How to prevent planes from pitching down when I roll?

[Seanburg2006]

Whenever I build an aircraft, it always begins to pitch down when I roll. I was wondering if there is anything I can do to fix this?

[Boris-Barboris]

It is impossible to trim a stable plane's pitch in such a way that it would keep the level flight AoA on all speeds and altitudes, hence you either trim pitch before activating SAS and tolerate the slight dirft (or retrim) when rolling or use the mod in my signature.

Edited March 7, 2020 by Boris-Barboris

[UncleManuel]

@Seanburg2006 Depends on the airplane design. Do you have any pictures?

And it depends how far the control surfaces are apart from CoL/CoG. That said it also has to be stated that KSP is not a very accurate airplane simulation. So not everything is 100% "correct"... 

[HvP]

You should start by making sure that your control surfaces are only assigned to discrete inputs.

By default, all control surfaces have yaw, pitch and roll turned on no matter where they are located on the plane. This means that your elevators in the tail will also try to roll the plane, compromising your pitch control - and your ailerons in the wings will also try to move the pitch of the plane, compromising roll control. If you haven't already, right-click each set of control surfaces and make sure that they only control the inputs they will be best suited to. The buttons in the right-click menu will allow you to disable any input that could interfere with their function.

In conventional aircraft designs this means that the ailerons at the wingtips only have roll enabled; the rudder in the vertical tail fin only has yaw control enabled; and the elevators in the horizontal tail stabilizers only have pitch enabled. There are designs such as delta wings, flying wings, and v-tail designs that need to mix these controls, but that will realistically mean some compromises.

Edited March 7, 2020 by HvP

[AHHans]

15 hours ago, Seanburg2006 said:

Whenever I build an aircraft, it always begins to pitch down when I roll.

I guess you are using SAS to keep the plane steady during "normal" flight (when you are not actively changing the direction with control inputs) and that your plane would pitch down on its own without SAS. (I think so because that's the most "natural" way to design a plane in KSP.(*)) When you have SAS active in "Stability Assist" mode, but give control inputs to change the course (e.g. press <Q> or <E> to roll) then SAS will temporarily deactivate to allow your inputs to control the craft. That will temporarily remove the pitch-up control input from SAS that kept the plane from pitching down, allowing the plane to pitch down while rolling. It will also "reset" the reference position that SAS uses to determine how much control input it generates and thus the plane needs to pitch down some more before SAS generates the pitch-up control input needed to keep the plane steady.

16 hours ago, Seanburg2006 said:

I was wondering if there is anything I can do to fix this?

You would need to trim the plane so that it will fly level without SAS. One way you could do this (since KSP 1.8) is to deploy the elevators and adjust the deploy angle so that the plane flies level without additional pitch input. But as @Boris-Barboris said: the trim setting depends on your flight condition and varies with airspeed and other parameters.

P.S. (*) I'm too lazy right now to explain why that is the case.

[Rocketology]

All aircraft, IRL and KSP, will pitch down in a roll unless speed is increased to increase lift. When the wings are parallel they are directly countering gravity, but as you roll/ bank the aircraft, your wings are no longer aligned directly against gravity, so you have a loss of lift against it, as you are now using some of the lift to pull the aircraft through the turn. 
IRL one of the first things a new pilot must learn is the coordinated turn, which is used to reduce side slip and  the pitching effect using the rudder and elevators, but will still need to increase throttle to maintain level flight. In KSP, using SAS, a coordinated turn isn't possible, even if trimmed as SAS cancels trim while active (SAS is OK for rockets, but sucks for planes).
And to have planes behave more realistically, turn off reaction wheels and set control surfaces "correctly". And in most cases you won't need 100% authority - you can tune these as needed with a few test flights. @Boris-Barboris has one of the best mods for aircraft out there, Atmospheric Autopilot, which you can set to actually do coordinated turns! Pair this with Kerbal Flight Indicators (light weight Head's Up Display for aircraft) and the sky is your oyster!
Last thought, if you enjoy building and flying planes in KSP and want to understand more, try out a good flight sim - it'll make understanding of flight easier and you'll make better planes in KSP and enjoy flying them that much more!