aren't Mk2 parts supposed to provide lift?

[king of nowhere]

We all know Mk2 parts are inferior to other parts in many respects. Here I found myself using them for the few advantages they have, mostly heat resistance.

but at least, for all their terrible drag and mass, they are supposed to provide some lift. they help making the plane flight.

that is, until i activated aerodinamic forces and saw this

they are lifting downwards?

not only are those dumb fuselages much more draggy than other parts, but they are also pushing my plane down?

why?

I swear, if I didn't need a pressurized cabin for 2, i'd have used anything else

[HebaruSan]

Does that persist if you pitch the nose above prograde instead of below?

[GoSlash27]

That's exactly what the problem is; they are pitched down relative the prograde vector. 

Best,

-Slashy

[king of nowhere]

15 minutes ago, HebaruSan said:

Does that persist if you pitch the nose above prograde instead of below?

no. i guess they are meant to be used like that. but then, i'm already using the minimum angle of attack i can make precisely with the wings (5°). Unless those parts are supposed to be used with 0 angle of attack. but everything i learned about making planes in this game cuncur that angle of attack is useful

[GoSlash27]

You can make more precise adjustments than that. Make sure to click the hexagon in the lower left hand corner to make it a circle.

Oh, and grab the rotation ring, don't use the keyboard.

Best,

-Slashy

Edited July 29, 2021 by GoSlash27

[HebaruSan]

1 hour ago, king of nowhere said:

no.

Good, thanks for confirming. A crude graphic to try to illustrate further what's happening, not to scale:

The wings are tilted "up" and so push up, the fuselage is tilted "down" and so pushes down, as shown in your screenshot. (Scare quotes because we're talking relative to the prograde vector, not absolute directions.)

1 hour ago, king of nowhere said:

i guess they are meant to be used like that. but then, i'm already using the minimum angle of attack i can make precisely with the wings (5°). Unless those parts are supposed to be used with 0 angle of attack.

I don't think it's fair to say they're "meant to be used" in any particular way. They're just responding to the air flow the way any lifting body with that orientation would. If you want the fuselage to push up, it has to tilt that way. Wings and fuselages don't magically generate "lift" in a particular direction just by virtue of existing.

1 hour ago, king of nowhere said:

but everything i learned about making planes in this game cuncur that angle of attack is useful

I hope you're open to continuing to learn more and more. 

Edited July 29, 2021 by HebaruSan

[OHara]

2 hours ago, king of nowhere said:

i guess they are meant to be used like that. but then, i'm already using the minimum angle of attack i can make precisely with the wings (5°).

If by 'like that' you mean that body lift pushes up/down when the part is angled is above/below prograde then yes of course. 

But, I don't think the designers succeeded in making body lift work in the game.  The high drag of the flat faces makes these lifting bodies never beneficial in my experience.

Some people use the Mk2 parts sideways, so the sharp edges go into the airflow, with very low drag, if you need to pitch up or down.

More people set just enough angle of incidence to wings on their Mk2 planes so that the Mk2 fuselage ends up perfectly prograde --minimising its drag-- when the plane goes through the sound barrier.

A few people use the blue lift vector in the SPH to make 1° rotations.  That vector shows you the lift force as if the airflow was coming from 1° below the nose, so you can rotate the whole plane with the smooth-rotation circle until the vector goes away, then snap-rotate wings in absolute mode to level, so they have 1° incidence.

[18Watt]

I have avoided angling my wings until I was recently convinced to try it.  My reluctance was because the smallest angle I could achieve was 5 deg.-  I hadn’t figured out the trick of turning off angle-snap to get 1 deg. changes.

For all but the slowest aircraft, I find 5 degrees is way too much- any gains I get from the wing angle is offset by the drag and negative lift of the fuselage, and that gets worse the faster I go.

I did find for some aircraft I was able to boost the efficiency by angling the wings 1-2 degrees up.  Even at those small angles, there eventually was a velocity were the gains disappeared.

Keep in mind that more lift from the wings does not always yield better efficiency.

12 minutes ago, OHara said:

More people set just enough angle of incidence to wings on their Mk2 planes so that the Mk2 fuselage ends up perfectly prograde --minimising its drag-- when the plane goes through the sound barrier.

This is similar to what I do.  Except my goal is usually to achieve orbit, so high velocity at high altitude is my goal (usually).  To achieve that, here’s what I like to see-

• At 20,000 m and a velocity of 1,400 m/s or better,
• Nose (and thus fuselage) is 2-4 degrees above prograde while in level flight, or slightly climbing.
• With MK2 fuselages, I usually get this without angling the wings at all.

 

[king of nowhere]

well, my goal is to make a plane that can land on various atmospheric planets, explore multiple biomes, and return to orbit. obviously it has very different optimization parameters compared to other planes, and a very important one is that it must be able to take off and land at low speed - good luck managing on irregular terrain otherwise. I'm going to experiment with 0 angle later, but that plane already has some difficulties taking off on laythe.

1 hour ago, HebaruSan said:

I don't think it's fair to say they're "meant to be used" in any particular way. They're just responding to the air flow the way any lifting body with that orientation would. If you want the fuselage to push up, it has to tilt that way. Wings and fuselages don't magically generate "lift" in a particular direction just by virtue of existing.

so, i assume that if i had used some other kind of fuselage, those lines would have been red, or yellow, and i would have just accepted them as normal drag. but the game models that specific aerodinamic effect as lift, hence it gets a blue line, which is what confused me.

31 minutes ago, OHara said:

Some people use the Mk2 parts sideways, so the sharp edges go into the airflow, with very low drag, if you need to pitch up or down.

 

huh. sounds like a nice idea. i don't want to know how the rest of the plane looks like, though

Quote

More people set just enough angle of incidence to wings on their Mk2 planes so that the Mk2 fuselage ends up perfectly prograde --minimising its drag-- when the plane goes through the sound barrier.

smart idea, but not applicable to my case. heck, i was on neidon there, it has a speed of sound completely different than kerbin's.

Quote

A few people use the blue lift vector in the SPH to make 1° rotations.  That vector shows you the lift force as if the airflow was coming from 1° below the nose, so you can rotate the whole plane with the smooth-rotation circle until the vector goes away, then snap-rotate wings in absolute mode to level, so they have 1° incidence.

this one actually interest me. i never knew of this trick. I've always been stuck with 5 degrees angle as the minimum that i could control with precision.

[GoSlash27]

2 hours ago, king of nowhere said:

well, my goal is to make a plane that can land on various atmospheric planets, explore multiple biomes, and return to orbit. obviously it has very different optimization parameters compared to other planes, and a very important one is that it must be able to take off and land at low speed - good luck managing on irregular terrain otherwise. I'm going to experiment with 0 angle later, but that plane already has some difficulties taking off on laythe.

so, i assume that if i had used some other kind of fuselage, those lines would have been red, or yellow, and i would have just accepted them as normal drag. but the game models that specific aerodinamic effect as lift, hence it gets a blue line, which is what confused me.

huh. sounds like a nice idea. i don't want to know how the rest of the plane looks like, though

smart idea, but not applicable to my case. heck, i was on neidon there, it has a speed of sound completely different than kerbin's.

this one actually interest me. i never knew of this trick. I've always been stuck with 5 degrees angle as the minimum that i could control with precision.

king of nowhere,

 I think you're finding out firsthand that 5° static incidence is way too much. You want just enough to have the nose pointed perfectly prograde when you hit 320 m/sec. Once you knife your way past 410 m/sec, that planes gonna take off like a scalded cat. With as much wing as you've got, that's probably 1 or 2 clicks on the fine adjustment setting.

 If you need more incidence while on the ground, you can adjust the landing gear to give you a nose high stance.

Best,

-Slashy

*edit* It's hard to tell from that photo, but if your engines aren't aligned with the fuselage, they should be.

 

Edited July 29, 2021 by GoSlash27

[fourfa]

Great thread! There are a couple of simple mods that can help with this design challenge: 

I use these in combination to get 1 degree angle snap, and sub-degree angle read outs. 

[GoSlash27]

On 7/29/2021 at 2:04 PM, OHara said:

But, I don't think the designers succeeded in making body lift work in the game.  The high drag of the flat faces makes these lifting bodies never beneficial in my experience.

OHara,

 I think they work pretty much exactly as they should. They produce lift, but with a lot more parasitic drag. This is as it should be since lifting bodies are not optimized airfoils. If you use them to produce lift (intentionally or otherwise), you have to expect more drag as a result.

 I think the vast majority of complaints about the Mk 2's "drag" are a result of this same sort of error. It's quite clean for its cross- section so long as you keep it pointed prograde.

Best,

-Slashy

 

 

[king of nowhere]

so, the ideal Mk2 plane would have the fuselage perfectly parallel to the airflow.

which, unfortunately, is not always possible, because it will depend on flight conditions.

i almost want to make a plane with rotating wings so that it can adapt the angle of attack to keep the fuselage in the right position

[OHara]

23 minutes ago, king of nowhere said:

so, the ideal Mk2 plane would have the fuselage perfectly parallel to the airflow.

. . . when you are in the flight condition where drag is most critical ---  which is often at the sound barrier. 

For landing and takeoff, you can place your gear to get a higher AOA as was said above, because drag is not so critical there.

And that is only in stock KSP, with the flat-bottomed fuselage.  You can turn the Mk2 parts on their sides and make a very . . . interesting looking aircraft.  Or you can try the mod FAR where body lift is a bit more realistic.

[18Watt]

1 hour ago, king of nowhere said:

i almost want to make a plane with rotating wings so that it can adapt the angle of attack to keep the fuselage in the right position

That’s not a bad idea.  
It sounds like you would like a plane which is efficient at speed, but also produces a lot of lift at slow speeds, to taking off from rough terrain is safer.   
Another idea is to have the plane sit a little (or a lot) nose-high on the ground- have the nose gear extend farther down than the main gear.   The wings will have a higher angle of attack during takeoff, and you’ll lift off at a slower speed without having to ‘rotate’.  

[Lt_Duckweed]

On 7/29/2021 at 2:15 PM, 18Watt said:

I have avoided angling my wings until I was recently convinced to try it.  My reluctance was because the smallest angle I could achieve was 5 deg.-  I hadn’t figured out the trick of turning off angle-snap to get 1 deg. changes.

For all but the slowest aircraft, I find 5 degrees is way too much- any gains I get from the wing angle is offset by the drag and negative lift of the fuselage, and that gets worse the faster I go.

I did find for some aircraft I was able to boost the efficiency by angling the wings 1-2 degrees up.  Even at those small angles, there eventually was a velocity were the gains disappeared.

Keep in mind that more lift from the wings does not always yield better efficiency.

This is similar to what I do.  Except my goal is usually to achieve orbit, so high velocity at high altitude is my goal (usually).  To achieve that, here’s what I like to see-

• At 20,000 m and a velocity of 1,400 m/s or better,
• Nose (and thus fuselage) is 2-4 degrees above prograde while in level flight, or slightly climbing.
• With MK2 fuselages, I usually get this without angling the wings at all.

 

To me this indicates a gap in understanding of how wing incidence is best used.  I fly to orbit with 5 degrees of incidence, 1650m/s at ~20,000m, with angle of attack of less than 0.05 degrees.  I generally expect my craft to maintain a L/D ratio of near to or over 4 from Mach 2 to Mach 6, with minimum transonic L/D of at least 2.5 or so as I cross  Mach 1.15 (point of highest transonic drag).

When using a plane with wing incidence, for a given atmosphere, each altitude (well, technically density) is tied to ONE unique optimal speed.   The correct flight profile is to stick as close as you can to prograde flight, managing vertical velocity as needed to climb through altitude at as near to the same pace you climb through velocity.  In practice, I accomplish this by barrel rolling anytime vertical speed climbs too high.

If you ever reach a point where you are flying with your prograde vector significantly above your craft attitude, you are going too fast in air that is too dense, and should have climbed faster by holding prograde.  You fix this by  letting the craft pitch up towards prograde a bit to gain altitude faster, then as you approach your ideal altitude, barrel roll to arrest vertical speed.

If you ever reach a point where you are flying with your prograde vector significantly bellow your craft attitude, you are flying too slow in air that is too thin, and should have climbed faster by scrubbing vertical speed with a barrel roll.  You fix this by  letting the craft pitch down towards prograde and lose vertical speed and altitude, you will initially overshoot, and "bounce" off of thicker atmosphere, then on the way back up barrel roll to arrest vertical speed at ideal altitude.

[GoSlash27]

8 hours ago, king of nowhere said:

so, the ideal Mk2 plane would have the fuselage perfectly parallel to the airflow.

which, unfortunately, is not always possible, because it will depend on flight conditions.

i almost want to make a plane with rotating wings so that it can adapt the angle of attack to keep the fuselage in the right position

It is exactly as OHara said; You want to minimize drag at the point where drag is most detrimental to your efforts. The rest of the time... you don't really care. In some of those cases, the additional drag may even be working in your favor.

 You have a lot of leeway in supersonic flight because your engines are producing buckets of excess thrust. You have a lot of leeway at low speeds because low speed= low drag. It's that one point where you're trying to transition between subsonic and supersonic where the drag must be minimized.  Everything else is either "don't care" or "bonus".

Just set the incidence so you're cleanest in the transsonic region at your chosen altitude, and the rest will all fall into line for you.

Best,

-Slashy