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how to determine wing size for an airplane


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after my previous project for an helicopter-based eve ssto failed, i'm trying again with a different project, this time as a spaceplane. the project calls for a propeller-powered flight until at least 15 km, followed by turning on rockets. the previous project failed because gravity drag costed too much fuel and i had nowhere near enough left to circularize orbit. hopefully, wings will keep sustaining my ship until the upper atmosphere. i've seen the concept is workable on a youtube video, but i don't want to take anything else from there, i want this to be my ship.

anyway, i know very little of making airplanes in this game, so i'll probably post a lot of questions in the coming weeks. first one is, i put together the fuselage with what i presume will be enough fuel to reach orbit. now i need wings, but how the hell do i know how many wings are enough? how about control surfaces?

all i know is that if i try to launch the plane on kerbin with rockets (i've yet to work seriously on the propellers) it will reach 60 m/s without taking off, then it will turn to the left (always the left, though it is perfectly symmetrical) and crash. an expected outcome for a first prototype. but i have no way of knowing if that's caused by lack of wings, lack of speed, lack of control, or whatever other issue.

in the wiki i can see a drag coefficient and a lift coefficient, but i don't know how to use them

Edited by king of nowhere
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3 hours ago, king of nowhere said:

then it will turn to the left (always the left, though it is perfectly symmetrical) and crash.

"Veering to the left" off the runway is a very common design problem. In fact, it's so common that it's listed in the FAQ on this particular forum. Unfortunately, there are dozens of possible technical issues that can cause this behavior. And each has its own fix. However, IMO the most common problem is that there is too much "ground drag" on the front wheel. One of the lessons of KSP is that to have passive stability (in the air or on the ground), you need low drag at the front, and higher drag at the back. If your problem is ground drag, then the cure is to switch the "Friction Control" on your front wheel to Manual, and then reduce it down to .6 or less.

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2 hours ago, bewing said:

"Veering to the left" off the runway is a very common design problem. In fact, it's so common that it's listed in the FAQ on this particular forum. Unfortunately, there are dozens of possible technical issues that can cause this behavior. And each has its own fix. However, IMO the most common problem is that there is too much "ground drag" on the front wheel. One of the lessons of KSP is that to have passive stability (in the air or on the ground), you need low drag at the front, and higher drag at the back. If your problem is ground drag, then the cure is to switch the "Friction Control" on your front wheel to Manual, and then reduce it down to .6 or less.

that's actually good to know, but that's the kind of small kinks i work out later. right now, what i really want is to figure out if three pairs of big-S delta wings are enough for a 200-ton vehicle, or too many, or too few. if they are enough, why my vehicle does not take off below 200 m/s, and if they are not enough, how many will i actually need.

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It's almost certainly enough wing to get you off the ground, so not being able to take off easily is probably down to something else. The first two things I would try would be moving the rear landing gear forwards, so you are pivoting nearer the middle of the craft than the tail, and also considering whether the gear you are using are chunky enough for the weight of the craft (in my experience, a heavy craft squashing it's inadequate gear out of line is the second most likely cause of veering on the runway, after the one described by viewing).

 

Whether it is enough wing for your eventual goal... 

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20 minutes ago, king of nowhere said:

right now, what i really want is to figure out if three pairs of big-S delta wings are enough for a 200-ton vehicle

I often use a single pair for a 40 tonne spaceplane. So 3 might be enough. 5 is certainly enough -- provided your design has ample wing incidence, the CoM and CoL relatively close, no preset downforce, etc. etc..

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My rule of thumb for Kerbin spaceplane SSTOs is 0.33 to 1.0 "lift units" (see part descriptions) per ton of craft. A lot of people will find that rather high, but it makes them easy to fly and land while fully loaded.

Since gravity on Eve is 1.7x Kerbin, but the air pressure at sea level is 5x, that translates to 0.11 to 0.34 "lift units" per ton for the same flight characteristics (if my assumptions are correct).

Since you're new to planes, I can HIGHLY recommend this as a primer: 

 

You've chosen a very challenging task, so my advice is to keep it as simple and symmetrical as possible, and minimize mass and DRAG at all costs. Don't go for "pretty".

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10 hours ago, FleshJeb said:

My rule of thumb for Kerbin spaceplane SSTOs is 0.33 to 1.0 "lift units" (see part descriptions) per ton of craft. A lot of people will find that rather high, but it makes them easy to fly and land while fully loaded.

Since gravity on Eve is 1.7x Kerbin, but the air pressure at sea level is 5x, that translates to 0.11 to 0.34 "lift units" per ton for the same flight characteristics (if my assumptions are correct).

Since you're new to planes, I can HIGHLY recommend this as a primer: 

 

 

i've read it - in fact, i was linked to this already in the past. unfortunately, it does not tell me why my thing won't fly - not below 200 m/s, at least. i tried the propellers and i can get to 60 m/s on kerbin, it's a good target speed for takeoff. for ease of reference i post a couple of pics

cmwH4UX.jpg

OUvAmVy.jpg

this second one also has the smaller rover that goes into the cargo bay

Quote

You've chosen a very challenging task, so my advice is to keep it as simple and symmetrical as possible, and minimize mass and DRAG at all costs. Don't go for "pretty".

i've chosen an even harder task, because a thing that just goes up and down does not suit me, i have additional specifications:

- vehicle has a 3-ton cargo bay containing a 3-ton rover. it must be possible to deploy the rover and getting it back into the cargo bay with a robotic arm (this adds 7 tons of dry mass, and forces me to make a bigger vehicle. plus the cargo bay must be high enough from the ground)

- vehicle must manage ssto on kerbin and laythe too. if the vehicle needs rocket braking, then it must be able to refuel on site.

- vehicle must have a clamp-o-tron senior

- vehicle should manage to lift off from water, or at least to reach land without additional fuel expenditure

then again, that's pretty much the only challenge i have left that i cannot solve with bigger rockets or more time

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5 minutes ago, king of nowhere said:

i've read it - in fact, i was linked to this already in the past. unfortunately, it does not tell me why my thing won't fly - not below 200 m/s, at least.

In a glance, it seems to me you have no wing incidence. (section 7 of Aerogav's guide) .  The main benefit is to reduce fuselage drag but incidentally may also solve your take off issues.

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1 hour ago, Spricigo said:

In a glance, it seems to me you have no wing incidence. (section 7 of Aerogav's guide) .  The main benefit is to reduce fuselage drag but incidentally may also solve your take off issues.

 

1 hour ago, eatU4myT said:

...things I would try would be moving the rear landing gear forwards, so you are pivoting nearer the middle of the craft than the tail...

i did both. 5 degrees angle incidence of the wings, using the shift-W technique described. it wasn't able to take off without rockets, but it did take off as soon as i activated rockets, at a much lower speed than was needed earlier. so, it mostly works. now i have to optimize a lot

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3 hours ago, eatU4myT said:

Once it's in the air, what is the slowest speed it can fly straight and level at a couple of hundred metres? And, can the propellers get you that speed on their own?

was difficult to estimate, until i finally realized i could just block the rockets from gimbaling.

after that, 80 to 100 m/s. maybe a bit more, the plane had a tendency to go down at those speed, and trying to pull up makes it stall.

EDIT: i tried on eve, and there i was able to lift around 60 m/s. which i could only reach going downhill, so i could not stay in the air. propellers are a pain because i have to change the blade angle constantly to adjust to velocity, and it's not clear exactly how much to adjust it.

Edited by king of nowhere
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So, disclaimer 1) I don't know much about the workings of propellers, and disclaimer 2) I know nothing about atmospheric flight on Eve.

That said, if the propellers aren't getting you to take off speed, I would presume you need either more of them (if that's how they work), or more lift to weight. I can't see any amount of optimization getting you to 15km!

Be very interesting to see how you get on with this - no plans to try anything as ambitious, but I'm certain you are going to make some interesting discoveries on the way that will be relevant to us all!

 

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13 hours ago, king of nowhere said:

i did both. 5 degrees angle incidence of the wings, using the shift-W technique described.

No, you didn't. That is how it would look: 

d39h3SI.png 

And that Engine cluster adapter is killing that craft. A painful death it seems.

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6 hours ago, Spricigo said:

No, you didn't. That is how it would look: 

d39h3SI.png 

 

and that is how it does look. the pictures are from before i tilted the wings

Quote

And that Engine cluster adapter is killing that craft. A painful death it seems.

i know they make problems, but i want a clamp-o-tron senior on the craft, and this setup allows me to put it on the back. put it on the front would be too much drag, and it can't be put radially, or at least i never figured out a way. otherwise i have to put the engines radially, i'm not sure it would be better. but i'll certainly try other models.

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6 hours ago, king of nowhere said:

 

i know they make problems, but i want a clamp-o-tron senior on the craft, and this setup allows me to put it on the back. put it on the front would be too much drag,

Is too much drag in the back too. Open/mismatched nodes are horrendous for drag on any orientation.

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28 minutes ago, Spricigo said:

Is too much drag in the back too. Open/mismatched nodes are horrendous for drag on any orientation.

yes, because on the back it generates suction. however, i assume it's still less bad than it would be in front. unfortunately, there isn't a shielded version of it, and i want to have it to couple with an orbiter. on a 200-ton vehicle, it won't be a single docking bay that makes the difference

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1 hour ago, king of nowhere said:

i assume

You don't need to assume, you can press F12 and take a look at the aerodynamic overlay.

You may also take a look at the debug menu, under >physics>Aero:

  • Display Aero Data in Action Menu
  • Display Aero Data GUI

In any case, at some time you need to start to make the compromisses that allow your craft to actually have a chance to make orbit. Keeping that exposed node is not making things easier.

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55 minutes ago, Spricigo said:

You don't need to assume, you can press F12 and take a look at the aerodynamic overlay.

oRXX5td.jpg

yes, i'm sure all those arrows mean... something.

ok, i can see that those arrows are telling me that the wings are pushing upwards, and i have some drag. none of that is exactly surprising.

and let's not start talking about the effects around the propellers


 

Quote

 

You may also take a look at the debug menu, under >physics>Aero:

Display Aero Data in Action Menu

Display Aero Data GUI

In any case, at some time you need to start to make the compromisses that allow your craft to actually have a chance to make orbit. Keeping that exposed node is not making things easier.

 

hey, now that's something actually useful. numbers, i can understand.

and what the hell, it's telling me that i have a total lift of less than 50 tons. going as fast as i can with the propellers (and not all that slow, i'm over 200 km/h, most planes have takeoff/landind speed slower than this).

according to this screen, i'd need to quadruple my wing surface. i'd rather not use 20 pair of wings; any other trick i may use?

by the way, it says total resistance 25 kN, of which only some will be caused by the clamp-o-tron. one single propeller blade gives me that much forward push, so i am right in assuming i can afford it. what i really can't afford is trying to make a spaceplane working in a most hostile environment, with a heavy payload, when i barely even know the basics about atmospheric flight. there's a reason i tried to make a helicopter first1. and now that i decided to go ahead with a fully reusable eve lander, i'm too headstrong to admit defeat, so i'm trying to learn spaceplanes.

now i tried to remove that docking port and put a nose cone in its place, and the resistance is actually a negative, so maybe it's not what i assumed and i can't have a hard number of how much drag it is providing. but the plane is 4 m/s faster on the runway. i don't see any difference in lift2.

1 also, "helicopterocket" is a much better name than anything i can conceive for a space plane.

2 if the docking port shows to be such a huge deal, i have a plan for it. I attach another docked clamp-o-tron, with a nose cone, and i will put a claw on the orbiter. upon docking, the orbiter will pick up the nose cone with the claw. upon landing, the lander will get reattached to the nose cone. the question is, are those 400 kg of dry weight i would add any better than the drag?

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I would highly recommend using the fan blades instead of props.  They have higher top speed, more thrust, and are more compact.

Also, this is Eve we are talking about.  It is punishing.  Unforgiving.  Cruel.  Forget the rover, forget the solar panels, forget the large isru, forget the docking port.

Eve does not give you the luxury of having luxuries.  The last time I made an Eve ssto it was 180 tons, and had a payload to low Eve orbit of 3 tons of liquid fuel.  Eve is HARD, there are probably less than 2 dozen players who have made successful Eve ssto's

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30 minutes ago, Lt_Duckweed said:

Also, this is Eve we are talking about.

Indeed. I'd consider dropping a 3 ton (really? why?) rover on Eve with a parachute, but a large ISRU, a big cargobay with a fat-ass rover, robotics, multiple engine types, all in an Eve SSTO spaceplane when you're new to planes in general? It's basically self-flagellation.

Most people consider it an achievement to get off Eve at all, let alone hauling that amount of junk.

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1 hour ago, steve_v said:

Indeed. I'd consider dropping a 3 ton (really? why?) rover on Eve with a parachute, but a large ISRU, a big cargobay with a fat-ass rover, robotics, multiple engine types, all in an Eve SSTO spaceplane when you're new to planes in general? It's basically self-flagellation.

 

yes. it is.

a good challenge that will keep me interested for weeks is indistinguishable from self-flagellation.:cool:

Quote

Most people consider it an achievement to get off Eve at all, let alone hauling that amount of junk.

just this morning i took on a mission to recover 27 tons of ore from eve. it's hauling a lot more junk than my plane project.

but it wasn't particularly challenging. it just needed a very big ship. I started stacking my asparagus, and once i tried and found it was not powerful enough, i just added another stack.

I already have a rover that can take off and land from any world without atmosphere, refuel, explore, it's fun to drive and almost indestructible. whatever stuff i want to do on those planets, i send my rover and that's it. and i have an helicopter that can go anywhere on worlds with an atmosphere. plus the mechanism to put it in a cargo bay. if i just wanted to do things in a simpler way, i'd strap some rockets to the cargo bay with an expendable staging mechanism.

having a rover that i can land and take off from eve in a reusable way is pretty much the only real challenge i have left. at least, the only one that interests me.

 

i would agree, though, that i should learn to make planes in general before trying an eve ssto spaceplane. but that's pretty much what i'm doing. right now my objective is simply making a plane of that size work. i already can make a small plane, unfortunately the square cube law means i can't just scale things up.

EDIT: i almost missed a perfectly good opportunity to quote

Quote

i choose to make an eve ssto and do the other things, not because they are easy, but because they are hard

 

Edited by king of nowhere
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17 hours ago, king of nowhere said:

i know they make problems, but i want a clamp-o-tron senior on the craft, and this setup allows me to put it on the back. put it on the front would be too much drag, and it can't be put radially, or at least i never figured out a way. otherwise i have to put the engines radially, i'm not sure it would be better. but i'll certainly try other models.

FYI, you can use a BZ-52 Radial Attachment point to attach a senior docking port to the side of a craft. It's the part in the 'Structural' section that looks like a regular docking port. Using this method leaves a gap between the side of the craft and the docking port, but I use the 'Move' tool to push it flush.

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13 hours ago, Lt_Duckweed said:

I would highly recommend using the fan blades instead of props.  They have higher top speed, more thrust, and are more compact.

 

ok, done. that was good to know, i gained a bit more of speed

 

7 hours ago, Grogs said:

FYI, you can use a BZ-52 Radial Attachment point to attach a senior docking port to the side of a craft. It's the part in the 'Structural' section that looks like a regular docking port. Using this method leaves a gap between the side of the craft and the docking port, but I use the 'Move' tool to push it flush.

nice, i've been looking for such an option.

 

QEhV3gU.jpg

aaand... we have first takeoff on eve!

i reached 1700 m before running out of electricity (even activating infinite electricity apparently didn't work without some source of power on board, and i hadn't yet put the rtg). the plane has a slight tendency to spin clockwise around its axis that must be compensated manually, but it is otherwise capable of flight and stable enough.

in the next days i will have to see how high it can go, and if that height is enough to reach orbit with rockets. i will also need to find a good way to turn the plane when it is landed, in case i can't pick up speed because i face a mountain. i need to find ways to deal landing on water. and i need to learn to survive an atmospheric reentry. but for today, that's enough progress

Edited by king of nowhere
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Aircraft design rule of thumb:   If it looks like an airplane, it should fly like and airplane.

Can also be read as: An aircraft in flight handles just about as good as it looks. 

 

There's no mystical reason for there being a direct relationship between an airplane's eye-pleasing appearance and it's general airworthiness. It is all but a simple matter of proportions.  It just so happens that the same geometric concordances that make an airplane wondrous to behold, are also those that make it fly properly.

 

 

So the question of "Will it fly?" can have it's answer pretty reliably approximated by replacing the inquiry with "Does it look like a proper airplane?".

You'll find that in almost all cases, these questions will have the same answer.

 

 

Planes that look weird/ungainly IRL almost all depend on fly-by-wire or some form of artificial stability enhancement; Compare an Airbus A320 to a Boeing 737 (Not MAX)  and you may notice, how the very aesthetics of their designs openly tell which of the two really needs to have a computer behind the flight stick to make it a decent ride.

 

So it's mostly not about how much wing a thing needs before it'll fly - But more of a "Where should the wings go" decision. 

Having more or less lift area will certainly change how your craft flies. A smaller wing implies proportionally faster takeoff and landing speeds, plus a bunch of other consequences;  All of these things can be played around with fairly loosely (you'd be amazed what can fly when it's built right) as long as the placement of centers of mass, lift and drag are correctly arranged.

 

Simple formula:

 

For a plane with the tail behind the main wings, place the CoG somewhere between 25% and 33% of the mean wing chord (that's the average distance between the leading and trailing edges over the whole wingspan)

This can produce a natural pitch-down tendency, which you then angle the tail fin to correct. (rotate stabilizer until the blue marker lines up with the yellow one) 

Such a configuration is extremely stable, allowing you to fly even with your hands off for a while. Though you get a bit more drag for having the tail actually pushing DOWN to get the nose back up. (This is why the A320 looks awkward as if its wings were too far forward. They really are, this being more fuel-efficient, and FBW takes care of how it handles anyways....)

 

Note however, if you try this with a Canard type design, it'll probably flip over and crash on its back.

"Putting the tail on the nose" is a very effective way to remove the "tail pushing down" extra drag - Yet again, this design is FAR less stable than any one where the tail comes after the plane.

A flyable canard can be built, however, and even be relatively stable (though never as much as a conventional tail) if done right.

 

For stable-ish Canards:  Put the CoG somewhere between half and one wing chord's worth AHEAD of the leading edge.  Yes, a stable canard should have the CoG actually outside the wing chord section. 

Stability results from having enough wing area on the rear that the center of drag is placed well back, whilst ensuring the center of lift remains aligned with the CoG - That makes the nose pull towards straight if you trim it right and let go of the stick. 

It's tricky sometimes to get this right, and takes some fiddling, but once you get it stable enough so you can fly it, it's lift/drag ratio gets you the best bang for your buck

The key is to find the balance between the front fins generating enough lift (in level flight) to counter the pitch-down force caused by having the wing way behind the CoG

The big challenge is to do this in such a way that when you stall, the nose still comes down before the tail does (or it'll tumble backwards all the way to crash site)  

 

 

Now, A pure flying wing design is the trickiest of all to get right - but that is the holy grail of aerodynamic efficiency. In KSP, you'll need to have various different wing bits along the span, and angle them just right so that the tip is swept back far enough to add a bit of downforce behind the centers of lift and mass.  It's possible, but make sure you got a nice lineup of pilot volunteers for test flying as you tweak the thing until it eventually becomes controllable.

BTW:  If you stall out of control on a flying wing aircraft, just eject.  There's usually not enough altitude anywhere between space and "splat" for it to safely recover....

 

 

Flying is really all about doing it in a controlled way - This is actually the big breakthrough that allowed the Wright Brothers (of Earth, some obscure backwater planet in the less developed parts of the galaxy) to first achieve powered flight.  Fact is:

 

Anything going fast enough can achieve flight. It's being able to decide how it comes back down that makes it an aircraft.

Furthermore, being able to do it a second time makes it a pretty good aircraft.

Edited by Moach
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