Thinking about making the switch to FAR.

[UnusualAttitude]

My latest design has a triple rudder - three Standard Canards - and is still experiencing issues with side-slip (either that or I've got a bigger issue - plane has a tendency to want to flip around and fly backwards once I get going good). I'll try swapping the middle one out with a wing piece tonight and let y'all know how that turns out - it was that same one I swapped out last night that crashed FAR.

Yup, standard canards just won't cut it as tail fins unless it's a very small, light design.

Also, bear in mind that the farther your fin is from the CoM, the more effective it will be at reducing sideslip. That's where mid-mounted swept wings (see Wanderfound's design) are superior to the tailess delta: the delta's centre of mass may be just too close to the fin for the latter to provide sufficient leverage.

Solutions to improve delta wing yaw stability include dihedral (tilting up the wing tips) and winglets (a vertical wing section on the end of the wing).

UA

[Van Disaster]

Also, bear in mind that the farther your fin is from the CoM, the more effective it will be at reducing sideslip. That's where mid-mounted swept wings (see Wanderfound's design) are superior to the tailess delta: the delta's centre of mass may be just too close to the fin for the latter to provide sufficient leverage.

Not just fins, any control surface - all the surface is is a lever system where the amount of deflection is the force, but it's a common fault to not pay attention to leverage and then wondering why control surfaces can't keep a plane pointing straight. The shape of a wing can also be seen in terms of leverage, but that's a little harder to put into a simple description.

Edit: took an illustrative shot; this is a pretty small plane, I stuck a mk1 fuselage piece and a stock tailfin on the top for comparison. This is the minimum size fin I'd look at using, and even that is not terribly laterally stable - although this is actually a spaceplane & it needs a large fin for low density atmospheres. I'd probably use a big fin anyway, though.

Edited March 11, 2015 by Van Disaster

[NathanKell]

To talk about DCA, first we need to talk about Q (dynamic pressure). All lift and drag is scaled by Q, and Q = 0.5 * density [in kg/m^3] * velocity^2 [srf velocity, in m/s]

Control surfaces work by applying lift and drag. So the faster you go, and the denser the air is around you, the stronger the control effect you will get. Sometimes that's too strong, especially because if all you have to fly with is the keyboard, you can't just nudge the stick a bit, it's either no deflection or full deflection.

DCA helps deal with this: you set a reference speed and altitude (and thus a reference Q) and your control deflection is scaled such that, say, holding down W always results in the same control force, not the same control deflection. Since over-controlling can lead to departing from controlled flight or even things breaking off, DCA can be a big help.

[capi3101]

Okay...so I swapped out my procedural tail last night with a Swept Wing A, which had an Elevon 3 attached to the backside for the central rudder. FAR said the plane would destabilize around 25k/Mach 3 - M_w was in the red and the simulation for w "wildly diverged". FAR was right on the money. At this point I'ma thinkin' I'll go back to a Standard Canard, since in that configuration I at least fault my piloting and not the design itself. I'll have sideslip but it's better than having a plane that has the potential to pitch up and instantaneously break up...like it did last night......

I'm trying to think if I've got any specific questions for y'all this morning - maybe this one: what exactly is the significance of the V *(L/D) parameter in the flight data? Is it some kind of way of telling me how fast I can expect the plane to get to or something?

[Wanderfound]

Pitch-up problems imply not enough horizontal stability. Beef up the horizontal tailplane, shift canards rearwards, pull CoM forwards.

Not sure about V*(L/D). Velocity times Lift over Drag, but I can't think of what it would imply apart from general high speed slickness.

[eddiew]

If I remember right, Mw is the longitudinal (pitch) indicator, and says you're going to have problems keeping the nose up (or maybe down). 25km @ mach 3 I'd mostly expect Mw to be in the red since that's really quite slow for such a thin air density and most planes would start to wilt. 25km @ mach 4 would be a much better indicator, and you might find the red goes away

If you're still showing red at mach 4, then there's a few factors to look at:

- are your pitch surfaces underpowered? (you can increase the deflection or give them more leverage)

- are you low on TWR? (below 0.7 on the runway is usually hard work with stock parts, 0.8 and up is comfortable, 1.1+ is a free ticket to orbit)

- is your design very draggy? (I remember you had those 'ballast' tanks... please say they're gone now )

- is your L/D below 1? (need more wing area!)

With procedural wings (and control surfaces) this is much easier to tackle since you can easily make them longer/deeper/wider and see how it affects the numbers without having to tear everything off and start over

Pitch-up problems imply not enough horizontal stability. Beef up the horizontal tailplane, shift canards rearwards, pull CoM forwards.

Dammit, ninja'd! I'm still pretty sure that my points are valid though

[capi3101]

I do suspect the problem's in the tail plane - three engine design and the exhaust from the outboard engines are hitting the elevator surfaces. I might try putting the elevators up onto the tail and see if that makes things any better; right now they're all level with the engines.

The RCS ballast tank isn't exactly gone - it's just "inline" now (by which I mean I have a Mk2 Monoprop tank up near the front of the fuselage for the sole purpose of being able to adjust the CoM). The payload's CoM doesn't exactly match up with the CoM of the plane either; I imagine that's a contributing factor to do my current issues as well.

[Van Disaster]

V*(L/D) is used to work out when you're going to run out of fuel, I think.

Pics are worth a thousand words! the coefficients are detailed in the help button & the FAR wiki, but Mw is the pitching force applied by the plane's downward movement - I assume this is relative to the plane, so any AoA will have a downward component even if your altitude doesn't change. Positive means pitching the nose up will encourage more nose pitch up, so if you don't catch it by applying an equalizing pitch-down force it'll backflip, too high negative and you won't have enough authority to counter it & raise the nose. CoM behind CoL will mean a positive pitch moment is being applied all the time ( levers again ) which is a naturally divergent state and needs active countering somehow - designing a plane which changed the ratio of lift provided by the wings & stabilizers and had positive Mw at low AoA and negative at higher AoA would actually be a rather interesting design, but I don't know how to do that

If it's red at Mach3/25km I suspect your predicted pitch angle is so high that the relative position of CoL & CoM has gone the wrong way; might want to either get there more slowly & build up a little more speed, or add some more wing. The payload's CoM won't matter, if it's part of the plane it's all one system & the resultant CoM is what matters ( until you unload it! ).

@eddiew: if you have a > 1 twr you don't even need a runway, might as well go straight up & forget the wings, even.

[capi3101]

Thought I'd post some screenies of my current plane (finally) so y'all can tell me where I'm screwing up. I did move the horizontal stabilizers up onto the tail; they were down there level to the engine. At this point I find taking off a might difficult.

The plane spun out a little after I took that Mach 3 screenie - I think what happened there was that I moved the nacelles/wings a little further back to compensate for the change of the center of mass - which means I took off the outboard engines, and didn't think to wash the plane through Intake Build Aid again. Classic asymmetric thrust-induced spin out. The plane also had its rear wheels too far forward and was popping wheelies on the runway, and yet when I moved the gear back I couldn't get it off the runway before 150 m/s.

Suggestions for this specific design would be welcome - so far all the corrective actions I've taken have resulted in a plane that has been progressively less flyable (or so it seems, anyways).

[Van Disaster]

My feeling would be to replace the horizontal controls by a bigger fixed front stabiliser with seperate control surfaces as a start.

[eddiew]

Ohhh... now I see why you're having a hard time. Drones are kind of a pig vs crewed planes due to lacking a good counterweight at the front. A cockpit makes a great way to haul the CoM forwards and give you some stability in the air, but a mid-wing drone is much harder to arrange. IMO this doesn't look too far off, however!

You could consider a square wing-piece on the inner edges of the outer engines - something to fill the big gaps between the three engines. They'll clip into the MK2 nacelles, but that doesn't matter; long as you place them on the engines, they'll align sensibly. That space is crying out for something to occupy it, and it'll give you extra lift and control surface space at the far back, with the option of reducing wings.

With that TWR and size, I'm pretty sure that a pair of standard deltas would be enough for the main wings if those gaps are filled in, cutting your mass and drag

Another option; dump the MK2 nacelles for a pair of 1.25m tanks. The cylinders have better fuel capacity to weight ratio, and while they lack a lifting body effect, I don't think you're short of lift. The two spare intakes could move to the nose. It'd give it a bit of a pig snout, but it should work just fine

One more; you could replace the central engine with a turbojet. They're slightly higher thrust than rapiers, and use a fraction less air, and max out slightly faster, so they're a great option for your last-breath thrust at max altitude

And if you're not doing it it, it's well worth setting up some action groups to adjust the engines as you climb. I'd suggest:

1 - toggle inner engines on/off

2 - inner engine air/rocket mode

3 - toggle outer engines on/off

4 - outer engines air/rocket mode

As you climb, watch your air intake % and move through 1, 3+1, 4 so's to maximise your air breathing power while not stretching it into the risk zone (you can generally go to 2-300% use as reported by KER without instability). That central engine can't cause asymmetric thrust so you can let it flame out on its own while the outer ones are burning in rocket mode.

*edit* I'm also not sure that the use of canards for the horizontal tailplane is helping; you're making the airflow around the tail very variable. My suggestion of filling in the gaps above should let you remove these completely however, since they do the same job - or you could put some small deltas with control surfaces there in their place.

Edited March 13, 2015 by eddiew

[eddiew]

Late thought; Wanderfound was specifically looking for a volunteer to supply an unstable spaceplane so he could do a fixit tutorial video:

http://forum.kerbalspaceprogram.com/threads/90747-Kerbodyne-SSTO-Division-Omnibus-Thread?p=1782827&viewfull=1#post1782827

Might be interesting for everyone to see this bird get the treatment

[capi3101]

Sounds like a good idea. I'll post my screenies and see what he thinks.

[Wanderfound]

Thought I'd post some screenies of my current plane (finally) so y'all can tell me where I'm screwing up.

At the moment it's kind of a mutant crossbreed between a delta and a conventional airframe, and isn't working as either. There isn't anywhere near enough wing/stabiliser area at the rear of the plane.

There's a choice to be made at the start of fixing it: convert it to a true delta, or make it work as a conventional wing/tail design?

To convert to delta: pull the wings back until their trailing edge is level with the rear of the engines and fill in the gaps at the rear of the fuselage with wing surface and elevators. Get rid of the canard tailplane. Then spend some time fiddling with canard and rudder size and position to get rid of the last red bits.

For conventional airframe: shift the lateral engines forwards until CoM is centred in the cargo bay. Thin out the chord on the wings, and increase the size of the tailplane to be about half to two thirds the size of the wings. Put landing gear just behind CoM, jacked up on hardpoints if you're worried about tailstrike.

I'd be inclined to take the second option. Even if you get the aero right on the delta (trickier than with conventional airframes; the rearwards biased CoM reduces your stabiliser's leverage), you'll still have off-CoM cargo bay problems.

Edited March 13, 2015 by Wanderfound

[Van Disaster]

Or the third option is to shift a significant amount of lift to fixed canards - which is just the reverse of a conventional tail. This is less stable in some ways but does come with the advantage that to pitch up you add more total lift rather than reducing total lift, and also you can set it up so the canard stalls first which will drop the nose & automatically unstall everything again. Whatever you do I think you need to spread your lifting surface out more longditudinally.

[eddiew]

There's a choice to be made at the start of fixing it: convert it to a true delta, or make it work as a conventional wing/tail design?

This is good advice, tbh Although with drones, I find it easier to make deltas than conventional shapes, due to the lack of weight at the nose. Last time I tried a midwing drone

. Although it does fly really well and is still my go-to heavy lifter

[capi3101]

Hey all, been on Spring Break. Don't worry - I've still only had one successful flight to orbit...but it's largely due to lack of trying.

This morning I came across this; in the same thread where I found that link, I found a suggestion that a plane's fuselage should be 70-75% the length of the wingspan, the nose moment should be 20% and the tail should be 40%. It occurred to me that in KSP 0.90 I have definite data on fuselage length - so are those ratios going to be guidelines I can go with? I think that PDF is talking about model planes but the principles should still be applicable, right?

Still having issues with stability in the 25-30k region getting up to Mach 4 - but if the "tail should be 40%" is referring to the length of the horizontal stabilizers, that's pretty much what you guys have been telling me all along.

[capi3101]

Alright - pics of the latest iteration of the Horned Owl:

Craft uses moveable wings as both the main wing and as the canard and tail stabilators; the first pic has the specific data for the wings while the second one is showing the stabilators. The tail is just a Swept Wing A with an Elevon 5 cobbled on. Craft shows stable up until 30k/Mach 4, when the beta and r axes become unstable. It does undergo noticable mach tuck around Mach 0.8, manifesting as a minor stall of the canards which last until the craft goes supersonic.

Pretty much followed Wanderfound's advice to turn the design into a conventional aircraft design - I don't think there's any advice he gave me that I ignored.

I realize my biggest issue right now is the tail - it's too small; those figures I found yesterday were a pretty big help. I'd use another procedural wing for the purpose of building the tail, except that A) it may just be my local install, but I can't add more than one set of control surfaces to a procedural wing at the moment - and the ones I add have to be close to the fuselage, and I'd like to keep the design as a cruciform tail if I can (unless one of y'all has a better suggestion). The numbers I calculated suggest the area of the tail should be 13.48 square meters, with 3.37 square meters of that devoted to rudder.

Another thing I've come to realize is that I really have no idea what the dimensions of the stock parts are, by which I mean I have no idea what they're surface areas are. There's some vague numbers that suggest control parts almost universally have 0.95 square meters of surface area, for all the more that good that does me; I had a tough enough time getting that single Elevon attached to the tail as it is. Anyways, is there a compendium anywhere that contains that data? I'd still like to try to put together an all stock FAR craft at some point.

[Van Disaster]

You have a wing loading that's approaching 1t/m^2 there ( perhaps 0.8ish if you include the horizontal surfaces which might not always be contributing positive lift ), that's a tad high - the Shuttle had what was called "very high" wing loading at 585kg/m^2, and that only flew downwards. If you want the area of any surface piece, stick it on a fuselage & look at the FAR static derivative tab, it will have the total wing area.

Mach tuck won't start until the wing airflow reaches mach 1; the wing cross-section FAR uses is I believe a reference supersonic shape so I don't think there should be too much oddness in the near transonic area, certainly not at 0.8.

Model planes have a really limited flight regime in general and I suspect some rather crazy power/mass ratio ( and also rather crazy strength/mass ratio ) - certainly use it for rough guidelines but I suspect using that doc as absolute rules is not going to give great results.

[NathanKell]

Also I would really, really not recommend using moving wings for your main wings. And, despite being the guy who did some updates for pwings...get B9 Procedural Wings and use them instead. So much better for general use.

[capi3101]

Hmm...my understanding of wing loading is that it largely affects takeoff speed and maneuverability; lower wing loading means lower takeoff and landing speeds and increased maneuverability, but heavily loaded wings are more suited for higher speed flight. 0.8 tonnes per square meter is comparable to the wing loading of an airliner.

As far as the B9 procedural wings are concerned - is that something I can download and install seperately from the rest of the B9 pack; as I've stated before, I don't have a lot of excess memory as it is.

[Crzyrndm]

it's only wing parts (add my vote for them over the original P-wings. They're apparently not working for some people, but they're so much more user friendly when they do work) Edited March 23, 2015 by Crzyrndm

[NathanKell]

Most airliners I've seen are at only 0.65t/m^2 even at max takeoff weight, not normal takeoff weight. They also have higher-Cl wings; wings in FAR are assumed to be optimized for the supersonic, so they produce less lift, relatively, than those high-lift airliner wings. Also, FAR doesn't support blow flaps or slats, not even fowler flaps; your arsenal of high-lift devices is very limited, and it's those that let an airliner take off at MTOW.

Wing loading affects what angle of attack you must fly at (and take off at), and may require doing above 0.6 mach (200m/s) before takeoff. KSP's parts are very dense, so aircraft are often much heavier than they look, and stock (with its uberlift from soup, just as much as uberdrag) trains one to use too little wing, too.

[Van Disaster]

The nearest real life references for spaceplanes would not be normal airliners; try Concorde/Tu-144/SR-71 or any other simple-wing large high-flying heavy aircraft, if there are any. I'd rule out the XB-70 because it used compression lift, and the B-1/Backfire/Blackjack because they have variable geometry.

The big issue with high wing loading - or to flip it around, insufficient wing area - is when you're up at 25km trying to hit mach 5 to transit to rockets; not enough wing means you'll be at some huge AoA just when drag is building up quickly.

Edited March 23, 2015 by Van Disaster

[capi3101]

Wing Loading is that L/W stat in my flight data, right? Should I mention that it usually ranges from 0.4-0.6 from the time I takeoff to the time the plane inevitably spins out? Or is it telling me something different?