Aircraft Speed

[nholzric]

Hey, I'm wondering if you can explain why one aircraft goes faster than the other. In the following screen shots I'm at about 1.6km altitude. The second aircraft tops out at 334 m/s, the first aircraft is throttled down to 334 m/s and I have the various air intakes visible.

This guy has about 365U of airflow per Panther engine. It's hard to see in the picture but he also has an antenna, barometer, and atmosphere sensor hanging off the belly (more drag?). Since this guy is pretty much at stead state his one Panther producing 45.9 kN of force means his drag is about 45.9 kN.

This guy has about 430 U of airflow per Panther. Inside his cargo bay he has an antenna and batteries but I assume those don't add to drag with the cargo bay closed. This guy is also pretty much at steady state and his two Panthers are producing a combined 149 kN so his drag is about 149 kN.

Is the reason the second guy goes so much slower that he has 3x the amount of drag - such that 2 Panthers on the second guy can't outperform the first? Is this the correct line of analysis or is there something else I should be looking at? Thanks!

[bewing]

Yes, the MK2 parts have 3 times the drag of MK1 parts. Don't use MK2 parts if you want to go fast. It's not the air intakes that are doing it. It's your MK2 fuselage that's slowing you down.

[Val]

10 hours ago, nholzric said:

Inside his cargo bay he has an antenna and batteries but I assume those don't add to drag with the cargo bay closed.

This is likely the problem. Those antennas and batteries are causing drag.

With 1.2 they changed how cargo bays shield surface attached things. Anything that is attached to the cargo bay walls are treated as if they are mounted outside the craft.

To get surface attached items shielded in a cargo bay, they have to be attached to something that is attached to either of the internal attachment nodes.

At least, that is my experience.

 

10 hours ago, nholzric said:

This guy has about 430 U of airflow per Panther.

Generally, that craft has too much intakes. You should never need more than one intake per engine when using an intake from the same tech node as the engine was unlocked in.

Exceptions are:

You need 2 Adjustable Ramp Intake (Radial) per engine.

Shock Cone Intake can support 4-5 engines, with a little throttle management, while accelerating on the runway, until above 20 m/s.

[DrLicor]

4 minutes ago, Val said:

This is likely the problem. Those antennas and batteries are causing drag.

With 1.2 they changed how cargo bays shield surface attached things. Anything that is attached to the cargo bay walls are treated as if they are mounted outside the craft.

To get surface attached items shielded in a cargo bay, they have to be attached to something that is attached to either of the internal attachment nodes.

At least, that is my experience.

As @Val stated, the stock aerodynamics are causing drag for things in the cargobay too. However it improved a lot with 1.2. 

Despite those improvements I just stick with FAR. This gives me the most realistic aerodynamics possible for KSP in my opinion. 

[AeroGav]

Intake air quantity won't affect top speed.   An engine either has sufficient air, in which case it runs normally and produces power according to it's speed/altitude modifier curves,  or it does not.  If air is insufficient, the engine surges and splutters very noticeably.  That is especially the case on a craft with more than one engine.   The first thing you notice is the craft yawing side to side sharply and going out of control, due to engines on different sides of the plane cutting out then coming back on again.

One other thing -  we can't see the back end of your mk2 plane.

I noticed it has two sponsons/nacelles on the wing, with adjustable ramp intakes on the front.   I presume the Panthers are attached to the rear of these stacks?

In that case, what is at the back end of the main fuselage?

Open nodes create huge drag.  Every fuselage stack must end with a tail cone, nose cone flipped 180 degrees, or an engine nozzle.   And if your engine nozzle has a rear attach node (all of the liquid fuel rocket engines, and the rapier) you ideally need to attach a cone the same diameter as the engine, and use the offset tool to move it out of sight inside the engine, to optimise drag.  

[N_Molson]

Get inspiration from real airplanes, I found the Concorde design really efficient in KSP. It has everything : ultra-sleek fuselage, very aerodynamic cones in front of and behind the fuselage, a single tail fin, and a slighlty curved delta-wing. Plus Panther-like engines (after-burning), fed by adjustable air intakes.

http://ichef.bbci.co.uk/news/660/cpsprodpb/3D7F/production/_85634751_85634750.jpg

[Val]

6 minutes ago, AeroGav said:

Open nodes create huge drag.

No. Open nodes do not create drag.

An open nodes is an opportunity to add a more aerodynamic shape (more pointy) to the end of a stack. But unless the part you add to the node has less drag, then it makes no difference whether an attachment node is open or not.

[AeroGav]

13 minutes ago, Val said:

No. Open nodes do not create drag.

An open nodes is an opportunity to add a more aerodynamic shape (more pointy) to the end of a stack. But unless the part you add to the node has less drag, then it makes no difference whether an attachment node is open or not.

I don't know if this is just semantics.

When you enable the option to show aero data in action menus,  you are correct in that the attach node itself does not have any drag, because it is not a part.    However,  an FT400 tank for example,  shows much higher drag if the front or rear nodes are left open.  There is no difference whether it is the front or rear part in terms of drag, but obviously leaving both open is worse.

Of course, if you cover the nodes with something that is also blunt, then you reduce the drag on the FT400, but you've created a drag problem on the new part instead.  Which is why all stacks must start and end with something pointy - a cone, an intake, or at worst, an engine.

[Raideur Ng]

14 hours ago, bewing said:

Don't use MK2 parts if you want to go fast.

Wait what? While I don't know the drag numbers of the Mk2 pods, they don't appear to be very draggy. Anyone got numbers on if the Mk2 pods are indeed worth using or if you're only shooting yourself in the foot with drag?

[nholzric]

Ah ha, I almost didn't post this because it seemed like too simple a question but this is an interesting discussion - thanks everyone!

• @Val Thanks for the tip about air intakes. Is there anything in game that would clue me in to how many air intakes are enough - other than engines stalling out at high altitude? Other than the adjustable ramp and shock cone exceptions what you're saying is more than one intake (from the same tech level) per engine is just causing drag.
• An engine's performance does not depend on the amount of air supplied, as long as it is "enough" it will run and if it's not enough it will either supper and/or flame-out.
• The batteries and antenna aren't attached to the cargo bay walls but they are attached to the wall of the tank in front of it. Visually they are in the cargo bay but I can imagine from a coding perspective how they would act like they were mounted on the surface of the craft. I know how to fix the design to remove these parts' drag.
• @AeroGav Sorry about the pictures not showing the back - at least the pictures are not in the dark!  I do have a reversed nose cone to cap off the main body. Assuming total engine output at constant altitude/speed is a good measure of drag then we can run the experiment of if empty nodes matter. But even better, concerning this areo data option you mention, I've seen the different colored vectors showing lift and drag coming from each part but does this areo data option show net scalar values for your craft? If so, cool, I should have been using that.
• @Raideur Ng Agreed, it's too bad the "fast looking" MK2 parts apparently have a lot of drag. I wonder if this is a design/balance thing (meaning there's some parameter in the part saying drag=lots) or if it's an artifact of a wider frontal cross section and you and I are just fooled by the aesthetics.

[AeroGav]

4 minutes ago, nholzric said:

Ah ha, I almost didn't post this because it seemed like too simple a question but this is an interesting discussion - thanks everyone!

 

• @AeroGav Sorry about the pictures not showing the back - at least the pictures are not in the dark!  I do have a reversed nose cone to cap off the main body. Assuming total engine output at constant altitude/speed is a good measure of drag then we can run the experiment of if empty nodes matter. But even better, concerning this areo data option you mention, I've seen the different colored vectors showing lift and drag coming from each part but does this areo data option show net scalar values for your craft? If so, cool, I should have been using that.
• @Raideur Ng Agreed, it's too bad the "fast looking" MK2 parts apparently have a lot of drag. I wonder if this is a design/balance thing (meaning there's some parameter in the part saying drag=lots) or if it's an artifact of a wider frontal cross section and you and I are just fooled by the aesthetics.

Reversed nose cone - just to clarify, you are using a type 2 to 1.25m adapter (either short or long) to link that 1.25m nose cone to the type 2 body?  Because when linking parts of different diameter together, you have to use an adapter or you get most of the "open node" drag penalty

I think your top speed test, provided you fly at same altitude, is valid.  You can of course get more data.  Press ALT + F12 to open up the debug menu, go to Physics, the Aero.  I'm a fan of enabling the aero data gui, it helps me see how efficient my flight profile is.  

Am i getting enough lift to balance my weight, is pitch/climb rate rising or falling? This data can give you early notice to add or remove some pitch trim with ALT and W or S,  well before you notice it visually.

AoA and Lift:drag ratio  help you to work out what AoA your airframe flies most efficiently at.

And Thrust vs Drag is crucial for planes, it's like TWR for rockets.

The other checkbox is simpler - enable aero data in action menus.

You can right click on individual parts and find out how much drag they are generating.

----------------------------------------------------------

mk2 fuselage vs mk1

They really shouldn't have three times the drag,  i don't see how it's justified.

If you look inside the cargo bay, you can see that it's less than twice the frontal area.  The widest bit in the middle is just the mk1 fuselage 1.25m cylinder, with a bit of wing/body blending at the sides.   The wing/body blending is the only thing adding frontal area compared to the mk1 variant.

Game balance?  Well, the fuel parts hold no more fuel than mk1 parts of same length, the extra wing/body blending area  must not be used.

The crewed sections do hold twice as many kerbals, for twice the weight and three times the drag.

 

[nholzric]

11 minutes ago, AeroGav said:

Reversed nose cone - just to clarify, you are using a type 2 to 1.25m adapter (either short or long) to link that 1.25m nose cone to the type 2 body?  Because when linking parts of different diameter together, you have to use an adapter or you get most of the "open node" drag penalty

That's correct, I have an adapter from type 2 to 1.25m and then a 1.25m nose cone.

I'm really curious how much this open node, or mismatched node, drag matters. We understand it intuitively, but just like the antenna causing drag in the cargo bay I wouldn't be surprised if our intuition and stock game mechanics diverge in a measurable way.