Angle of incidence?

[Arugela]

I have a big cargo plane. I'm trying to get it to orbit. (technically have in the past before changing wings.) I changed the wings so the front tips are at the original location and the back of the wings is slightly higher in elevation and are aiming slightly towards the ground(The front of the wing points down.). Is this a negative or positive incidence? Or does the angle have to be relative to do with the wings location above or below the center of the craft?

What is this wing design supposed to do in general? I was hoping it would help keep the angle of attack lower to get into orbit at higher speeds from reduced drag. Not sure if incidence does this at all now. I think it help keep the plane nose up though. It has some interesting effects like auto keeping the nose up during reentry and helps the plane auto slow down without inputs which is nice. Not sure on the rest.

I have the wings angled so the back of the wing is above the front of the wing by the smallest possible margin. The tail wings are then in the opposite direction, I think, as to counter the COL adjustment so it's directly behind the COM. The COL ball is directly touching the back of the COM ball like they are kissing. It actually has decent control as far as I'm used to.

Wether I look up positive or negative incedence they all use diagrams of a plane with the front of the wing angled up towards the air... Can't find good info on the opposite and the terminology and lack of clarity as to it's meaning is extremely confusing and frustrating.

From what I've found I am referring to the wing mounting incidence. Although this game does not flex the wings much unless I'm missing something.

Edited February 25, 2020 by Arugela

[Guest]

That's a negative angle of incidence and it will make your plane stick to the ground. Take-offs will be very tricky and if and when you get into the air, you'll need to nose up a lot to stay there, which is un-aerodynamic. Wings should have a positive angle of incidence, i.e. the leading edge is higher than the trailing edge. This means that when you're cruising, your fuselage will be level and cause the lowest possible amount of drag.

To tune your angle of incidence, pick the cruise speed you're targeting (or, for an SSTO, a good average of the part you're accelerating through the thicker atmosphere). Then tune the angle of incidence so that your nose stays pointed exactly prograde for as much as possible. If your nose is continuously above prograde, it means you need to increase your angle of incidence; if it's below it, you went too far and need to decrease it.

[Arugela]

Does negative incidence help at high speeds at all. I"m doing an SSTO.

I think the negative incidence help with making body lift while getting out of the atmosphere as it point you up. And my plane has a very lift heavy body.

Edited February 25, 2020 by Arugela

[XLjedi]

I tend to look for minor adjustments to elevator AOI when at speed.  Then there's a tradeoff for how much and when you want the flight to be leveled.  Some would choose to level out at max speed, others might go for a middle-of-the road assignment and just let the SPS smooth out the difference.  So depends purely on your own preferences.  You can also create your own trim tabs in the form of a slight deployment setting for an elevator or aileron.  I've managed it all any number of ways in the past.

[AHHans]

47 minutes ago, Arugela said:

Does negative incidence help at high speeds at all. I"m doing an SSTO.

No.

You need a positive angle of attack on the wings for the wings to generate any lift. (At least in KSP, which is rather simpleminded in this regards.) Having a slightly positive angle of incidence on the wings can help you reduce drag by giving you that positive angle of attack on the wings while the main body of the craft is pointed straight into the direction of motion and thus has the least amount of drag.

At higher speeds you need only a smaller angle of attack in order to have the wings generate enough lift to keep your plane in the air. So, indeed, the optimal angle of incidence for a high-speed aircraft is smaller than for a low(er) speed aircraft. But in both cases it is positive.

1 hour ago, Arugela said:

The COL ball is directly touching the back of the COM ball like they are kissing.

That is actually somewhat risky. The CoL display in the editor shows only the center of what KSP considers lift, not the sum of all aero-forces. And the latter are what is important for stability. For "slow and low" flying aircraft that is usually fine, as the "lift" forces dominate. But for "high and fast" craft the drag becomes more and more important. So if you have lots of parts with more drag than lift - like your typical fuselage sections - in front of the craft and the lift-generating wings more at the back of the craft, then the actual center of the aero-forces will move forwards when you get higher (into thinner air) and faster. I'm not saying that this is a problem for your craft, but it is something to look out for. (AKA remember if your craft does become unstable and flip out when you get into thin air.)

[Snark]

3 hours ago, Arugela said:

I have a big cargo plane.

Could you post a screenshot?  It's pretty hard to make judgments about things without being able to see how they're arranged.  Picture worth a thousand words, and all that. 

[Lt_Duckweed]

At hypersonic velocities, wings produce the optimum ratio of lift to drag with a positive angle of incidence of 5°.  Any more or less than this will give you a worse lift to drag ratio.  Your goal when flying an SSTO is to always have the fuselage as close to 0 angle of attack as possible, and have the wings as close to 5° as possible.

This is an example of a drag optimized SSTO, and thanks to the 5° wing incidence it can get to orbit with no oxidizer. https://www.imgur.com/gallery/fTWS13F 

Edited February 27, 2020 by Lt_Duckweed

[fourfa]

I'd agree with the above - there's a very simple method, which is set all main wings parts to +5 deg AOI.  If that's not enough lift, add wing segments at the same +5.  If it's too much, remove some.

Keep elevator surfaces level (if we're keeping it simple).  It's nice for handling if the main wing stalls before the tail.

If you need more wing partly because it's significant fuel storage (Big-S segments) it's OK to tweak around with 3-4 degree AOI, IMHO. But optimizing involves a lot of repeatable test flights and can be boring (for some) or relaxing and interesting (for me)

[TrueNorth]

How do you tweak AOI in increments less than 5 degrees?

Edited November 15, 2021 by TrueNorth
1 to 5 degrees

[18Watt]

18 hours ago, TrueNorth said:

How do you tweak AOI in increments less than 1 degree?

Turn off Angle Snap in the SPH.  It's at the lower left corner in the SPH, and normally looks like a hexagon.  Click on it to make a circle, this means Angle Snap is off.

With Angle Snap ON, rotations occur in 15 degree increments.  If you hold left-shift, the angles change in 5 degree increments.  So the smallest angle you can achieve is 5 degrees, which is usually way too much.

With Angle Snap OFF, rotations can happen at one degree increments (maybe less).  With most spaceplanes about 2-3 degrees positive angle of incidence is plenty.  This will make the plane more efficient at high-Mach speeds (3-5), where you want to accelerate rapidly.  It is not absolutely necessary however- all of my spaceplanes get to space just fine without positive angle of incidence.  They just do a little better with a little positive AOI.

[TrueNorth]

Turning off Angle Snap just makes everything not snap, and my parts still turn in 90 degree or 5 degree intervals. Am I doing something wrong?

[OHara]

Turning off Angle Snap and entering the rotation mode (keyboard shortcut '3') and then dragging the red/green/blue circles with the mouse should work for you, if you play on PC (and I think the UI and general idea is the same on console with different input mechanism).

[Arugela]

The problem with the mouse is that once you change the craft there is no going back. We need access to exact values with an input like they added more recently to other parts of the game. If you need to go back and change things later it starts to mess the craft up when you adjust things later. Particularly after coming back from a save of said manual angle adjustment. We really need a gui with values we can input into showing what we are doing and what we already have.

Edited November 16, 2021 by Arugela

[fourfa]

For that problem (more  control and precision) I use the mods Editor Extensions (allows any angle snap you like; I usually have 1 / 5 / 15 degree options active) and Part Angle Display (high precision readout of part angle, simple!)