Air Intakes!

[aesthetics]

How well do downward facing air intakes perform compared to traditional forward facing ones?

Edited April 21, 2014 by aesthetics

[Claw]

You'll get a lot less performance, but the exact amount will depend on AoA, speed, altitude, etc.

If you're asking for design of things like VTOL, the performance drop isn't really a factor for lower altitudes. It probably won't be a problem unless you're trying to leave the atmosphere at high altitude .

[aesthetics]

It is actually for a wingless VTOL, I'm having trouble getting it to high enough speeds to get orbital.

[Shna_na]

It's more a case of direction relative to velocity. You are using a VTOL, so having one that faces upwards will provide you with more performance when rising, but not so much when accelerating laterally. Having a forward-facing one would reduce performance when climbing, but increase it when flying laterally. Try having one facing upwards and one facing forwards.

[aesthetics]

The way it is built, all thrust is aimed down towards the bottom of the craft, so intakes should be facing up towards the sky? In previous crafts trying that method proved ineffective, I'll post examples once I get home .

Edited April 19, 2014 by aesthetics
sp

[Claw]

The way it is built, all thrust is aimed down towards the bottom of the craft, so intakes should be facing up towards the sky? In previous crafts trying that method proved ineffective, I'll post examples once I get home .

Yes. Generally speaking you should orient your intakes opposite of the direction of the engines so that they directly receive incoming air.

Here is an example of a somewhat non-standard VTOL I made. The intakes are on top and the engine is on bottom. Some people also make VTOLs that transition to normal flight. In that case, I would put the intakes facing forward rather than up. Although it sounds like you're making a VTOL that only flies vertically and doesn't transition to horizontal flight. So you would want intakes facing up if you're trying to do high speed/high altitude flight.

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[aesthetics]

this is the build in question. I used a bit of clipping to place some intakes on top, and each of the intakes facing down, is x2. I'm guessing i should flip at least one of them?

Edited April 19, 2014 by aesthetics

[Taki117]

Those intakes don't do anything. Especially if you are going up.

[aesthetics]

the ones on top only seem to increase in flow if I am moving straight up, if i am tilted slightly forward, it drops off completely.

[sdj64]

One good thing about intakes in the wrong direction is that they don't produce any extra drag. But you will lose out on most of the air.

[Nao]

Sigh why can't people check first before answering...

Those intakes don't do anything. Especially if you are going up.

Nope, they work quite well!

One good thing about intakes in the wrong direction is that they don't produce any extra drag. But you will lose out on most of the air.

Nope they aren't! The drag value goes down to 0.00 but the intakes still produce standard intake drag.

You'll get a lot less performance, but the exact amount will depend on AoA, speed, altitude, etc.

Not really, back facing intakes produce constant airflow regardless of AoA. While forward facing ones lose efficiency with increasing AoA. This way for example for standard SSTO rear facing intakes produce almost the same airflow as forward facing ones when the plane is tilted by ~30deg (I can't remember the precise number of back facing intake efficiency thou).

For example this is an SSTO that uses 16 intakes and can achieve orbital speeds inside the atmosphere while carrying enough fuel to have 2000m/s deltaV in orbit. No intake is facing forward. (roughly the same can be achieved on 16 intakes placed forward on cubic struts)

[Claw]

Sigh why can't people check first before answering...

If you're going to pull out snippets of quotes and provide information, at least don't make it sound like we are lying.

In your own post you said that my comment of AoA doesn't matter, then go on to make it sound like forward facing intakes are worse because of AoA. I get your point, but 1 forward facing intake is still more efficient than 1 rear facing.

Your craft is a fine example of getting around that. 16 intakes is on the "air hoggy" side, but is a fine method to overcome the efficiency problem. You can achieve the same results with two forward ones.

I'm not discounting your methods, I'm just saying that this game offers more than one version of "correct".

[Claw]

this is the build in question. I used a bit of clipping to place some intakes on top, and each of the intakes facing down, is x2. I'm guessing i should flip at least one of them?

After seeing your craft, what I would say here is that you're probably not suffering from an intake problem. It's probably more of a flight profile problem. It appears that you have three TurboJets and seems you have plenty of intakes.

Generally, you'll want to get up to the 20-25km point as quickly as possible. That's where your engines will produce the most thrust and the atmosphere is pretty thin. I try to hit 350 m/s at 15km, and 650 m/s at 20km. Start to shallow your climb to keep accelerating as much as possible in the 20-25km band. You'll want to be at least 1,000 m/s around 24 km. Make sure you keep climbing up though though because if you start to descent it becomes a pain to get climbing back up.

At some point you'll need to start throttling back so that your engines don't flame out. You may even want to consider putting the two outboard engines on an action group to shut them down (toggle them actually, so you can use the same key to turn them back on). That will prevent asymmetric thrust flameouts which will cause your craft to go out of control. If you shut down just the outer two, you'll have more air available to the center one and you can keep the throttle up higher. I usually aim to be around 1,600 m/s at 30km. Keep working the speed up as you climb, but try not to stagnate at one altitude for too long or you just end up wasting fuel.

It becomes a balancing act of flying fast enough to keep intake air high, but not stagnating and burning off all your fuel.

Edited April 20, 2014 by Claw

[Nao]

If you're going to pull out snippets of quotes and provide information, at least don't make it sound like we are lying.

In your own post you said that my comment of AoA doesn't matter, then go on to make it sound like forward facing intakes are worse because of AoA. I get your point, but 1 forward facing intake is still more efficient than 1 rear facing.

Your craft is a fine example of getting around that. 16 intakes is on the "air hoggy" side, but is a fine method to overcome the efficiency problem. You can achieve the same results with two forward ones.

I'm not discounting your methods, I'm just saying that this game offers more than one version of "correct".

Yeah, I'm in a bad mood today, so sorry about the bad tone.

But the thing is, if you are using intakes for ascent of a plane, their efficiency doesn't usually matter in low atmosphere, while high up where lift is smaller and you pitch up the plane the AoA effect on intake efficiency starts to be important factor. And as i've said rear facing intakes can have the same efficiency as forward facing ones working at an angle (~30deg which is also around the best glide angle for wings) So unless you angle the forward facing intakes downward (or angle wings upward) rear facing intakes can work just as good, and doesn't mind you doing any maneuvers (or being used almost horizontally).

Yes the plane is an airhog but to get up a mass of ~13t into orbit would take 10-16 forward facing (circular) intakes not two.

edit: Agree with your second post 100%.

Edited April 20, 2014 by Nao

[Claw]

Yeah, I'm in a bad mood today, so sorry about the bad tone.

No sweat.

(~30deg which is also around the best glide angle for wings) So unless you angle the forward facing intakes downward (or angle wings upward) rear facing intakes can work just as good, and doesn't mind you doing any maneuvers (or being used almost horizontally).

The best lift angle for wings is 25 AoA. Past that they lose lift. Also, I don't really want to fly up at 25 AoA because it's a lot of drag (most lift is not always the best place to fly). I try to keep high altitude AoA down around 15. Once above 24 km, AoA is less of a factor for wings and more so for engines and keeping that upward trajectory. And at that point I usually tend to decrease my AoA. (At least for my standard "to-orbit" profile.)

Yes the plane is an airhog but to get up a mass of ~13t into orbit would take 10-16 forward facing (circular) intakes not two.

Okay, I'll take your word on this one. I almost always use ram intakes and I can get 13t into orbit with 1 TurboJet and 2 ram intakes. (By the way, I usually avoid it but personally have nothing against "air hogging")

Edited April 20, 2014 by Claw

[Nao]

The best lift angle for wings is 25 AoA. Past that they lose lift. Also, I don't really want to fly up at 25 AoA because it's a lot of drag (most lift is not always the best place to fly). I try to keep high altitude AoA down around 15. Once above 24 km, AoA is less of a factor for wings and more so for engines and keeping that upward trajectory. And at that point I usually tend to decrease my AoA. (At least for my standard "to-orbit" profile.)

There is a lot of silly stuff going on in KSP stock aero model... For normal wings i agree on 25deg being the sweetspot, although i continue to use that pitch at high altitudes if allowed by design (the decrease in performance on normal planes is probably due to engine and intakes working at an angle too, rather than inefficiency of wing itself). As for control surfaces, they are crazy (reach highest lift at 90deg pitch, lift vector seems to be not perpendicular to wing surface, infiniglide etc)

Okay, I'll take your word on this one. I almost always use ram intakes and I can get 13t into orbit with 1 TurboJet and 2 ram intakes. (By the way, I usually avoid it but personally have nothing against "air hogging")

Welp Circular have 80% efficiency of rams, and the point of that craft was to be a replacement for Aeris4A in BSC challenge (never submitted), so it had somewhat too much intake air for the mission to allow room for piloting errors.

Oh and i've just remembered where i tested the info about drag that sdj64 mentioned: (Water increases drag value many times, so it's easy to see changes with different intake positions/states)

So the thing goes like this: If the intake is closed it has little drag value (0.02 i think) that doesn't change with pitch at all (forward/backwards/sideways - no change). If the intake is open, drag coefficient increases with speed but the intake facing again doesn't matter, even thou the amount of airflow generated will vary. The drag value in tooltip is probably based on separate equation not connected to physics, and nobody bothered to check it for all states so it tends to show wrong values when working outside of intended purpose/direction.

[sdj64]

Alright, sorry about the mis-information. All I knew about was the tooltip value and I assumed that was correct. I never tested the same design with forward versus backward intakes so I didn't know if it made an actual difference.

On related note, but may be slightly off-topic: do wings facing backwards or sideways compared to their regular orientation still produce the same lift?

Edited April 20, 2014 by sdj64

[Nao]

On related note, but may be slightly off-topic: do wings facing backwards or sideways compared to their regular orientation still produce the same lift?

Yes wings can work the same when placed backwards just fine. Control surfaces don't thou I think. They still produce lift (possibly the same scalar value) but the direction vector can do some weird things (I haven't tested this properly but I'm pretty sure placing cannards backwards make them work differently).

On that note the amount of lift the wings produce is proportional to the longitudal vector component of such wing, that is if the wings is rotated 45deg along vertical axis it will produce less lift.

One important thing coming from that is the yaw problem. If you place the wings symmetrically rotated i.e. backwards (giving them more sweep) they will not only work at reduced efficiency, but they will create roll forces with any amount of yaw, since one wing will produce more lift than other (yawing right will roll the craft right with positive wing sweep). The same thing happens IRL but since in KSP wings don't produce lift when sideways to airflow, the effect is stronger)

Other fun fact about wings: they produce lift at the center of mass, and for most wings it is at their base, so for example if we have two wings, one angled up, one down placed close to the roll axis, one on each side, they will produce small amount of roll torque, but if we just flip them around, attaching by their tips, the torque will be huge even thou the wing surface is roughly in the same place.

Edited April 21, 2014 by Nao
grammar errors

[Claw]

(As we hijack the thread...)

Yeah, Nao is right. Wings backward are fine but control surfaces backward are not.

And yes, control surfaces increase lift all the way up till 90 deg AoA. So if most of your craft's lift comes from control surfaces, you can get all kinds of wacky response out of it. (This is the one thing in stock aero that I wish would get fixed ASAP. The rest I can live with for a while.)

[Nao]

Hmm, returning to topic more.

Your comments Claw got me thinking, because it is indeed possible to lift ~10tons on one engine and two ram intakes SSTO to a reasonable speed.

After some testing it was revealed that backward installed intakes change efficiency with speed for some reason. They have around 80% efficiency at rest and only ~20-25% at 2000m/s.

I'm not sure how that works, but at first i suspected that the base of "100m/s" that the intakes have when at rest is added for normal facing intakes and subtracted from airflow speed on rear facing ones. But at 2000m/s it is only 10% of speed difference. So unless there is a "increasing trend" (need better word) in a speed - air scoop efficiency curve of intakes, it doesn't make much sense.

And also from little testing i had time for, it looks like the efficiency does not change linearly with speed. :|

Welp i've found many interesting glitches concerning intakes thought the earlier patches (like free engine power increase/flameout blocker from closed intakes, that kind of became stock in the 0.23) but it's the first time I see this... wonder it it was changed recently or was there all along.

[aesthetics]

Thank you all for the inputs, I believe Claw was right with my craft, poor piloting skills! A common issue for me. under the hood is actually two sets of engines. Above the turbo's I have a set of 48-7s's to push me the last leg up. After modifying my flight pattern and removing excess intakes, I made orbit with fuel to spare!

Edited April 21, 2014 by aesthetics

[Claw]

Thank you all for the inputs, I believe Claw was right with my craft, poor piloting skills!

I didn't say it that way! Besides, there are no poor piloting skills, only piloting skills that don't match the situation. (Or maybe that should be a situation that doesn't match piloting skills...)

Anyway...Congrats on getting it to orbit!

[aesthetics]

I didn't say it that way! Besides, there are no poor piloting skills, only piloting skills that don't match the situation. (Or maybe that should be a situation that doesn't match piloting skills...)

Anyway...Congrats on getting it to orbit!

Thank you, thank you! Poor piloting skills is my own assessment! First day of playing the game, engineering was a cake walk, crashed an obnoxious amount of rockets into the Mun with way too much fuel to spare. Builder first, pilot last!