Why don't biplanes work?

[Snikersnee]

Even if I line up the CoM and the CoL, it flips head over heels. Why? Too much lift? Is that a thing? Tilting the wings doesn't seem to work, help?

[Jovus]

Have you tried putting the CoL [i]behind[/i] the CoM?

[Red Iron Crown]

Pic of the craft in question (in the SPH with CoL and CoM indicators on) would be very helpful in figuring out what's happening.

[ghpstage]

[quote name='Play_LORD']Even if I line up the CoM and the CoL, it flips head over heels. Why? Too much lift? Is that a thing? Tilting the wings doesn't seem to work, help?[/QUOTE] Have you accounted for the drag on your wings?
If its a classic biplane with wings connected straight to fuselage and a second set of wings above that then the drag on the upper set will cause a torque that will try to turn the nose upward.

[Pecan]

Drag was also my thought, but more simply that it could have the CoD ahead of CoM, which will make it want to flip even if the wings are balanced vertically.

Edit for posts below: An amusing/bemusing selection from Google images - [url]https://www.google.co.uk/search?q=push-prop+biplanes&espv=2&biw=1680&bih=987&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwid-6CF66HJAhULXhQKHT_CAcYQsAQINg[/url] Edited November 21, 2015 by Pecan

[Alshain]

Biplanes are typically front end props, with all their engine mass up front. If you are building stock, balancing the CoL may be very tricky.

[Red Iron Crown]

[quote name='Alshain']Biplanes are typically front end props, with all their engine mass up front. If you are building stock, balancing the CoL may be very tricky.[/QUOTE]
That's a good point. A Wheesley in the nose with the thrust reversed is probably the way to go if trying to make one that looks like a traditional biplane.

[ElMenduko]

What do you mean they don't work? I've made a jet biplane work, but it had its engine on the back.

What's difficult to make is a traditional biplane, with a front-mounted propeller engine. Balancing the CoL and CoM is difficult because most engines weigh a lot (even for propeller monoplanes, sometimes the wings almost have to touch the propeller to balance it properly).

By the way, is there any way to see where the Center of Drag is?

[BenCushwa]

This is one of those instances where a "Center of Drag" indicator would be handy. Even if your CoL, CoT, and CoM are fine, if your CoD is offset too far in one direction, it'll flip your plane over just as quickly as any of the others. Try offsetting CoM/CoL/CoT to compensate for what I'm guessing is your offset drag.

EDIT: [quote name='ElMenduko']By the way, is there any way to see where the Center of Drag is?[/QUOTE] Heh. We were just thinking of a CoD indicator at the same time. Nice.

[Alshain]

[quote name='Red Iron Crown']That's a good point. A Wheesley in the nose with the thrust reversed is probably the way to go if trying to make one that looks like a traditional biplane.[/QUOTE]

Ok, now you have given me ideas. Good job :P

OP, there is also [URL="http://forum.kerbalspaceprogram.com/threads/76668-1-0-5-Kerbal-Aircraft-Expansion-%28KAX%29-v2-5-3"]KAX[/URL] and [URL="http://forum.kerbalspaceprogram.com/threads/24551-Firespitter-propeller-plane-and-helicopter-parts-v6-1-1-%28December-23rd%29-for-KSP-0-23"]Firespitter[/URL] if you like mods. Bear in mind I'm not suggesting it can't be done in stock (since you haven't said much I don't know that you actually are trying to do it in stock) but it's going to be harder if you use a rear-mounted engine to get the same 'look' from the wing position.

[pincushionman]

Because we don't have the parts to pull the CoM far enough forward most of the time. Real biplanes have "puller" propellers and engines [i]way[/i] at the front.
*EDIT* Ninja'd. Wow, I'm wtiting slow today.

[Pecan]

[quote name='pincushionman']Because we don't have the parts to pull the CoM far enough forward most of the time. Real biplanes have "puller" propellers and engines [i]way[/i] at the front.
*EDIT* Ninja'd. Wow, I'm wtiting slow today.[/QUOTE]

LOL, and missing all the push-prop designs possible :-)

[cantab]

Stock aerodynamics ignores interaction between wings, so there's no reason a biplane shouldn't work. The usual rules apply - CoL somewhat behind CoM, the easiest design is with a separate rear tailplane and tailfin, and make sure the plane can pitch up while on the runway to get the lift to take off.

As others mentioned if you're making a replica you'll have a job getting the weight up front. You'll need the engines up front - maybe try a couple of Junos. Most of the fuselage should be empty, keep the fuel up forward too. And if need be you can use ore tanks to hold ballast.

[Snark]

So there's something I recall reading about biplane designs that I'm curious whether it works in KSP (i.e. whether the aero model is complex enough to reproduce the effect, and/or make it worthwhile):

One of the cool things that a biplane can do is a subtle piece of design that makes them more stall-resistant by giving better stability on the pitch axis at low speeds. They mount one of the two wings (I think the top one) slightly forward of the other, and also tipped slightly up to give it a higher AoA.

What that does is this: when the plane starts approaching a stall situation (where the AoA climbs too high for the wing to generate lift): the [I][U]front[/U][/I] wing stalls first (because of its slightly higher AoA). Since it's the front wing losing lift first, that moves the CoL aft, which tends to pitch the plane down, which corrects for the stall.

Has anyone tried building this in KSP? Does it work?

[Alshain]

[quote name='Snark']So there's something I recall reading about biplane designs that I'm curious whether it works in KSP (i.e. whether the aero model is complex enough to reproduce the effect, and/or make it worthwhile):

One of the cool things that a biplane can do is a subtle piece of design that makes them more stall-resistant by giving better stability on the pitch axis at low speeds. They mount one of the two wings (I think the top one) slightly forward of the other, and also tipped slightly up to give it a higher AoA.

What that does is this: when the plane starts approaching a stall situation (where the AoA climbs too high for the wing to generate lift): the [I][U]front[/U][/I] wing stalls first (because of its slightly higher AoA). Since it's the front wing losing lift first, that moves the CoL aft, which tends to pitch the plane down, which corrects for the stall.

Has anyone tried building this in KSP? Does it work?[/QUOTE]

Worked with FAR in 0.90. Haven't tried since. They took away the wing that was best for Biplanes. You can still use the modular one, but it doesn't look as good lol.

[NathanKell]

Since KSP wings do stall now, that trick will indeed work.

[Scarecrow88]

Biplanes work just fine.

[IMG]http://www.88qv.com/KSP/Bipe02.jpg[/IMG]

[IMG]http://www.88qv.com/KSP/Bipe03.jpg[/IMG]


As this is stock, it does use a jet engine at the back, which may help resolve some of the CoL/CoM issues mentioned above.

[numerobis]

One thing is that your CoL depends on your angle of attack. I've built planes that were perfectly well balanced when flat, but were terribly unstable: as soon as I pulled up I'd have CoL ahead of CoM.

[pincushionman]

[quote name='Scarecrow88']Biplanes work just fine.
...
As this is stock, it does use a jet engine at the back, which may help resolve some of the CoL/CoM issues mentioned above.[/QUOTE]
What did you do to move the center of mass forward? Do you just have that much fuel at the front?

[GDJ]

I've had plenty of Biplanes work. They work very well. I put the COL above the COG, and COT right in the middle. Firespitter's Biplane does this, and eventho' it's slow, it's very VERY maneuverable and a blast to fly in and out of the KSC buildings. :)

[Scarecrow88]

[quote name='pincushionman']What did you do to move the center of mass forward? Do you just have that much fuel at the front?[/QUOTE]

I can't remember for definite, but I seem to recall placing four of the cylinder shaped RCS tanks just behind the air intake at the front. At least I think it was this plane that I did that on. I'm off to bed now, but will have a look in the morning to confirm.


ETA: Just checked the design, and I actually used 6 of the cylindrical RCS tanks to move the CoM forward. Edited November 23, 2015 by Scarecrow88

[swjr-swis]

[quote name='Scarecrow88']Biplanes work just fine.[/QUOTE]

Agreed that biplanes work fine. Even in tiny configurations.

Minimal biplane:
[URL="http://imgur.com/a/x3a4s"][imgur]x3a4s[/imgur][/URL]


Still tiny, but a bit more elaborate:
[URL="http://imgur.com/a/2Xr5s"][imgur]2Xr5s[/imgur][/URL]

A blast to fly too! Edited November 24, 2015 by swjr-swis
Just learned about the imgur tags...

[Snikersnee]

Ok, sorry for not responding. I didn't get any notification that anyone replied, so i didn't bother checking. Pretty sure the problem was center of drag, and with a little tweaking, i fixed the problem, so thanks.