bending... ;)

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Probably more than you want to know about the droop itself:

1. Most rotors have a "droop stop" at the hub. This keeps the blade from hitting the fuselage when the rotor first starts turning. That means the blade will extend straight out or at a small angle out from the hub with little bend.

2. The outboard end of the blade doesn't curve as much as the middle. That is because there is less weight outboard to cause bending as you move outboard on the blade.

3. The most curve is very roughly somewhere between 25% and 50% of rotor diameter. The inboard end doesn't curve as much because of 1 and the outboard end because of 2.

This may provide an image: http://www.codecogs.com/users/23287/Cantilever-Beams-101.png

If not, see the first illustration at: http://www.codecogs.com/reference/engineering/materials/beams/cantilever_beams.php

Due to the effect on droop of the type of rotor hub and the stiffness of the rotor blade, the above does not hold for all helicopters.

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I bent mine by hand and dipped them in boiling water from the kettle, held it in place for a minute or so, then popped them in the freezer overnight.

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  • 2 weeks later...

This has been a really interesting thread - many thanks, guys. I've now got the rotor assembled, all 36 parts or so, despite three of the blades breaking off during cleaning up. Once I assembled it, I realised that KP had moulded in a certain amount of droop, which was pretty nice engineering for the time. At the moment it appears that this will be sufficient, but I will check again once the time comes to mount the rotor - might need to add a bit more then.

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  • 1 month later...

I just finished an 1/72 Airfix Kamov 25 and sucessfully bent all six blades by hand at room temperature - no problems, they were even partially painted. You need to study reference photos to get the correct droop profile.

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This is my method:

1. Nick wife's hair dryer( or use own if you have any hair left)

2. Attach blades to central boss...place boss in blutac and fix to block that gives enough clearance for droop

3. Place clothes pegs to blades.. Closer to boss= less droop .... Further along towards tip = more droop

4. Warm carefully with hair dryer( 10-12 inches away)

5. Leave to cool

Make sure you use all the same type of pegs obviously!

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If I may suggest, a safer and (more importantly) more controllable variation on the "boiling water" method:

Find an ordinary cooking spoon with a curve that matches your desired blade droop. Spoons are ideal for this, since one part of the "bowl" is usually a sharper curve, that tapers off to a more gentle or almost straight contour. A few good sturdy rubber bands, and you're ready to go.

From my recent 1/144 Sea King:

GEDC5214R_zpsa91e1e4d.jpg

GEDC5215R_zpsfb40e438.jpg

Bring a pot of water to a boil and take it off the heat, dip the spoon with rotors in for a slow 30-count (approx. 30 sec.), then run the spoon under cold water for about half that time, and they're done. It gives you a uniform curve on all the blades, and it's fast.

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Good idea, but depending on scale, you might need a huge spoon for the Mi-17 or Mi-26 :) What large kitchen utensils are heatproof and have a non-constant curvature like a spoon bowl?

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