tetryds

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Hey y'all, quick question - while I was at my in-laws I designed a plane that had crappy pitch authority, one of those that didn't really want to pitch up (but did) at takeoff and eventually got into an un-recoverable dive after a bit (and overheated and exploded). I know the usual tricks for fixing this at this point, but I was wondering if there was a good way of predicting this kind of behavior from the stability derivatives prior to takeoff (you know, fix the problem ahead of time instead of getting up there and realizing the problem exists). So, how about it - is there a value in the stability derivatives that should say to me "your pitch authority is going to go from suck to blow at this point"? It'd be a relatively high magnitude green number - I'm thinking Mw but I'm not wholly sure.

When you are looking at the FAR Control & Analysis tab in FAR, the one with all of the numbers, change the speed and altitude numbers to the altitude in KM and speed in mach that you are having the problem at. I am pretty sure you are going to see a Down Vel Derivatives authority number go well into the red. If the Mw number is in the red at a set speed you will find that you are having a loss of pitch authority. The easiest fix for this is canards.

dnPuHqI.jpg

But what does the aircraft look like do you have any pictures?

Edited by Hodo

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@Hodo: Canards to fiz positive Mw is a bad idea, he would need to tweak it for artificial stability I wouldn't suggest that.

Moving the wings back is the easiest way to fix Mw.

*I am just talking about fixing a red Mw by the way.

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Maybe I didn't ask the question well - at what magnitude of Mw should I expect to see lawn dart behavior for a given speed and altitude? And is the Mw what I should be looking at to try and predict that behavior?

I did think to check the AoA graphs as well; I've read that most successful designs keep the dM/dα between 0.5 and 0.65. But to see if that's how the graph plays out, I kinda need to know the numbers involved.

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I did think to check the AoA graphs as well; I've read that most successful designs keep the dM/dα between 0.5 and 0.65. But to see if that's how the graph plays out, I kinda need to know the numbers involved.

No numbers involved. Just check which AoA you can reach when pulling all the way. Possibly also compare that to the AoA needed for level flight.

EDIT: In case it helps:

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Edited by FourGreenFields

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Maybe I didn't ask the question well - at what magnitude of Mw should I expect to see lawn dart behavior for a given speed and altitude? And is the Mw what I should be looking at to try and predict that behavior?

I did think to check the AoA graphs as well; I've read that most successful designs keep the dM/dα between 0.5 and 0.65. But to see if that's how the graph plays out, I kinda need to know the numbers involved.

Kind of what FGF said, but it's a bit more complicated than that.

Sometimes you cannot get a decent pitch authority because your control surfaces are going to stall.

It's not too obvious to detect control surface stalling using the graphics, if the L/D max changes way too much between pitch = 0 and pitch = 1 you may be stalling the control surfaces.

Well, Mw itself is not the whole story, this also depends on the moments of inertia, gimbal, etc.

So yes, look at the graphics, if you overshoot with too little deflection you are too unstable, if something around 20 degrees is still not enough you are too stable.

Can you provide a picture of your design, also showing an AoA sweep from 0 to 45 degrees at Mach 0.6 and stability derivatives at 5km Mach 0.6?

Edit: oh look apparently there is something about elevator control deflection on my signature.

Edited by tetryds

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Alright - I could've sworn I took more screenshots than I apparently did (in fact I know I hit the F1 key more times than this). I didn't get the stability derivatives at Mach 0.6/5k but I did get the AoA sweep at least; I figured that'd give y'all something to look at. If need be I'll get the derivative screenie tonight; meantime here's the Condor:

Not much to say about this plane other than what's in the comments. She doesn't want to pitch up very easily after takeoff but she'll fly well enough right up to the point where I want to kick on the rockets, at which point she's fully deflected upwards and still going down. For good measure I did prep the derivatives/AoA graph/sims for those conditions (Mach 3/20 k) and I got those screenies...probably not helpful but you never know.

Should probably mention that the wing/tail/fin are all B9 procedural parts. The tailplane is an all-moving stabilator; the wings have an aileron and spoiler attached. I'm aware of the inadequacy of the fin/rudder.

Edited by capi3101

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Alright - I could've sworn I took more screenshots than I apparently did (in fact I know I hit the F1 key more times than this). I didn't get the stability derivatives at Mach 0.6/5k but I did get the AoA sweep at least; I figured that'd give y'all something to look at. If need be I'll get the derivative screenie tonight; meantime here's the Condor:

Not much to say about this plane other than what's in the comments. She doesn't want to pitch up very easily after takeoff but she'll fly well enough right up to the point where I want to kick on the rockets, at which point she's fully deflected upwards and still going down. For good measure I did prep the derivatives/AoA graph/sims for those conditions (Mach 3/20 k) and I got those screenies...probably not helpful but you never know.

Should probably mention that the wing/tail/fin are all B9 procedural parts. The tailplane is an all-moving stabilator; the wings have an aileron and spoiler attached. I'm aware of the inadequacy of the fins.

Well, no wonder this thing won't move. Loads of lift on the rear, and relatively high mass up front. The elevators aren't that far from the wings, which means they won't help you stay in the air much.

Add canards. On the tip of the cockpit and/or behind the cockpit where the cross-section is smallest. That should make this thing a lot more agile.

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Hey guys,

I've built two aircraft that are performing fairly well with FAR, and now I'm trying to optimize them. Specifically, I'm trying to optimize a fighter for CONSISTENT manuverability at mach 1.5 and low altitude both before AND after it has dropped its payload (BDArmory two AIM-9s, four AIM-120s, and four AGM-65s).

JOGzk33.png

I'm also trying to optimize a recon aircraft for consistent manuverability at mach 2 and low altitude with a full fuel tank AND a nearly empty one.

NHPgaoj.png

Anyway, I don't know what kind of information or math I need to optimize these aircraft. Does anybody have any advice?

(If you're wondering why I'm putting so much emphasis on consistent manuverability, it's because I'm flying with a keyboard, not a joystick.)

Edited by CursoryRaptor15

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Hey guys,

I've built two aircraft that are performing fairly well with FAR, and now I'm trying to optimize them. Specifically, I'm trying to optimize a fighter for CONSISTENT manuverability at mach 1.5 and low altitude both before AND after it has dropped its payload (BDArmory two AIM-9s, four AIM-120s, and four AGM-65s).

http://i.imgur.com/JOGzk33.png

I'm also trying to optimize a recon aircraft for consistent manuverability at mach 2 and low altitude with a full fuel tank AND a nearly empty one.

http://i.imgur.com/NHPgaoj.png

Anyway, I don't know what kind of information or math I need to optimize these aircraft. Does anybody have any advice?

(If you're wondering why I'm putting so much emphasis on consistent manuverability, it's because I'm flying with a keyboard, not a joystick.)

For those speeds I'd simply work with trimming for elevator control. Alt + w/s.

Else you'd probably need to tune the thing so the CoM doesn't move.

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Hey guys,

I've built two aircraft that are performing fairly well with FAR, and now I'm trying to optimize them. Specifically, I'm trying to optimize a fighter for CONSISTENT manuverability at mach 1.5 and low altitude both before AND after it has dropped its payload (BDArmory two AIM-9s, four AIM-120s, and four AGM-65s).

http://i.imgur.com/JOGzk33.png

I'm also trying to optimize a recon aircraft for consistent manuverability at mach 2 and low altitude with a full fuel tank AND a nearly empty one.

http://i.imgur.com/NHPgaoj.png

Anyway, I don't know what kind of information or math I need to optimize these aircraft. Does anybody have any advice?

(If you're wondering why I'm putting so much emphasis on consistent manuverability, it's because I'm flying with a keyboard, not a joystick.)

One of the biggest problems I can see you running into is the lack of length to the craft. This with the external weapons load causing a shifting drag issue when they are used, I would extend the body of the craft and switch to an internal weapons bay setup. This would keep the craft streamlined and would extend the craft length to smooth out your wave drag flow. This inturn will make the craft faster with less power. You can then adjust the agility of the craft from that point at a given speed pretty simply.

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Well, no wonder this thing won't move. Loads of lift on the rear, and relatively high mass up front. The elevators aren't that far from the wings, which means they won't help you stay in the air much.

Add canards. On the tip of the cockpit and/or behind the cockpit where the cross-section is smallest. That should make this thing a lot more agile.

Did as you suggested (added canards) and it did help matters; I was able to get the Condor into orbit. Also swapped out the cockpit for a Mk2 inline, then added a Mk2-1 Adapter up front, attached a service bay to that so I could put the probe core into it, then an NCS Adapter to that, an Avionics hub onto that and finally an extended Communotron 16 to the end of that.

Re-entry has proven...interesting. I've been making un-kerballed space flights and the probe core I've been using is an OKTO. Without fail, the OKTO has been the first (and only) part to explode on re-entry, this despite it either being nestled snugly in a closed cargo bay (and later in a closed dedicated service bay). I'm thinking that's probably some kind of bug at work - the rest of the plane survives intact up until the point where it usually falls into the drink (you know, because the probe core exploded and so I lose control over the craft at that point). I'd ask for help on this one, except I'm still in 1.0.2 and still using Garabidean (SP - not using the latest FAR version)...

I suppose that's a question - can you still use Glauert with 1.0.2? Because if you can, I have no reason not to upgrade...

Edited by capi3101

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Re the octo heating up; the whole thermal mechanism is pretty dubious at the mo. Large parts have a habit of rapidly dumping excess heat into nearby smaller parts, and this seems to include an ability to flow through/into cargo bays. The small parts then detonate because they have a lower heat tolerance. I get the same thing - reaction wheels inside cargo bays can explode for 'no reason'.

Any reason for not moving to 1.0.4 though? There were some bugfixes and the heating overall is more manageable than 1.0.2...

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Any reason for not moving to 1.0.4 though? There were some bugfixes and the heating overall is more manageable than 1.0.2...

Mods compatibility; there are several I use that don't say they're ready for 1.0.4 yet. That and I just haven't done it yet.

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Re the octo heating up; the whole thermal mechanism is pretty dubious at the mo. Large parts have a habit of rapidly dumping excess heat into nearby smaller parts, and this seems to include an ability to flow through/into cargo bays. The small parts then detonate because they have a lower heat tolerance. I get the same thing - reaction wheels inside cargo bays can explode for 'no reason'.

Any reason for not moving to 1.0.4 though? There were some bugfixes and the heating overall is more manageable than 1.0.2...

I found a fix for my random parts exploding due to heat.... Deadly Re-Entry. Fixed most of my heating issues.

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The simplest way to fix part overheating is to reduce the conduction multiplier in the debug window->physics->thermals.

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So I am back in the cargo hauling SSTO business, I needed something to ferry passengers, science and cargo into orbit so this is what I came up with.

iVIUKr0.jpg

Iu1pKZa.jpg

100km x 100km orbit as per HARM standards. It made a repeat flight the next day with 2 passengers and a lab in the cargobay and a mineral scanning satellite. Unfortunately the engine on the satellite died due to the overheating bug..... but the satellite works at the altitude it is at.

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Wonderfully inspirational thread,

some incredible designs!:3

I've started to get a hang a bit of prop aircraft with FAR and am having another fase of my twin-boom fetish so I have created a SAAB 21 inspired exploration plane, the D20, for my early career. I've been incredibly surprised at how stable and responsive it flies.

A very slow climber and stalls out at about 4.5km, but a lot of that is probably to blame on the weak engine (SXT's K15 at 1.25m scale).

It cruises at around 125 m/s and is capable of reaching over 200 m/s in dives from which it recovers quite well.:)

J7hZJ8Jm.jpg

MYcU2o3m.jpg

4My7YDtm.jpg

0rG1jmLm.jpg

Any tips on how to make it climb better, reach better heights?:)

Slowing it down for landing is a slow process and I still often hit the ground at a scary ~125 m/s. I am assuming flaps will help a lot with that once I get my actiongroups unlocked.

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Wonderfully inspirational thread,

some incredible designs!:3

I've started to get a hang a bit of prop aircraft with FAR and am having another fase of my twin-boom fetish so I have created a SAAB 21 inspired exploration plane, the D20, for my early career. I've been incredibly surprised at how stable and responsive it flies.

A very slow climber and stalls out at about 4.5km, but a lot of that is probably to blame on the weak engine (SXT's K15 at 1.25m scale).

It cruises at around 125 m/s and is capable of reaching over 200 m/s in dives from which it recovers quite well.:)

http://i.imgur.com/J7hZJ8Jm.jpg

http://i.imgur.com/MYcU2o3m.jpg

http://i.imgur.com/4My7YDtm.jpg

http://i.imgur.com/0rG1jmLm.jpg

Any tips on how to make it climb better, reach better heights?:)

Slowing it down for landing is a slow process and I still often hit the ground at a scary ~125 m/s. I am assuming flaps will help a lot with that once I get my actiongroups unlocked.

AfaIk if you set default action groups in the FAR settings, and place the control surfaces after you did that, they work even without action groups unlocked. Else, try side slipping, or turns (or up-down wiggle).

Better climb: Moar powar (such engine much wow) or less weight. You could probably add engines instead of the nose cones.

Other than that, I do believe that reducing wing mass/strength should be possible, considering it isn't realy necessary to turn tight at high speeds in this thing.

EDIT: May I ask why you're using wing sweep on this thing? Swept wings are usually worse than straight wings at subsonic speeds, unless you have to use it for some other reason (flying wing, etc.)

Edited by FourGreenFields

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@Artfact: Weight is the major issue when building propeller airplanes.

We don't notice, but often you can end up building a 10 ton monoplane with a single propeller and hope that it is going to go anywhere.

Another thing is that you don't need a lot of fuel on propeller planes, after you slim it down, especially the wings, fuel will be a major factor.

For the wings, don't go for anything higher than 0.6 mass ratio on them, I would recommend even less, but that is only if you slim it properly.

Since you like propellers that much you may be interested on this: http://forum.kerbalspaceprogram.com/threads/130035

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AfaIk if you set default action groups in the FAR settings, and place the control surfaces after you did that, they work even without action groups unlocked. Else, try side slipping, or turns (or up-down wiggle).

Better climb: Moar powar (such engine much wow) or less weight. You could probably add engines instead of the nose cones.

Other than that, I do believe that reducing wing mass/strength should be possible, considering it isn't realy necessary to turn tight at high speeds in this thing.

EDIT: May I ask why you're using wing sweep on this thing? Swept wings are usually worse than straight wings at subsonic speeds, unless you have to use it for some other reason (flying wing, etc.)

Well, the original design (which I can't match due to my low tech level atm) has swept wings.:) Although they might not be as dramatically as mine. Will try replacing those with slighter swept procedural wings in the next version.:)

http://img.wp.scn.ru/camms/ar/167/pics/90_10.jpg

Saab 21 really was quite a weird and beautiful design.

And I'll be looking forward to what it can do with MOAR POWER then, once I've unlocked some better engines.:cool:

@Artfact: Weight is the major issue when building propeller airplanes.

We don't notice, but often you can end up building a 10 ton monoplane with a single propeller and hope that it is going to go anywhere.

Another thing is that you don't need a lot of fuel on propeller planes, after you slim it down, especially the wings, fuel will be a major factor.

For the wings, don't go for anything higher than 0.6 mass ratio on them, I would recommend even less, but that is only if you slim it properly.

Since you like propellers that much you may be interested on this: http://forum.kerbalspaceprogram.com/threads/130035

I'll have a check at the weight, I haven't payed much attention to it indeed. 400 LF indeed is way too much. Although I do have the CoM on quite a sweet spot at the moment.

Bad-T looks pretty awesome! I'll keep an eye on it, thanks.:)

I'm keeping off using BdArmory for now, though I'm getting more tempted with every video Paolo puts out!

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I like making FAR planes more than spaceships! Here's what I came up with using far too many mods.

I name them after flightless birds because of low expectations. Here are two of my favourites.

F-1B Kakapo, a light fighter inspired by the F-5, F-16 and F-20. Its a smooth flyer, the BDArmory AI has no trouble with it.

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F-3C Ostrich. A hideous gen4.5 "eurocanard" of sorts. The handling is extremely AoA-happy, but still smooth if you have a careful hand.

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No reaction wheels used, even for the really daft 180.

Edited by Shazbot

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Made this a while ago, thought I'd share it. Proving that FAR is no barrier to making crazy stuff fly!

19870043605_24e7217130_b.jpg

A V/STOL miner and fuel hauler for my ISRU Space Program, inspired by the Caproni Ca.60. Fits in the level 2 runway and SPH limits. Capable of VTOL when lightly loaded, or STOL operation with full tanks with a takeoff speed around 60-70 m/s. Maximum speed around Mach 0.8, it has Wheeslies as well as turboprops but this is obviously not a supersonic design. Flies well except for a tendency to phugoid, nothing I can't control but it means unlike some of my other planes I can't walk away from this one and let it fly itself.

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I made a new jet in my flightless bird series, the F-5B Rhea. I made it mostly with procedural fuel tanks. Apparently they're lighter than procedural structural elements, so I just took a load of them and removed a lot of fuel, and hey presto, a lightweight plane! The procedural parts allowed me to make a much sleeker plane than my previous designs.

So yeah, it's another light fighter, this time with some additional inspiration drawn from the Mirage F1, in addition to other jets of a similar role.

I also turned the difficulty up a notch by setting area ruling to full, and it still reaches Mach 2.2 with the same SNECMA Atar as the F1, courtesy of AJE. It did however require an intercontinental flight to reach that speed!

The handling is sublime. Easy to fly, pretty high AoAs, almost 9g turns at speed. It doesn't do cobras, at least with the low fuel amount I fly it with. It might reach higher AoAs with more weight.

I'll put missiles on it at some point. For now, I'm really digging the way this thing flies.

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Made this a while ago, thought I'd share it. Proving that FAR is no barrier to making crazy stuff fly!

(image)

A V/STOL miner and fuel hauler for my ISRU Space Program, inspired by the Caproni Ca.60. Fits in the level 2 runway and SPH limits. Capable of VTOL when lightly loaded, or STOL operation with full tanks with a takeoff speed around 60-70 m/s. Maximum speed around Mach 0.8, it has Wheeslies as well as turboprops but this is obviously not a supersonic design. Flies well except for a tendency to phugoid, nothing I can't control but it means unlike some of my other planes I can't walk away from this one and let it fly itself.

That's insane. I love it.

Edited by Shazbot

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