all aircraft pitch nose down

[thevegimobil]

Hi,

so everytime i make something designed for horizontal flight i always need to add a forward control surface or the craft pitches nose down all the time. This is really annoying trying to build cool aircraft that all have to have a forward canard or something.

I know the aerodynamics has been off from the start but i was wondering if there is a way around it, FAR perhaps?

Thanks

[CosmicScientist]

Are you looking at the Center of Mass or Center of Lift when building your airplanes? They need to be lined up to avoid the craft flopping around.

If not that, can you give a screenshot of your aircraft?

[Baconluver25]

This just means that your center of lift is behind your center of mass, you want the center of life to be just inside the center of mass, but still behind it.

[katateochi]

Baconluver25 beat me to it! yeah, what he said!

Not trying to say thou that planes are easy thou, if you plotted the frequency of me cursing it would show an upward trend along the length of the runway.

[Bigcheecho]

This just means that your center of lift is behind your center of mass, you want the center of life to be just inside the center of mass, but still behind it.

I always thought CoM in CoL would result in chaos.

[Baconluver25]

I always thought CoM in CoL would result in chaos.

I always get the center of lift inside the bubble of center of mass, but also still behind it, that usually works out well.

[loknar]

If you're serious about aerodynamics, vanilla is no-no. Install FAR. http://forum.kerbalspaceprogram.com/showthread.php/20451-0-21-Ferram-Aerospace-Research-v0-9-5-5-Aerodynamics-Fixes-For-Planes-Rockets

[Tiron]

Are you looking at the Center of Mass or Center of Lift when building your airplanes? They need to be lined up to avoid the craft flopping around.

If not that, can you give a screenshot of your aircraft?

Er, no, lining them up perfectly is a VERY bad idea. It causes extreme instability and makes attempts to control the craft all but futile. I suppose it MIGHT work if you had really weak controls forces versus the weight of your craft, but it's still sub-optimal even if it does. Note that IRL, Jet Fighters do this sort of thing ON PURPOSE because it makes them more maneuverable...but then they have Flight Computers that are constantly making adjustments to keep them in controlled flight. The new SAS *might* have half a shot at handling it on some craft, but I suspect it's too slow and tries to correct small movements too lightly to manage it.

Center of Lift behind the Center of Mass is good for spin recovery ability but bad for maneuverability and also gives that nose down tendency. Moving the CoL closer to the CoM increases maneuverability and reduces the nose down tendency, but also reduces spin recovery capability.

Adding canards moves the CoL forward, which is why you're seeing it help.

[lammatt]

i found the stock control surfaces are kind of weak.

B9's control surfaces are a lot powerful, and u can afford the center of lift trailing quite signficiantly while still allowing maneuvers and easy take off

[espm400]

i found the stock control surfaces are kind of weak.

B9's control surfaces are a lot powerful, and u can afford the center of lift trailing quite signficiantly while still allowing maneuvers and easy take off

The stock control surfaces are very weak indeed. If you look at the following example, you'll notice the F/A-18 has stock control surfaces. It's very stable, yet not very maneuverable. The rest use B9 and are all very agile (the F-15 is incredibly maneuverable).

Here are some examples of CoM/CoL:

B-1B:

My old F-15 analogue (been updated since the pic):

As the name says, an HST testbed (it pitches up on it's own):

It's been a while, but IIRC using Alt+directional control will trim your control surfaces, allowing you to hold a pitch up attitude.

[Blaster]

Having done a lot of testing, I have noted that wing placement wreaks a lot of havoc with lift at speed.

I put together a large SSTO using a Rocko 32 tank for a fuselage and a cupoula and hitchhiker as a command section, four jets and two rockets at the back. The double wings are 2 connectors and a delta, with a delta on its own in front of that. With rearward lift it has to pitch up almost full to keep climbing at 25 deg, however, once you break 500 m/s it levels out, and by 1000 m/s it flips up and over. Every time. The lift imbalance happens with speed, as slowing down causes the plane to level out again. I tracked the problem down to having the forward wings. It does not matter if its canards or deltas. The question is, why does the plane with rearward lift bias that has to be pitched up to fly suddenly start pitching up at speed, with no amount of pitching down or RCS to keep it from going rear first?

http://twitch.tv/blastermaster555 The video there demonstrates what I am talking about

[ChevronTango]

Having done a lot of testing, I have noted that wing placement wreaks a lot of havoc with lift at speed.

I put together a large SSTO using a Rocko 32 tank for a fuselage and a cupoula and hitchhiker as a command section, four jets and two rockets at the back. The double wings are 2 connectors and a delta, with a delta on its own in front of that. With rearward lift it has to pitch up almost full to keep climbing at 25 deg, however, once you break 500 m/s it levels out, and by 1000 m/s it flips up and over. Every time. The lift imbalance happens with speed, as slowing down causes the plane to level out again. I tracked the problem down to having the forward wings. It does not matter if its canards or deltas. The question is, why does the plane with rearward lift bias that has to be pitched up to fly suddenly start pitching up at speed, with no amount of pitching down or RCS to keep it from going rear first?

http://twitch.tv/blastermaster555 The video there demonstrates what I am talking about

if its a slow gradual pitch up I find its caused by the SAS not following the curve of the planet. if its a more sudden pitch up it could be something to do with fuel weight changing as the flight progresses. Multiple tanks draining in odd orders can shift the CoM , past the CoL and cause odd behaviour not present in the flight earlier. By reducing speed you reduce lift on the wings and hence reduce the moment which may explain your phenomenon

[zarakon]

One tip that's not particularly obvious..

If you hold shift and use WASD while placing parts, you can rotate them by small increments. You can use this with your control surfaces to place them at angles such that they counter your nosedive problem

[Blaster]

if its a slow gradual pitch up I find its caused by the SAS not following the curve of the planet. if its a more sudden pitch up it could be something to do with fuel weight changing as the flight progresses. Multiple tanks draining in odd orders can shift the CoM , past the CoL and cause odd behaviour not present in the flight earlier. By reducing speed you reduce lift on the wings and hence reduce the moment which may explain your phenomenon

You would think, but if you watch the video, you will see the only fuel tank is the rocko 32, which is rearward. It should pitch down more, not less, as that tank exhausts its fuel and the plane's rear lightens up. Phenomenon 2 is a sudden inversion from pitching down to pitching up dependant on speed and related to having frontward wings like canards, despite a CoL behind the CoM