Hydrofoil Building Help

[FahmiRBLX]

I'm currently working on my hydrofoil boat for the Ellano Challenge, but testing ends up the boat either lifts off it's back onl, tipsy or flips. Anyone could help?

 

Craft will be uploaded to KerbalX and linked to here soon.

[KerbolExplorer]

Try adding weight in the front of the boat to evoid making it flip

[Gaarst]

Hydrofoils are impossible to truly balance and a "basic" water wing will be unstable at some point. What you can do is shape your foils so that your craft rotating in a direction gives negative feedback so that they naturally counter-act the rotation.

For roll this means some dihedral, yaw means vertical foils at the back (mind the roll they may cause), pitching means having a 'main' foil towards the CoM and a smaller one towards the back, acting as a plane tail (you want more pitch moment towards the back); dihedral may help for pitch as well but only if you have significant AoA built in your foils. Even with that hydrofoils still require a lot of tweaking, there's no magical answer and they are a lot harder to get right than planes.

Edited August 28, 2018 by Gaarst
Emphasis

[bewing]

I don't think I agree with Gaarst. Maybe look at this one: 

 

In the water, you get roll stability with a wide stance. You get yaw stability by having more drag at the back than at the front. And you get pitch stability by having your elevon hydrofoils at the front producing lift, and using flat wing parts at the back as passive foils. If the front comes up out of the water, then the front loses all its lift and comes back down. If the front drops a little then more of the foil is underwater -- you gain a ton of lift, and the front rises back up again.

(I posted my fixed up 1.4.5 version of that craft on Steam as the Perigo_Spase_Bote, IIRC.)

[Gaarst]

59 minutes ago, bewing said:

I don't think I agree with Gaarst. Maybe look at this one: 

And I do think I agree with you because I think we are saying about the same thing. 

 

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In the water, you get roll stability with a wide stance.

My suggestion for dihedral is so that the higher foils are out of the water and generate less lift. You're right that if you have a wide stance, they will get out, but for a narrower wing a bit of dihedral helps (dihedral also helps in that even the submerged part of the foil will generate less lift if it is at an angle). The thing about dihedral causing conflicting forces makes sense but it's a high speed thing, I haven't reached the point where I am building supersonic hydrofoils, and it looks like neither has Fahmi.

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You get yaw stability by having more drag at the back than at the front.

Basically what I said. Vertical surfaces don't have lift (a bit of buoyancy but whatever) so they don't mess up your balance but they generate drag (more so if you are yawing) at the back.

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And you get pitch stability by having your elevon hydrofoils at the front producing lift, and using flat wing parts at the back as passive foils. If the front comes up out of the water, then the front loses all its lift and comes back down. If the front drops a little then more of the foil is underwater -- you gain a ton of lift, and the front rises back up again.

I didn't really think about it but yeah passive wings at the back do make more sense than simply moving some of your lift to the back.

 

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(I posted my fixed up 1.4.5 version of that craft on Steam as the Perigo_Spase_Bote, IIRC.)

I don't want to be that guy, but could you post a craft file here? I'm now in the mood to make hydrofoils, and since you appear to have more experience than me with these I'd like to see your craft. Also I don't have KSP on Steam, that too.

[bewing]

35 minutes ago, Gaarst said:

I don't want to be that guy, but could you post a craft file here?

https://pastebin.com/raw/M4XqVyX4

In V1.4.5 the wheels pull a little to the left when it's rolling. Action 2 to deploy the hydrofoils. Action 3 to put the hydrofoils into flight mode.

Best to take off before getting much over 400 m/s. Above that, small vibrations while still on the water will cause RUDding.