Is adding wing incidence an advanced technique ?

[AeroGav]

I'm trying to put together a spaceplane tutorial.  I'm trying hard to keep things simple as possible, and not over engineer the sample craft I'm planning to use.

Looking through KerbalX and the forums,   the kind of aircraft most first timers try to create is based on mk2 fuselage parts, which have very high drag.

Should I have people use wing incidence right away, or is that likely to set up traps for the unwary or make them just switch off thinking "this is too hard !" ?

My initial plan is to broach the subject of wing incidence in an appendix , showing how the craft's performance can be improved.

I'm just warning people about the drag and telling them to keep the mk2 fuselage as short as possible, storing what can be stored in the wings or mk1 parts there instead, and to  fit large wings , and try to fly as high as possible whilst  keeping AoA low.

Test flights of the flat-winged craft show that it reaches orbit ok, but doesn't  quite have the fuel to reach Minmus,  and it doesn't accelerate with much enthusiasm.  Adding payload or extra fuel would make it hard to get to orbit, so it has to stay under 30 ton gross.      

 

Adding incidence halves the craft's drag, but makes the blue CoL indicator act in a disconcerting way , and means you got  to start talking about mods like editor extensions or CorrectCoL.  You also have to warn people about applying the danagers of  having a  canard at lower incidence angle than the main wing, causing main wing to stall first and flip out of control

 

[katateochi]

Learning about wing incidence was a big aha moment for me, but I found out about it quite late in my kerbaly areo endeavours. I'd say it is an "advanced technique" and as you can get by without it, it's probably best for new builders to focus on the simpler things first. That way they can get familiar with how to balance the craft properly and learn about the other ways to reduce drag first. Then bring it in as a later topic and show, on a craft that's already been well optimized, how much of an improvement it makes.  

One thing I don't feel like I have a guide for is how much to angle the wings. It feels like a bit of a dark-art and having some guidelines as to how much angle is needed would be very helpful. 

[GluttonyReaper]

Yeah, I'd say it's pretty advanced, and you can get by without reasonably okay if you don't use Mk2 parts. My suggestion would be to recommend not using Mk2 parts... but include a link to a tutorial on wing incidence, with a disclaimer that it's a bit more advanced. That's what I would like to see in a tutorial anyway

[SelectHalfling0]

This is the second time I've ever heard of wing incidence. I would like to see a wholesale guide on it too.

Also came across a neeto little article about the Corsair-2 Jet, which has variable incidence.

[AeroGav]

18 minutes ago, GluttonyReaper said:

My suggestion would be to recommend not using Mk2 parts..

YUP..

 

Inline Cockpit generates 3x as much drag as the big S delta wing, as does short cargo bay etc,

They generate lift yes, but only one tenth as much as the big S wing, for 3x the drag...

Edited September 12, 2017 by AeroGav

[Xavven]

IMHO, wing incidence is such a fundamental part of aircraft design that it should absolutely be included in any plane or spaceplane tutorial. Regardless of whether you use wing incidence, you already have to get the COM and COL in the right places, and you already have to explain AOA as a basic piloting concept. You need to explain how the COM shifts as fuel is burned and how this can affect your plane's stability, trim, and control authority, as it does to planes IRL. Plus, planes with no wing incidence have to pitch up considerably to fly straight and usually have their wing roots in silly places -- they don't even look like real planes.

People are probably going to turn to tutorials when their first spaceplane attempts fail, so you need answers to common problems, such as inability to take off, plane flipping uncontrollably,  tendency to turn around and fall backwards when reaching high speeds and altitudes, inability to pitch up, etc. All of these things can be helped or fully corrected with incidence.

[Snark]

I think the right approach to take would be to include it, but put it in a section at the end (after the absolute must-have sections), and clearly message it at the start of the section thus:

1. "This is what wing incidence is."  (illustration)
2. "You don't actually have to do it.  You can build planes just fine without it, if you want."
3. "Doing it correctly takes a little bit of fiddling, but can significantly improve your performance."

Organization of info in a tutorial is crucial.  Spaceplanes are complicated, and I expect that a good tutorial for them will end up being quite long and will need an awful lot of detail.  It's very easy for something that big and complex to overwhelm and frighten off the uninitiated.  "Holy smokes, that looks hard!  Never mind!"  Proper organization can make a world of difference:  it improves the "accessibility" of the information.  It becomes easier to read and less frightening, while still presenting the same total amount of information.

My suggestion:  Organize it in prominent sections.  Start with an "overview" section at the beginning, which includes a short bullet list of the other sections, with a brief phrase describing each.  At the start of each section (e.g. "Choosing an Intake" or whatever), have a brief paragraph explaining what the section teaches you to do, and (importantly) how necessary this section is (i.e. "you'll need to understand this or else <bad thing>", or "you can skip this when you're getting started, but it's useful later to improve your performance", or whatever).

[regex]

2 hours ago, katateochi said:

One thing I don't feel like I have a guide for is how much to angle the wings. It feels like a bit of a dark-art and having some guidelines as to how much angle is needed would be very helpful. 

It kind of is a "dark art" in that you have to edit and roll out the plane several times before you get it right. How much angle is entirely dependent on the craft, too.

[AeroGav]

50 minutes ago, Xavven said:

Plus, planes with no wing incidence have to pitch up considerably to fly straight and usually have their wing roots in silly places -- they don't even look like real planes.

The problem is that it does mess with the appearance of mk2 designs.    A big feature of them is the wing/body blending,  and the fuselage includes a chine to snap the wing segment on to.   If you angle the wing upward at 5 degrees,  it gives the plane a "broken" appearance.   I sometimes get comments like , "Its great but it looks a bit rough and unfinished, can't you line the pieces up better" etc.

On a mk1 it's less of a problem, except that passengers can't always see out the windows

[Snark]

7 minutes ago, AeroGav said:

I sometimes get comments like , "Its great but it looks a bit rough and unfinished, can't you line the pieces up better" etc.

Yep.  I suppose in a tutorial section that talks about wing incidence, it should probably include a little disclaimer that "this can make your planes look a little odd sometimes, but basically you have to choose which is more important to you: looking pretty or flying well".

I think it's important to call that out specifically, because it does a service to the reader.  Not everyone has the same priorities.  Some people really care about looking clean and beautiful, and would be willing to pay a performance hit to achieve that-- others don't give a wet slap about looks and just want to get the biggest payload fraction or highest speed possible.  By explicitly calling out "here's what the tradeoff is", it lets the reader choose for themselves what's important to them.

[maceemiller]

I am useless at building planes of any description.....flying them im ok but building them is hard. Would like to see your guide.

[regex]

50 minutes ago, AeroGav said:

The problem is that it does mess with the appearance of mk2 designs.    A big feature of them is the wing/body blending,  and the fuselage includes a chine to snap the wing segment on to.   If you angle the wing upward at 5 degrees,  it gives the plane a "broken" appearance.   I sometimes get comments like , "Its great but it looks a bit rough and unfinished, can't you line the pieces up better" etc.

Have you tried clipping smaller surfaces into the body to control incidence or using, say, the chines to provide that? Outboard wing surfaces? Canards providing the incidence? I imagine there are lots of ways around this so you can retain the blending and/or not cover up windows.

E: Since that seems like the real issue you're running into...

Edited September 12, 2017 by regex

[Foxster]

3 hours ago, regex said:

Have you tried clipping smaller surfaces into the body to control incidence or using, say, the chines to provide that? Outboard wing surfaces? Canards providing the incidence? I imagine there are lots of ways around this so you can retain the blending and/or not cover up windows.

E: Since that seems like the real issue you're running into...

Yup, good points. 

Providing this "feature" to an aircraft is not necessarily just about adjusting the wings. 

[The Space Dino]

SSTO noob here, I will check your guide when it's done.

Ah, don't use mk 2 parts? Noted. Btw during some flight I've noticed that mk 1 crew cabins have quite an amount of lift.

[AeroGav]

1 hour ago, The Space Dino said:

SSTO noob here, I will check your guide when it's done.

Ah, don't use mk 2 parts? Noted. Btw during some flight I've noticed that mk 1 crew cabins have quite an amount of lift.

Well ,  the guide, or first iteration of it at least , is up.

I've tried to include pictures so it's not just a wall of text.

Previously when attempting this kind of thing, I've started from "first principles", but it's such a massive subject, I never finished writing the damn thing before it became obsolete,  and i'm not sure anyone would want to read such a gigantic essay in any case.

To make it easier for myself,  I limited the scope of this one 

• to a specific type of ssto i'd seen a few people struggle with recently
• assumed a higher level of starting knowledge with the reader, or that they are prepared to google / wiki basic aeronautical terms like "angle of attack" and "mach number".    I'm assuming they know how to construct basic rockets and aircraft already,  and that they are aware of the theory behind spaceplanes - get most of your velocity on jet engines , and that jet thrust declines with altitude and varies with airspeed.

Can always add a glossary to explain the basics later.

[Helmetman]

I like to build planes with incidence. But I don't like to build spaceplanes with wing incidence.
The thing is, when I go really really fast, inclined wings will somehow create a lot of drag, while it should reduce it right?

Simple planes (15km and below under Mach 1.0) have increased speed and range due to minimized drag.

But when at full speed @ Mach 3+ they start to act like airbrakes. Planes that once could reach orbit now get limited to a lower top airspeed in the upper atmosphere. My brain tells me it is the increased lift of the fuselage at speed.
The fuselage has increased lift at higher speed I think it is the incidence + additional AoA which is usually high in the upper atmosphere that increases the drag at greater speed where it would not with 0 degree inclined wings. Yet, people still create spaceplanes with inclined wings right? How do you manage? And where do I fail?

 

 

[AeroGav]

21 minutes ago, Helmetman said:

I like to build planes with incidence. But I don't like to build spaceplanes with wing incidence.
The thing is, when I go really really fast, inclined wings will somehow create a lot of drag, while it should reduce it right?

Simple planes (15km and below under Mach 1.0) have increased speed and range due to minimized drag.

But when at full speed @ Mach 3+ they start to act like airbrakes. Planes that once could reach orbit now get limited to a lower top airspeed in the upper atmosphere. My brain tells me it is the increased lift of the fuselage at speed.
The fuselage has increased lift at higher speed I think it is the incidence + additional AoA which is usually high in the upper atmosphere that increases the drag at greater speed where it would not with 0 degree inclined wings. Yet, people still create spaceplanes with inclined wings right? How do you manage? And where do I fail?

 

 

I kind of addressed this question in the last part of my guide -

Theory - Choosing an Incidence angle

Optimal supersonic lift/drag ratio occurs at about 5degree AoA.     So, we could angle our wings up at 5 degrees,  then just put the nose on prograde hold so the body is at 0 AoA and the wings are at their optimal 5.      This would give great efficiency during the closed cycle mode part of the climb,   but as you remember from part 6b, there are times when this produces far too much lift.        During the speedrun, we were trying to keep our altitude below 17km.   When passing through mach 1, we were trying to shallow dive.    

On the basic version of the aircraft, AoA was as low as 1.5-2 degrees in parts of the speedrun.   If our wings had 5 degrees built in incidence,  we'd actually have flown with the body at an AoA of more than 3 degrees nose down.

So, the ideal figure is probably a compromise. You could argue that 3-3.5 degrees of incidence is probably the midpoint between our AoA in the speedrun and that used during the close cycle climb,.  Whatever value is chosen between 0 and 5,  it is guaranteed the aircraft will perform better with some incidence, than with none.

 

I think what might be happening is that during the speedrun , you're getting too much lift and so are having to aim the nose below prograde to stop the plane climbing.   The body ends up with a large negative aoa and this creates drag.    Or you're simply bouncing up to altitudes where your jet engines run, but no longer produce enough power to hit top speed.    It do my speedrun in level flight at 15-17km on Whiplash engines, with RAPIERs i'll do it at 20-22km.

Once you go closed cycle mode, you no longer have any reason to hold the plane down deeper in the atmosphere than it wants to  go.   Set prograde hold on SAS for minimum drag  and let the wings, and their incidence, lift you out of where all the air resistance is.    BTW I mostly do oxidiser - free space planes, that get their "closed cycle/rocket mode" thrust from NERVs.    The TWR is somewhere between 0.3 to 1 and 0.6 to 1,  so you need optimal lift/drag ratio to get to orbit .    Hard to make this work without using incidence on a draggy mk2

[Helmetman]

@AeroGav Thanks. I'm a little new here or else I'd probably read your guide already.

I never use Mk 2 so I can't say much about it. It is either Mk 1 or Mk 3, because I can adhere to the drag of Mk 2 fuselages. < That much I can say.
Now you were spot on about the negative AoA. I tackled this by slowing down initially to lower the prograde marker. Good to know what is causing this now. 

Thinking about this. Maybe if I fly upside down into space I get positive AoA and a better ascent profile 

I would only have to finetune the incline to get balance right for my specific ascent profile if it's the only thing causing this. Gonna test this now
Wish I thought of this before...

 

[Dark Lion]

Is this a spaceplane tutorial, or hyper-efficiency spaceplane tutorial? I can't honestly say I knew what you're on about when I first read this, @AeroGav... In my short time in KSP I've launched multiple SSTOs of each modular size (but never more than 120-ish parts, due to CPU suffering) only one of which actually needs the use of wing incidence as I understand it. It's a Mk2 "Batwing SSTO," which I fashioned so that it resembles the fast-travel jet from the Arkham City game (maybe Arkham Origins, I don't remember.) The forward canards are still at 0° deflection in the hangar, but deployed fully on takeoff. As speed increases, I use the authority limiter to manipulate that specific angle incrementally as such so that it aids the climb to my speed-run and falls back to 0° just before reaching the critical closed-cycle portion of flight. A minimal angle of attack handles all else hence to space. 

Having made me aware of the literal difference between "wing incidence" and "angle of attack," I thank you for your indirect enlightenment, but must encourage you to be a little more explanatory when you expect people to participate in your poll. Especially if you aim to present a tutorial with the result. I read through the thread, then still had to Google "wing incidence" just to get on your level.

You've spoken ill of my beloved Mk2 sexiness being such a drag... I can't help but insist that you try it for yourself. Perhaps you'll return with a link to the tutorial of how to make it more efficient? Or prehaps to demonstrate how to build the same craft, closer to replica, and without Mk2/hella-draggy parts? I'm sure you're busy, but when you find the time:
kerbalx.com/DarkLion/Batwing-SSTO

[AeroGav]

9 minutes ago, Dark Lion said:

Is this a spaceplane tutorial, or hyper-efficiency spaceplane tutorial? I can't honestly say I knew what you're on about when I first read this, @AeroGav... In my short time in KSP I've launched multiple SSTOs of each modular size (but never more than 120-ish parts, due to CPU suffering) only one of which actually needs the use of wing incidence as I understand it. It's a Mk2 "Batwing SSTO," which I fashioned so that it resembles the fast-travel jet from the Arkham City game (maybe Arkham Origins, I don't remember.) The forward canards are still at 0° deflection in the hangar, but deployed fully on takeoff. As speed increases, I use the authority limiter to manipulate that specific angle incrementally as such so that it aids the climb to my speed-run and falls back to 0° just before reaching the critical closed-cycle portion of flight. A minimal angle of attack handles all else hence to space. 

Having made me aware of the literal difference between "wing incidence" and "angle of attack," I thank you for your indirect enlightenment, but must encourage you to be a little more explanatory when you expect people to participate in your poll. Especially if you aim to present a tutorial with the result. I read through the thread, then still had to Google "wing incidence" just to get on your level.

You've spoken ill of my beloved Mk2 sexiness being such a drag... I can't help but insist that you try it for yourself. Perhaps you'll return with a link to the tutorial of how to make it more efficient? Or prehaps to demonstrate how to build the same craft, closer to replica, and without Mk2/hella-draggy parts? I'm sure you're busy, but when you find the time:
kerbalx.com/DarkLion/Batwing-SSTO

What does it look like ?  The craft you're trying to recreate ? I'm not familiar with the game.

It's hard to make changes to your craft without knowing that,  because i can't judge if any visual changes are acceptable

[Dark Lion]

My version: https://imgur.com/DisQ1LR
The original: https://imgur.com/3cvdeRU

@AeroGav I'm sure it's obvious by the pics here, visual changes were necessary to make the small jet into an SSTO, so of course they're acceptable, given they still maintain the wing design and general shape. I did the best I could to make my version as "kerbal" as possible, while still being a viable SSTO. (There's a tiny docking port to refuel in LKO and science suite in the cargo bay, not to mention room for 5...)

KerbalX again, since the last one didn't link proper: https://kerbalx.com/DarkLion/Batwing-SSTO

 

Edited September 13, 2017 by Dark Lion

[AeroGav]

17 minutes ago, Dark Lion said:

My version: https://imgur.com/DisQ1LR
The original: https://imgur.com/3cvdeRU

@AeroGav I'm sure it's obvious by the pics here, visual changes were necessary to make the small jet into an SSTO, so of course they're acceptable, given they still maintain the wing design and general shape. I did the best I could to make my version as "kerbal" as possible, while still being a viable SSTO. (There's a tiny docking port to refuel in LKO and science suite in the cargo bay, not to mention room for 5...)

I had a quick look at your vessel, RCS build aid says you did a good job of balancing the fuel tanks (CoM doesn't move one inch full to empty) but CorrectCoL thinks your plane is unstable in pitch.         Well,  if i really want to go to town,  I might use a lot of mk1 or 2.5m parts (the latter especially, are really good for drag) then cover them in wing panels. Yikes

 

Edit - I don't mean to be rude, but have you considered a nose job ?

mk2 to mk1 adapter, into 2 mk1 cockpits, then a heatshield and a fairing

after using the offset tool to move the cockpits back into the adapter and clip the bubble canopies into each other a bit..

This has no effect on drag, need to see if that's actually any less drag than what you had before though.

Edited September 13, 2017 by AeroGav

[Dark Lion]

@AeroGav, admittedly, this one SSTO took me a full day worth of experimenting to get balanced like that. Fuel dispersal was indeed a serious issue in the first few iterations, but was ultimately overcome by simply extending the nose a bit. No part of the wings deviate from the longitudinal plane, but the dihedral wing (is it still called dihedral if the slope angles down? Inverse dihedral?) facilitates the need of those forward canards to keep the nose level on takeoff. The only mod used in its making was Kerbal Engineer Redux, so I'm not sure what CorrectCoL's beef is with the pitch... The only issue I've had there stems from poor piloting. High-altitude flight isn't the best place for acrobatics you actually control LOL! but maybe that has something to do with it? If you can, would you mind showing me an image of what exactly CorrectCoL shows you on the Batwing? I play KSP on a laptop, so I can't really afford the RAM to experiment with all these great mods just yet.... Love to know if what I'm missing is really worth the RAM and will help me build things right on the first go...

[AeroGav]

@Dark Lion 

Actually it's not as bad as i thought.   It is fun to fly because the handling is so neutral, it's not always forcing you back to prograde and stopping you having fun.   But  I did manage to get into a deep stall and crash it.

The line should slope downhill the whole way.   The green line is kind of flat from -15 AoA to +10 AoA so there's no tendency to push you back onto prograde.   Test flying, it seems to settle at 6.5 AoA left to itself but if you cut the throttle in level flight it does not go into a shallow glide maintaining same AoA - instead it holds the nose level with the horizon while the flight path drops and AoA increases, till it stalls.      I wouldn't worry too much.

[AeroGav]

27 minutes ago, Dark Lion said:

but the dihedral wing (is it still called dihedral if the slope angles down? Inverse dihedral?)

That's called "Anhedral"

I reckon you can get away with a bit of anhedral (it reduces roll stability) because you have a lot of dihedral on those huge v tails.

To my eyes though the anhedral is on the top surface of the inboard wing - it's a blended wing / body design, but the bottom surface is flat and the top surface slopes down, rather than having the top flat and the bottom slope up.

The worst aerodynamic problem with making a 100% faithful replica, is that centre of lift is waay to the back.       Sure, you can make the CG go back as well, but how you gonna control pitch without massive canards ?  Trailing edge elevons will be v close to CoM,  almost no lever arm to work with.