Intake drag question

[annikk.exe]

I've heard that closing air intakes reduces drag.

This was news to me. Excited by the possibility, I set up an aircraft with about a dozen intakes then went for a test flight.

I flew straight at about 2000m altitude, then tried toggling most of the intakes off.

I expected to see a minor speed increase due to reduced drag. However, I was not able to observe and change at all in the craft's speed.

What is the deal here? How does closing intakes help? What test can I perform to verify that closing intakes does something?

[Richy teh space man]

It's at higher altitudes when the atmosphere starts thinning and jets become useless that you get the added benefit of closing intakes.

[annikk.exe]

Thanks. Can anyone suggest a test where I could verify this? Small rocket engine at 30,000m perhaps?

[Guest links123]

You can only test it at higher altitudes

[Rhomphaia]

You can only test it at higher altitudes

Just did a test at 200m, with a plane with 2 ram scoops and 2 radial intakes. closing the ram scoops gave a 0.5% increase in speed. since the drag when open increases with air speed, the effects would be more noticable at higher altitudes where you are going faster, but they are still observable at low

[Traches]

Better test: mount a pair of intakes symmetrically, preferably further outboard than inboard, and close only one of them. If it tries to yaw towards the open intake, you've got your proof!

Practically, I don't think it makes a ton of difference. I usually stick them in my "gogogo" action group (shut down jets, close intakes, turn on rockets) for spaceplanes.

[Rhomphaia]

Better test: mount a pair of intakes symmetrically, preferably further outboard than inboard, and close only one of them. If it tries to yaw towards the open intake, you've got your proof!

Practically, I don't think it makes a ton of difference. I usually stick them in my "gogogo" action group (shut down jets, close intakes, turn on rockets) for spaceplanes.

also a workable test.

though in this case it is better to close them both at the start, then open one. Since with both open it won't go straight up either. with both closed it goes straight until you open one where it immediately starts to yaw around it.

[John FX]

If you put them facing the wrong way they still intake air but cause no drag...

[numerobis]

If you right click on the intake it'll tell you the drag. You get 0.3 drag when closed, and 0.3 plus whatever it displays when open.

The reason you're not seeing much difference is either:

- intakes are small, so even if their drag coefficient is 11.5 times more than other parts, they aren't a huge source of drag relative to the rest of your aircraft.

Or:

- at low speed, intake drag increases according to speed^3 (rather than squared for most parts). That means that at low speed, there's very little difference between open and closed intakes.

I don't understand people saying you should close intakes at high altitude. What's the argument? I'd think that at high altitude you want intakes open, to get airflow -- why else would you have put those intakes on the aircraft?

Edited December 6, 2013 by numerobis

[Rhomphaia]

If you right click on the intake it'll tell you the drag. You get 0.3 drag when closed, and 0.3 plus whatever it displays when open.

I don't understand people saying you should close intakes at high altitude. What's the argument? I'd think that at high altitude you want intakes open, to get airflow -- why else would you have put those intakes on the aircraft?

I think what they mean is, you close the intakes when you transition from jets to rockets.

[annikk.exe]

With a sufficient number of intakes (6-7 per turbojet engine) I've found I can keep firing my turbojet very gently all the way up to 70km. Above 50km the turbojet provides perhaps 0.5kn, but the drag is so low up there that even such a small thrust as that is often enough to maintain the apoapsis.

Using that technique I've been able to get a periapsis with turbojets and come within about 50m/s delta v of a low kerbin orbit. So this is why I made the thread.. I'm wondering why everyone is in the business of closing intakes..

[Zephram Kerman]

Some consider 6-7 intakes per engine "sufficient", while some others consider more than 1 to be "airhogging". It's a debate that's already been done to death. (I hope your thread doesn't turn into another rehash of this.) In my sandbox, I like the extra challenge, so the rule is 1:1, but I'll bend that to make something work.

Anyway, back to the topic. In a "normal" space plane, the challenge is to transition into orbit when the jets run out around 16 km and 2600 m/s. At that altitude, with increasing speed, to reduce drag by closing intakes makes a significant difference.

[arq]

I find the drag difference to be negligible. Speed is irrelevant, as the drag coefficient only affects drag force linearly (even if the velocity effect is quadratic or cubic), it's the same percentage increase (except that the drag coeff increaeses with speed, but it caps at 2.00 very quickly). Get Kerbal Engineer Redux or some other mod to measure drag and you'll see the difference is tiny. Technically it exists, but it's worth more to me to keep it simple than to save 1% of my fuel on my way up. I just toggle all my intakes in the same actiongroup as my jets.

[Zephram Kerman]

Yes, it's only worthwhile to close intakes when the jets shut down.