Could a Quadruplane Be Effective on Duna?

[Edax]

I've been experimenting with making SSTO biplanes and found them generally an excellent way of generating more lift for a safer landing while maintaining a compact shape. I have in the past landed a monoplane SSTO on Duna, but the insufficient lift forced me to use a retro-burn parachute landing which was crazy-dangerous. This has lead me to wonder, would a quadruplane'a extra lift (from say, 4 sets of swept wings) be able land on Duna safely without parachutes, rockets or thrusters?

Edited August 17, 2015 by Edax

[KerikBalm]

Sure, why not.

KSP doesn't model the benefits of having higher aspect ratio wings, so you don't lose any aerodynamic efficiency from stacking wings like that

[Sharpy]

Actually, the heaviest transport planes have multiple main wings clipped into each other. So two wings at exactly the same location still provide twice the lift.

Though aligning center of drag with center of thrust will be tricky.

Edited August 17, 2015 by Sharpy

[r4pt0r]

Actually, the heaviest transport planes have multiple main wings clipped into each other. So two wings at exactly the same location still provide twice the lift.

Is that why my attempts at huge space planes always fail? Because I don't use any exploits?

I thought a wing that was occluded from the airflow provided no lift in KSP?

Edited August 17, 2015 by r4pt0r
didn't want to offend any thin skinned people

[Sharpy]

Well, you can always combine the wings in line one after another. And this applies only to absolute monsters, like the airliner with some 200 seats across a multitude of passenger cabins. You can still get your average MK3 plane on one pair of wings.

Radially attached parts are not occluded, remember?

[r4pt0r]

I guess I always thought lift providing surfaces were different.

hmmm...

[Teilnehmer]

KSP doesn't model the benefits of having higher aspect ratio wings, so you don't lose any aerodynamic efficiency from stacking wings like that

What about FAR? What kind of problems do biplanes (quadruplanes) have in the FAR aerodynamic model?

[mikegarrison]

In the real world, biplanes/triplanes/etc. were a design solution for a specific problem -- the wing skins were made of fabric. In order to give the wings structural strength without being insanely heavy, they needed a truss structure. Also, they needed extremely low wing loading, and they got that by cutting the wing loading in half, or in thirds, or even in quarters by increasing the wing area by using multiple wings. About the only place you still see biplanes today is in aerobatic airplanes, and they use the design for the same reason the old airplanes did -- they want reduced wing loading and a strong, stiff truss wing structure.

Triplanes didn't get three times as much lift as a monoplane would have gotten. Instead they got the same lift (or less), but they spread it out over more surface area. In KSP it would work totally differently. Three stacked wings would be three times the lift.

I don't know how FAR would handle it. Interesting to find out, I guess.

Edited August 17, 2015 by mikegarrison

[Sharpy]

There is *some* occlusion to radially attached parts - after all, the asteroids have no nodes and still can occlude a trailing craft. But I don't think wings occlude anything ever.

[eddiew]

I don't know how FAR would handle it. Interesting to find out, I guess.

In 0.90, FAR penalised the second wing and gave it very limited extra lift. Don't believe I ever saw anyone share a design where it was of benefit to have stacked wings over having the same area side-by-side. They were certainly possible to build, but it was pretty much dead weight and extra drag, and most of the FAR showcase thread is full of highly optimised, hyper-sleek designs

Probably works the same in 1.0.x... dunno if there's a level of separation that would allow you to recover the lift. Wouldn't be surprised if FAR's aero modelling is that accurate

[mikegarrison]

In 0.90, FAR penalised the second wing and gave it very limited extra lift. Don't believe I ever saw anyone share a design where it was of benefit to have stacked wings over having the same area side-by-side. They were certainly possible to build, but it was pretty much dead weight and extra drag, and most of the FAR showcase thread is full of highly optimised, hyper-sleek designs

Probably works the same in 1.0.x... dunno if there's a level of separation that would allow you to recover the lift. Wouldn't be surprised if FAR's aero modelling is that accurate

The basic problem is that in terms of the overall flow, the wing creates a circulation around itself. But with stacked wings the circulation interferes with each other. If you stack them and offset them correctly, you can start getting some extra lift. That's how slotted flaps work. But visually that ends up looking roughly like one bigger wing, not a stack of wings.

Twin rudders have the same problem, and yet we do use them on airplanes (usually when one rudder would cause problems for some reason like hanger height, radar reflection, etc.). However, you mostly only see them either spread very widely apart or else on supersonic airplanes where anything outside of the mach cone of anything else can't interfere with it.

[Sharpy]

...also, the lift (it's still lift even if it's sideways, right?) generated by rudders is several orders of magnitude lower than that of wings. So two rudders interfering with each other, generating less than 2x the lift of a single rudder isn't much of a problem.

[KerikBalm]

There is *some* occlusion to radially attached parts - after all, the asteroids have no nodes and still can occlude a trailing craft. But I don't think wings occlude anything ever.

#1) As far as drag is concerned... are you sure?

#2) Heat occulusion is not the same as drag occlusion. You can definitely occuluder radially attached parts on a 1.25m stack with a 2.5m heat shield.

[Geschosskopf]

I've been experimenting with making SSTO biplanes and found them generally an excellent way of generating more lift for a safer landing while maintaining a compact shape. I have in the past landed a monoplane SSTO on Duna, but the insufficient lift forced me to use a retro-burn parachute landing which was crazy-dangerous. This has lead me to wonder, would a quadruplane'a extra lift (from say, 4 sets of swept wings) be able land on Duna safely without parachutes, rockets or thrusters?

These days, flying on Duna is EXTREMELY easy compared to what it was pre-1.0. The atmosphere helps you with both hands whether you're trying to use it (aerocapture, flying, using parachutes) or ignoring it (launching). However, there are still a few wrinkles to flying on Duna that can be traps for the unwary, so I recently put together a set of guidelines (not really a tutorial) on flying Duna in 1.0.4.

But to cut to the chase....

The main trick to flying on Duna is landing safely because the ground is very bumpy everywhere except in the bottoms of canyons and basins, airbrakes are virtually useless due to the thin air, and wheel brakes are virtually useless due to the low gravity. There is thus a premium on STOL performance and reverse thrust to stop before some lump in the terrain kills you. So yes, you need a lot of wing, but no, you don't need 4 sets of wings.

For example, this is a VERY good Duna plane (I'm currently flying it all over the place there now in my career save):

This weighs 12.16 tons on Kerbin and has 17.32 units of lift. On Duna, this gives it a stall speed of about 35m/s even at 4500m terrain elevation, thanks to having about 1.4 lift/ton. Yet despite having just a single Firespitter electric prop (4Kn/ton thrust), it can climb nearly vertically, go over 1000m/s, and hurl itself briefly above the atmosphere. The Firespitter prop also has a reverse gear which it uses to stop quickly. It might as well not have the airbrakes--they do almost no good at all. The only fuel is the tank in the cargo bay and that gives it enough juice to circumnavigate the planet several times.

This might sound OP, but the same plane hardly flies at all on Kerbin. It's got so little power and so much drag from all the lift that can't quite reach 100m/s or 10km altitude. And it has so much control authority (needed on Duna) that it's totally squirrelly on Kerbin. In short, it's about the worst airplane I've ever built to fly on Kerbin, which is where all its parts were designed for. But the combination of the current aero model and Duna's low gravity make it a phenomenal performer on Duna.

So, if you're going to fly on Duna, do yourself a big favor and use HyperEdit to test (aka "simulate" in a separate game) the thing actually on Duna. You will gain no idea at all of how it flies on Duna by flying it on Kerbin.

Edited August 17, 2015 by Geschosskopf

[Edax]

So, if you're going to fly on Duna, do yourself a big favor and use HyperEdit to test (aka "simulate" in a separate game) the thing actually on Duna. You will gain no idea at all of how it flies on Duna by flying it on Kerbin.

This is why I thought to ask the forum, since I use stock, and don't want to use hyperedit since I view that as cheating.

[Geschosskopf]

This is why I thought to ask the forum, since I use stock, and don't want to use hyperedit since I view that as cheating.

Well, with stock the main issue is power. Ions powered by fuel cells are your only viable option, but they lack the macho to get you off the ground, so you'll need some small rockets to assist in takeoff (and some more on the front to stop you during landing rollout). See Brotoro's "Developing Duna" thread in Mission Reports for how it did it (all stock).

As to HyperEdit, if you don't use that, then how does your space program do simulations? Not at all? How realistic is that? There's no shame in having HyperEdit in a separate sandbox game to quickly move ships to other planets so you can test their perrformance where you intend to use them. Not doing this is just asking for the thing not to work in your real game, meaning money and time down the drain, dead Kerbals and loss of rep, etc. Sure, you CAN use HyperEdit to cheat (restocking your fuel, moving your ship), but that's your business. It's highest and best use is as a simulator.

And this is ESPECIALLY important when it comes to Duna airplanes. The Duna environment is so different from Kerbin that you can't tell anything at all about how a plane will fly on Duna by testing it on Kerbin. And in a stock game, an ion-powered plane won't even leave the ground on Kerbin these days, so you can't even get the wrong impression of it . So if you don't do some testing first, you're going in totally blind and the plane will probably crash so fast that you won't even be able to identify its problems so you can fix the next one.

[Edax]

Well, with stock the main issue is power. Ions powered by fuel cells are your only viable option, but they lack the macho to get you off the ground, so you'll need some small rockets to assist in takeoff (and some more on the front to stop you during landing rollout). See Brotoro's "Developing Duna" thread in Mission Reports for how it did it (all stock).

As to HyperEdit, if you don't use that, then how does your space program do simulations? Not at all? How realistic is that? There's no shame in having HyperEdit in a separate sandbox game to quickly move ships to other planets so you can test their perrformance where you intend to use them. Not doing this is just asking for the thing not to work in your real game, meaning money and time down the drain, dead Kerbals and loss of rep, etc. Sure, you CAN use HyperEdit to cheat (restocking your fuel, moving your ship), but that's your business. It's highest and best use is as a simulator.

And this is ESPECIALLY important when it comes to Duna airplanes. The Duna environment is so different from Kerbin that you can't tell anything at all about how a plane will fly on Duna by testing it on Kerbin. And in a stock game, an ion-powered plane won't even leave the ground on Kerbin these days, so you can't even get the wrong impression of it . So if you don't do some testing first, you're going in totally blind and the plane will probably crash so fast that you won't even be able to identify its problems so you can fix the next one.

I learn by trial and error. Since I am designing SSTOs, they'll be plenty of delta-v to get back into orbit. I'm only really looking to design an SSTO with enough lift that it merely can land safely on Duna without the use of retroburns. I only need the stall speed around 70 m/s. My last SSTO stalled around 120 m/s, but it wasn't designed to fly on Duna to begin with. This really seems like a simple problem of just adding lift, and I'm trying to do that without big wings so that I can more easily land on rough terrain. I don't plan to fly around the planet cause the game tends to crash after 15 minutes of low altitude flight, so I suspect having to refuel in orbit. Duna to me is still magical cause I've only been on it once, and it was a learning experience, and I don't want to tarnish that with easy mode.

Edited August 17, 2015 by Edax

[Sharpy]

#1) As far as drag is concerned... are you sure?

#2) Heat occulusion is not the same as drag occlusion. You can definitely occuluder radially attached parts on a 1.25m stack with a 2.5m heat shield.

A fair point. Measuring heat is easier than measuring drag. I believed the two strictly related, with all other heat sources inactive, but I guess this may not be the case. (but yes, I'm sure about the heat occlusion, I struggled with airbraking an asteroid quite enough to see when my gigantors overheated and when they survived the airbraking.)

[FancyMouse]

I used to be pretty negative about hyperedit until I want to test stuff that can't be tested on Kerbin system. It really saves real-life time, let me focus on the more important part, and I feel fine by defining myself a principle like never save after a hyperedit. Having an isolated save is also good, but I'm just too lazy to move crafts around.

[cantab]

You can certainly test a Duna plane on Kerbin. Lighten your plane, probably by short fuelling it, to simulate Duna's lower gravity. Then ignore the altitude in metres and instead pay attention to the pressure where you are flying and how that compares to pressures on Duna. About 10 km up on Kerbin is comparable to datum on Duna for example. Of course there will be some differences, for example the tests will have reduced inertia as well as reduced weight.

[Fearless Son]

I'm only really looking to design an SSTO with enough lift that it merely can land safely on Duna without the use of retroburns. I only need the stall speed around 70 m/s.

If all you are doing is going in for a glided landing, I would recommend you try to use just a large set of fixed wings and minimize your other weight. Keep your fuel and engines minimal, and try to use fixed wings with built-in control surfaces. Just make it as minimal as you can reasonably get away with otherwise.

Bit of advice though, I recommend you set the wings high on the body of the glider. No airstrips on Duna means a landing in potentially rough and unpredictable terrain. Low wings are likely to clip on something in such a circumstance.

[xtoro]

Another alternative to high wings, which can mess up your lift/COM is to put the tiny retractable wheels on the ends of the wings. Adds a bit of weight but it has saved my wings many times when hitting rough uneven ground.

[Fearless Son]

Possible, but you also run the risk of knocking one end of the wing up when you need it to be level. Can send the plane spinning across the ground when you are trying to keep it on a linear surface deceleration.

Not a bad idea, but a trade off to consider.

[xtoro]

Yeah I should have mentioned that wing wheels work best if your wings are near the back and they can line up closely to your rear wheels. Otherwise you easily end up spinning around lol

[Edax]

If all you are doing is going in for a glided landing, I would recommend you try to use just a large set of fixed wings and minimize your other weight. Keep your fuel and engines minimal, and try to use fixed wings with built-in control surfaces. Just make it as minimal as you can reasonably get away with otherwise.

Bit of advice though, I recommend you set the wings high on the body of the glider. No airstrips on Duna means a landing in potentially rough and unpredictable terrain. Low wings are likely to clip on something in such a circumstance.

This might be a dumb question, but I'm not fully understanding the purpose of control surfaces on the wings. I put control surfaces at both ends of the space plane so that it can pitch up and down, so what is the function control surfaces on the wings given that their so close to the center of mass? Often times in-game they seen uncooperative, such as 2 sets of controls surfaces on a big-s wing will cause 1 set to increase lift, and the other set to decrease lift (again, on the same wing) when I'm trying to pitch the plane. This doesn't seem to make any sense logically.

Edited August 17, 2015 by Edax