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Wiggling Wings on Variable Sweep-Wing Aircraft


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After @swjr-swis posted his replica of the Bell X-5 on the USAF Museum Replica Showcase, I decided to try and make my own variable sweep-wing aircraft. I then looked inside his plane, as seen in the screenshot below, and decided to mimic it in "Tim C's Car" - which would be based loosely on the F-14 Tomcat.

 

R9G19HL.png

  • The interior of @swjr-swis's Bell X-5 replica.
  • I implemented the same angle limit and target angle (both in part settings and action groups) for my own aircraft.

 

However, when I put the finished product on the runway, the wings started wobbling up and down. It stopped after a few seconds - and after I confirmed that the H and N keys were working as expected - but it started again when I hit 70 m/s during takeoff. After using up almost all the runway on wet mode (yes, I'm using Panther engines) to take off, the wings that remained on the plane look like they're flapping - hence causing a loss of control followed by a crash.

  • I started with a Wing Connector B, D, and then a Small Delta Wing. The wings touched the runway and snapped off every time.
    • When I removed the D connectors, the outcome remained the same as prior.
  • I then replaced those wings with single swept wing parts, but still got wobbling wings - although there were way fewer instances of snapping off on the runway. 
    • Even when I tried autostrut - grandparent part and rigid attachment, it was no use.
  • Before you ask, I doubt the takeoff length is due to bad aerodynamic design. I copied the prototype but kept the swept wings fixed (as in no variable sweep-wing mechanism), and the plane took off just fine.

 

 

lwMyeWc.png

  • Tim C's personal supersonic jet - or "car," as named per family tradition - flapping its wings.

 

How can I get my supersonic plane's wings to stop wiggling all the time? Here's the craft file of the latest variable sweep-wing prototype if you want to mess with it: https://mega.nz/file/LfxjSa7A#WBfQEUkCpdAAqCubWl1yiX9hiSczPHBxbdLd-QHLDoE

 

Thank you all very much.

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Are you locking the hinges? My experience with hinges and pistons has been that they spaghettify when you put much pressure on them if they aren't locked. Maybe lock the hinges manually first and if that fixes the problem, then you know where to troubleshoot.

I mostly use pistons/hinges for landing gear. I've tackled the locking problem by either having 2 action groups (#1 toggles lock, #2 toggles the hinge) or with a KAL-1000 controller. With the controller, I use the same toggle locked and toggle hinge actions. Toggle locked triggers twice, once at the very beginning to unlock and again at the very end of the sequence to relock. Then the toggle hinge action occurs at 0.1 seconds. I also put the KAL-1000 mode to 'none-repeat' so that it restarts the sequence each time I trigger it. Of course, with my application, I always run this sequence in a vacuum or under parachute so there's very little stress on the unlocked hinges while moving. I'm not sure what happens if you unlock your hinges at hundreds of m/s and try to move them, but I guess you can find out. Good luck!

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11 hours ago, Mars-Bound Hokie said:

How can I get my supersonic plane's wings to stop wiggling all the time?

Start by removing pitch (and yaw, while you're at it) authority from the ailerons (elevon 5). In that position they have near to zero pitch leverage anyway, plus their 'hinge line' goes right through the CoM which very often causes KSP to randomly reverse their pitch action mid-flight. This one change by itself makes the plane controllable at least.

It will still miss considerable lift to perform adequately. The swept wings are horrible lift surfaces, despite their better-than-stock-average looks. Note that in my Bell X-5 replica, they are literally just for show: the actual lift is provided by the clipped Type D wing sections.

As for the flapping: it suddenly happened after what seemed like a tiny and completely unrelated change to my Bell X-5 too. I had to then revert several iterations because they *all* suddenly had the flapping bug. If I manage a bit of playtime I'll see what I can tweak on the Tim C Car.

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On 2/7/2023 at 7:46 AM, Mars-Bound Hokie said:

How can I get my supersonic plane's wings to stop wiggling all the time?

I remembered that I did a very similar plane a while ago, only missing the variable sweep wings because 1.3.1 didn't have DLC. I imported the plane into 1.12.4 today and decided to give it a try.

I ended up ditching the hinges for this one. Even using the bigger-size hinges, the joint strength was too low and it kept either flapping or altogether breaking off, even in straight flight. So then I tried with the rotors, and those seemed to work a bit better. They are draggier than the hinges and thus impact performance a bit more, and there is still a good bit of flexing on high G turns, but overall I think it still a success. The plane is extremely controllable at all times, and the wings stayed on even through some pretty rough testing.

efmqJgd.png

Feel free to re-use this solution for your builds. The entire wing assembly, including the 'counter-lift' sections that stay clipped inside the fuselage, is attached to the rotor. From the underside, looking from the back towards the landing gear, you can just grab the lower corner of the rotor and offset it for inspection or lift it off and save as a sub-assembly for reuse.

 

Some video footage showing Jeb putting the SWiS 41-F Blackwood Mk2 to the test:

 

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On 2/8/2023 at 10:18 PM, swjr-swis said:

there is still a good bit of flexing on high G turns, but overall I think it still a success. The plane is extremely controllable at all times, and the wings stayed on even through some pretty rough testing.

 

I could say the same thing about Tim C's car when I replaced the hinges with the M-12 servos and configured them as you did. In fact, I'd say there was some more flapping in my prototype than yours when I underwent the same maneuvers. Overall, thanks to your help and letting me look inside the Blackwood, I was able to get a stable prototype working.

 

Mxfk0Ir.png

 

Unfortunately, Tim C's jet did not achieve anywhere near the velocity I was hoping for (1400 m/s or higher). In fact, when compared to the Blackwood's test run, it seems pretty lame. If you take a look at the performance stats below, you'll see why.

  • For a bit of context, I tested the Blackwood's range flying at full throttle along the equator. More specifically, I was testing to see if it could fly around the planet's circumference (plus whatever ground would be added due to the planet's rotation) and I wanted to log a good cruising altitude and velocity. The Blackwood had flown almost 1,200 km away from the KSC before I was running on fumes and had to land.
    • I also had to keep the engines running in dry mode to avoid the power going out during gliding, since it only has 50 charge capacity. That's exactly why I have batteries in all my (serious) aircraft, so the craft doesn't lose power and die during gliding.
  • I did the same to Tim C's Car. Though it has nearly triple the range as the Blackwood, it ultimately fell ~380 km short of the KSC.
    • At least in the first test flight of the new prototype, which was before I put 156 Lf units back in the rear tanks.
      • The second run was successful, albeit I was running on fumes near the end and had to glide.

 

(UPDATED) Jet Cruise Stats
  Blackwood Tim C's Car
Average Airspeed (m/s) 810 (W) 795
Average Altitude (km) 17.5 (W) 17
Range (km) ~ 1,200 ~ 4,712 (W)
  • Tim C's stats have been replaced with the results from the second test run, which had the extra fuel added
  • The range for Tim C's car was calculated as follows:

C00YniK.jpg

The 1.5 part will be explained later.

 

I was looking forward to building a jet that could match, or even surpass, Bill's Car in terms of speed while being able to complete the circumnavigation. Sure, my wings still have a bit of flapping (at least when the wings are out), and it's even more obvious in sharp maneuvers,  but it's nowhere near as bad as it was before. As long as the plane is stable, there shouldn't be a problem.

  • Plus, it's pretty "kerbal" of us to design a supersonic jet whose wings can "flap."

 

Anyway, here are some pictures of Tim C's (second) test flight. Though the first run wasn't too bad, I figured I can do better.

 

4tO71QI.png

  • Takeoff went well. No (excessive) wiggling, and it got airborne at a reasonable speed.
  • After I got airborne both times, I switched the afterburner to supersonic mode and swept the wings to begin the circumnavigation flight.

 

XlmlgPF.png

  • Pretty neat sunset shot, huh.
    • Would make for a beautiful desktop background, actually.
  • This was taken during the first test run. Though I said I would take the stats for the second one, I'll leave pictures I think look cool.
  • After the aircraft autopilot finally got its act together, it flew at a steady flight path along the equator at the speed and altitude mentioned in the above table. 
    • Took longer to figure out where in the first test flight than the second run.

 

Dcfmqu9.png

  • Another cool snapshot that was taken at the first test run.
  • I had to cut off the engines when I noticed that I had 95 Lf left. I may not have been able to make it to the KSC in this case, but at least I can do an off-road landing test.
  • Just like in the Blackwood test flight, I had to go into glide mode. This plane was very easy to control.
    • However, I didn't have to keep the engine running continuously to preserve electric charge.

 

SipFTGw.png

  • (FIRST TEST RUN)
  • Tim C standing in front of his prototype in the desert.
    • Picture taken 393.8 km away from the KSC (so close).

 

All pictures following this line will be from the second test run.

 

UkCKKSd.png

  • Tim C's jet flying nose-first into the night.

 

Fi8lXfK.png

  • 100 km away from the KSC at over 17 km altitude.
    • You can barely make out the lights beneath the horizon.
  • I was at 100 units LF left when this shot was taken, so I cut the engines and extended the wings so I can glide. Surprisingly, I was able to glide all the way there without much of a hitch
    • I did have to turn the engines on (but in regular mode) when I saw that I was flying off-center to the runway and soon before touchdown.

 

OSfQX8c.png

  • Almost there.

 

CiMYb4z.png

  • Tim C giving a salute after a successful circumnavigation flight around Kerbin.
  • Rare footage of adult Tim C Kerman with a standard KSP haircut.

 

H96DZpt.png

  • Overall performance stats for this flight.
  • As seen in the upper right corner of the screenshot, Tim C was almost out of gas.
    • Like low enough to get a stern talking-to from the FSDO at the very least.
  • Unlike his dad's personal supersonic jet, he was NOT able to make it around the world in less than 80 minutes. Rather, it took him nearly 90.
    • That's why I included 1.5 hours in the range calculation.
      • (Circumference of the planet) + (distance the KSC moved in the 1.5-hour period) = ground distance traveled = range
    • His "uncle's" jet also beat his time and cruising altitude, but at great risk of exploding and his flight also ended with a nearly-empty tank.  
      • Jeb's not actually Tim C's uncle; he's just close friends with Bill, so that's why Bill's and Val's kids call him "Uncle Jeb."
    • His mom's jet's circumnavigation time is unknown.

 

So, anyway, that's it for Tim C's Car's test run. I'm welcome to suggestions on how to improve performance, if you all have any. Overall, thank you for your help - especially you, @swjr-swis.

 

Craft file: https://kerbalx.com/Mars-Bound_Hokie/Tim-Cs-Car

Edited by Mars-Bound Hokie
Craft file
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