Aircraft failure analysis

[NFunky]

Hey guys, I was wondering if there's a way for me to upload a craft file for someone to test fly for me. I've been having trouble with my latest aircraft, but I can't figure out why it's not working. The plane used to fly, but after some tweak or other, it now explodes on liftoff.

I've got low vision, so I can't for the life of me see what's happening right before the blast. I check the F3, but all it'll tell me is which parts exploded first. I've been trying to watch liftoff from right next to/under the plane, but everything looks good to me, right up until it doesn't, so I'd really appreciate another set of eyes.

Thanks

Edited January 8, 2015 by NFunky

[Red Iron Crown]

You can upload your craft to http://kerbalx.com/. You could also share a pic of the plane in the SPH with CoM and CoL indicators on and get some advice about it.

[Alshain]

Use DropBox or a comparable free storage service.

EDIT: Ninja'd but yeah I would start with an image using any image host first (imgur is a good choice because the forums support it natively.) There may be something we can see without even flying the craft. Make sure to give us that log that tells us which piece is exploding first.

[capi3101]

Seconding Alshain's suggestion - a pic and the F3 Data would be helpful.

[NFunky]

Wow, I'm always surprised how fast I get responses on this forum. Sorry it's taken me so long to post again. It took me a second to figure it out, but I now have a DropBox. This link should lead to the craft file and a screenshot from the SPH.

https://www.dropbox.com/sh/a0p2oopehljjr4p/AACuns5xhb0NAHVhjNX5YNtWa?dl=0

As far as the F3 data is concerned, it seems to be slightly different each time. I'll run it again though, then upload the screenshot.

[Dr-Drunk]

I haven't test flown your craft yet but from what I can tell from the screenshots it would seem you're tail-striking. Atleast it seems to suggest the first bit of damage was there.

One point in concerns to your design though.. You've got the CoM and the CoL awefully close to eachother for a 3 tank setup. I'd imagine that once the tanks start draining your CoM would move behind the CoL and leave you unable to control the aircraft verry well.

Gonne load up KSP again and give your plane a shot so I'll get back to you in a bit.

Edit:

Okay so I gave the plane a quick testrun just now. (For future referance you might wanna include a modlist so people can actually load your crafts btw. )

Takeoff went well enough but the tail does go awefully close to the ground so I had to nudge it up ever so slightly. Personally I fixed the issue by moving the CoL back a bit and placing the rear gears slightly back. Hope it helps!

Edited January 7, 2015 by Dr-Drunk

[Hodo]

DrDrunk is right about the tail strike your craft is hitting its tail on take-off.

As for your CoL vs CoM.

That is fine. Your craft would be a bit twitchy but not that bad, and actually would handle better at supersonic speeds due to the CoL shifting back on the wings.

The only thing I have to say is a major issue that would cause some problems is the shear number of radial intakes you have on that aircraft. In FAR airhogging does not work. It is best to drop all those additional intakes all they do is create additional drag and more mass to the craft. The only other issue is those engines. Ideally you want to use turbojets for any SSTO operation or RAPIERs.

On take off you dont want have more than 10-15deg nose up on most aircraft, as you would have a tail strike. I know on all of mine I rarely have more than 15deg nose up to achieve take off after I hit V1 speed which is about 70-80m/s or 150-170kts on most of my designs.

[Wanderfound]

Post a FAR stability analysis at 10,000m/Mach 1. I'd be surprised if you don't have pitch-up problems; you're very light on for horizontal tailplane.

And, as Hodo said re: intakes. Use less, and move them further back. You don't want that much drag up front, especially with a basic jet powered plane that can't reach high altitude. If you want to go near space, you need turbojets or RAPIERs.

Edited January 7, 2015 by Wanderfound

[NFunky]

Oh man, I'm sorry I forgot about the modlist. I've got a bunch, but the relevant ones are FAR, DR, KIDS, KJR, KER, and RealChute. I'm also at a point in the tech tree where I have basic jets, but no turbojets. This plane is just supposed to get me to the poles and other biomes on Kerbin.

So I've moved the CoL back and reduced the intakes from six to three, and I can't seem to move the gear back any further, but I'm still blowing up on liftoff. I must still be missing something. Here's my dropbox again. I've got the A2 version of the craft and pics of it in the SPH and the post-crash report.

https://www.dropbox.com/sh/a0p2oopehljjr4p/AACuns5xhb0NAHVhjNX5YNtWa?dl=0

[Hodo]

Oh man, I'm sorry I forgot about the modlist. I've got a bunch, but the relevant ones are FAR, DR, KIDS, KJR, KER, and RealChute. I'm also at a point in the tech tree where I have basic jets, but no turbojets. This plane is just supposed to get me to the poles and other biomes on Kerbin.

So I've moved the CoL back and reduced the intakes from six to three, and I can't seem to move the gear back any further, but I'm still blowing up on liftoff. I must still be missing something. Here's my dropbox again. I've got the A2 version of the craft and pics of it in the SPH and the post-crash report.

https://www.dropbox.com/sh/a0p2oopehljjr4p/AACuns5xhb0NAHVhjNX5YNtWa?dl=0

Ok then you want to go simpler in your design. It isn't going to need 3 jet engines that is a bit of an overkill.

The other issue is your rear/main gear are to far back. You want them ideally just behind your CoM. And your first design was far better then the last one. The one with the CoM and CoL closer together was the best of the two. The other would be TO nose heavy.

This is a VERY old craft that was quite capable of going anywhere on Kerbin.

And it was an SSTO.

When you get more advanced in your tech level and in your skill you will start to design craft like this.

Notice the landing gear on this one are almost directly under the CoM, they are about .5m behind to be exact. This is to give it the best point of rotation for take off and landing without having to worry to much about tail strikes.

[Alshain]

Oh man, I'm sorry I forgot about the modlist. I've got a bunch, but the relevant ones are FAR, DR, KIDS, KJR, KER, and RealChute. I'm also at a point in the tech tree where I have basic jets, but no turbojets. This plane is just supposed to get me to the poles and other biomes on Kerbin.

So I've moved the CoL back and reduced the intakes from six to three, and I can't seem to move the gear back any further, but I'm still blowing up on liftoff. I must still be missing something. Here's my dropbox again. I've got the A2 version of the craft and pics of it in the SPH and the post-crash report.

https://www.dropbox.com/sh/a0p2oopehljjr4p/AACuns5xhb0NAHVhjNX5YNtWa?dl=0

Well, this is not what I'd call an ideal fix but try these wheels and use the tweakables to raise the front and lower the back. That should put some air under your wings on the runway and allow you to lift off without pulling back as far. It might make landing a little harder though.

There unfortunately is a limited things you can do other than that. The last and final option is to angle the wings, but that can affect flight so the gear trick is a better way.

On another note: What is your goal with this plane? If it's going to space, you need more engine (not basic jets), not to mention oxidizer. If it's going around Kerbin, you need less fuel. That thing is way overweight for an atmospheric plane in FAR. Planes in FAR glide long distances without the engines running, if your trying to circumnavigate the globe, it's possible to do that with only a few tanks and a much smaller plane.

Edited January 7, 2015 by Alshain

[NFunky]

I still just can't seem to figure this out. Here's a new version of my design with the CoM and CoL in a pretty good relationship, only three intakes, and the gear moved near the CoM. It still blows up though. I hit the spacebar, wait for the engines to spool up, release the brakes and then I don't touch the keyboard, but the plane still blows before it reaches the end of the runway or lifts off.

I'm sorry to say it, but I kind of have a hard time with pictures. A lot of the time I can't tell what is what unless I can rotate and zoom to get my bearings (the vision thing again). Would you be able to upload the craft files for those pics?

https://www.dropbox.com/sh/a0p2oopehljjr4p/AACuns5xhb0NAHVhjNX5YNtWa?dl=0

- - - Updated - - -

Oh BTW, I think I may have forgotten to mention that I'm playing with KIDS (Kerbal Isp Difficulty Scaler), which means my little 3 tank plane could only make it half way to the pole.

[Alshain]

I still just can't seem to figure this out. Here's a new version of my design with the CoM and CoL in a pretty good relationship, only three intakes, and the gear moved near the CoM. It still blows up though. I hit the spacebar, wait for the engines to spool up, release the brakes and then I don't touch the keyboard, but the plane still blows before it reaches the end of the runway or lifts off.

I'm sorry to say it, but I kind of have a hard time with pictures. A lot of the time I can't tell what is what unless I can rotate and zoom to get my bearings (the vision thing again). Would you be able to upload the craft files for those pics?

https://www.dropbox.com/sh/a0p2oopehljjr4p/AACuns5xhb0NAHVhjNX5YNtWa?dl=0

That doesn't sound like a tail strike then. Is the plane wobbling on it's wheels any or is it going straight. The log is showing the first damage occurs because of an impact with the runway. Something has to be causing that impact.

Oh BTW, I think I may have forgotten to mention that I'm playing with KIDS (Kerbal Isp Difficulty Scaler), which means my little 3 tank plane could only make it half way to the pole.

You did say that, my brain just didn't draw the connection.

Edited January 7, 2015 by Alshain

[arise257]

I'm going to counter the advice you've been given, and suggest that you set the rear landing gears on the wings, and move them as far back as possible. Making your pivot-point directly beneath the CoM is efficient (take off with less lift), but if you pitch up too hard, you're going to smash the tail of your plane into the ground. To me it's just not worth the frustration of worrying about that every time. Additionally, by moving them back you can land your plane more easily because you'll always touch-down the back wheels first. Another thing is that wing-mounting them will lower the tail relative to the nose, meaning you're already pitched-up about 8 degrees on level ground.

As for your jet...

Kill all but two of your intakes, and use only one engine. Your jet weighs way too much, use no more than 3 tanks -- I see nine. You also need to complete the line of Elevon control surfaces on the rear wing edges. Get rid of the delta wings with small control surfaces on your wing tips and replace them with a structural wing surface that will fit while keeping your CoL back -- add Elevons to that new surface. Get rid of the active control wings on the sides, near the engines -- move those to the front. They will give you the nose lift you need to get off the ground faster. Get rid of the goo canisters and material bays -- you'll never use them on Kerbin for contracts.

Another tip, get rid of those ISP difficulty mods. You're making it too hard on yourself.

[NFunky]

My bad, I didn't realize that. I crashed it again and here's the bottom part of that report.

https://www.dropbox.com/sh/a0p2oopehljjr4p/AACuns5xhb0NAHVhjNX5YNtWa?dl=0

[Alshain]

My bad, I didn't realize that. I crashed it again and here's the bottom part of that report.

https://www.dropbox.com/sh/a0p2oopehljjr4p/AACuns5xhb0NAHVhjNX5YNtWa?dl=0

My mistake. You had it right, sorry. For some reason I thought the log was newest event first. It's not.

I have one other suggestion, and it's a long shot. I once saw a Twitch or Youtube video where for some reason there was a bump in the middle of the runway. A strange glitch that caused the plane to "jump" because of a phantom edge collision. Given that your having this issue without touching the keyboard, it might be worth a try. Put your plane on the runway and throttle very little and taxi to the other end of the runway. Watch carefully for any strange behavior as it's rolling to the other end.

Edited January 7, 2015 by Alshain

[arise257]

Your logs are definitely reading like a wiggling wheel problem. Look at all your gears and make sure they are pointing straight forward and back. It's easiest to do if you rotate your plane upside-down, and pull it down into the floor so that wings are in-plane with the floor markings. You can use the tiled grid and markings to check/align your parts more accurately. Another thing that can help is strutting your parts together so that you rule out chassis wiggle. If the chassis flexes too much, it puts uneven lift/pressure on one side, resulting in a sliding, then exploding wheel.

[Hodo]

Your logs are definitely reading like a wiggling wheel problem. Look at all your gears and make sure they are pointing straight forward and back. It's easiest to do if you rotate your plane upside-down, and pull it down into the floor so that wings are in-plane with the floor markings. You can use the tiled grid and markings to check/align your parts more accurately. Another thing that can help is strutting your parts together so that you rule out chassis wiggle. If the chassis flexes too much, it puts uneven lift/pressure on one side, resulting in a sliding, then exploding wheel.

I can agree with the wheel wiggle issue. Sounds like your gear mounted on your wings is causing them to flex upwards throwing your landing gear out of alignment and thus sending you off in a random direction. Also looks like you are going a bit to fast on take off. Most aircraft should be off the ground around 70-80m/s. If you are going faster than that you should look at the amount of wing your craft has vs the the mass of your craft. Check the FAR diagnostic charts see what they kick back, what are the numbers in it. What do the graph lines look like when you sweep the AoA and the Mach numbers. It should be pretty stable and predictable, if it isn't you have a problem. If your numbers are in the red they you have a problem.

Right now I would start by strengthening the landing gear, I would move them to the body of the craft.

Doing as another person said and moving them to the tail of the craft is probably one of the most idiotic and wrong answers of the day.

[Alshain]

Doing as another person said and moving them to the tail of the craft is probably one of the most idiotic and wrong answers of the day.

Lol, you say that but I've seen it work. Never done it myself, it's not my go to strategy, but there was a plane in Wanderfound's contest with gear mounted as far back as it could possibly be. I thought for sure it wouldn't leave the ground without the lip of the runway to help it.... I was wrong.

EDIT: Here it is, KillAshley's Incubus. He made it work by using a higher gear in the front to angle the wings without actually changing the angle of the wings. Those rear gear are sitting on the engines. It certainly surprised me when it took off.

Edited January 7, 2015 by Alshain

[arise257]

I'll post my craft if it's so idiotic and wrong, but I've got a ton of very maneuverable jets that implement that design pattern.

[NFunky]

Ahhhh, finally! Between adding more wing area and moving the gear to the body, it takes off and flies pretty well. Thank you all for the help. This forum is the best!

[Wanderfound]

Lol, you say that but I've seen it work. Never done it myself, it's not my go to strategy, but there was a plane in Wanderfound's contest with gear mounted as far back as it could possibly be. I thought for sure it wouldn't leave the ground without the lip of the runway to help it.... I was wrong.

EDIT: Here it is, KillAshley's Incubus. He made it work by using a higher gear in the front to angle the wings without actually changing the angle of the wings. Those rear gear are sitting on the engines. It certainly surprised me when it took off.

You can also make it work by just having enough control authority to overpower the mass:

[Hodo]

I'll post my craft if it's so idiotic and wrong, but I've got a ton of very maneuverable jets that implement that design pattern.

To quote one of my former instructors.

"Just because it worked doesn't mean it is right."

You can get a car to take off if you put enough power behind it. Don't believe me look up Top Gear and the Reliant Robin Shuttle.

All you did was basically make a tail dragging aircraft like an old WWII plane. That is great when you have a prop at the front. But pretty bad when it comes to most modern designs. This is because you are actually handicapping your take off and landing speeds by not giving yourself proper rotation for take off or landing.

Most of my designs can land at about 60-90m/s depending on the plane. All of them can stop with in half or less of the distance of the runway, some can even stop LONG before that without having to stand on the brakes. I even have one aircraft that doesnt even need to use the brakes to stop and doesn't use chutes to land like a autogyro.

This aircraft can land in half of the distance of the island runway and come to a stop with its wheel brakes, or it can come to a complete stop without its wheel brakes in a little over 3/4ths of that distance without them using its thrust reversers on those turbines. It lands at 65m/s which is fast for something that size, but I have to tweak its flaps a bit.

This is what a proper landing gear position looks like.

[arise257]

Yeah... I'm running NEAR, and all of my planes land and take off in the same range. In the end, what matters is where you place your wings and control surfaces, and how much your craft weighs. You do spend more of your time in the air, afterall.

Edited January 8, 2015 by arise257

[Wanderfound]

Hodo does have it right; there are limited situations where rearset gear can work (planes with static pitch, planes with excessive TWR, lightweight planes with extreme control authority), but most of the time you want a conventional tricycle setup with the rear gear just aft of CoM.

The other approaches all have limitations: planes with strong static pitch are difficult to land, bulk TWR is wasteful, the control authority thing only works on lightweight aerobatic specialists.

If rear gear placement gives you tailstrike problems, there are two easy solutions: taller gear and tailstrike guards. Especially with the new editor tools, taller gear is easy: just offset them to the max, then place a structural intake or something to cover the visual gap (if you care about appearances). Small hardpoints work well too, and can also be offset. The new offsetting and rotation tools make the placement of tailstrike guards (landing gear positioned to contact the ground just before the engines strike) a lot simpler as well.