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Old Apr 02, 2012, 09:00 AM
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Crashed the plane. What went wrong?!

I've got a Wild Hawk, completely stock, that I've been flying around for about the past 7 months. I've gotten pretty good with it, and only have a small amount of hot glue, tooth picks and tape on it. However, yesterday was my worst accident yet with it!

First off, the video of what happened from the plane:

RC Plane Crash! (4 min 32 sec)


Jump to about 2:45 or so in the video to see where it starts.

I was playing around with the plane, after having just launched it for the first time that day. I flew it the day before, and had no problems, so I wasn't at all worried for yesterday. It started out fine, and I had no problems with it as well. I brought it up to a decent height (probably around 300' or so). I decided to take it into a steep nose dive, and then pull up near the ground. However, as it picked up some pretty good speed, I tried to pull up, and it failed to do so, simply rotating slightly, pulling a bit of the nose up, and slamming into the ground!

Thankfully (and surprisingly!), the plane only broke into a few pieces, and I've already gotten it put back together, and am waiting for a calmer evening in the next few days to make sure it still flies nicely. But, I'd like to try to figure out what I did wrong, before doing it again!

When I went to rummage through the carnage after the crash, I noticed that the tubing that surrounds the servo's control arm for the elevator had separated from the body channel of the plane. I thought it was probably due to the crash itself, but now I'm wondering if that wasn't part of the accident itself. All I can come up with is one of three explanations for the crash:

1) The plane picked up too much sped, and the elevator itself wasn't capable of overcoming the speed of the plane to be able to level itself back out.

2) The plane had too much speed, such that the air sped over the wings at a speed that created a vacuum near the elevator itself, so that it had no air at the elevator to use to level the plane back out.

3) The vibration of the tail / fuselage at such high speeds was enough to cause the tubing to pull away from the body, rendering the elevator's servo basically inoperable, which only slightly pulled the elevator up when I needed it to, which is what caused the plane to only slightly pull up and rotate, as what appears to happen in the video.

Now, keep in mind that I'm a complete amateur at this! I don't know a thing about aerodynamics, flying, etc. I'm still learning, and about the only thing I've learned from this experience is "don't do it again!". But I'd like to learn the "why", so that if I somehow get into a similar situation in the future, I might have a better idea as to how to save it in the end.

Thanks for any advice you have!
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Old Apr 02, 2012, 09:14 AM
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I can't help you with the cause of the crash, but I can offer some advice.

SLOWLY enlarge your flight envelope as you fly. That way you may be able to see that you've reached an air frame's max capacity before it cause a crash.
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Old Apr 02, 2012, 09:38 AM
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The faster you go, the more power is required to move the elevator.

My guess is your servo was probably not powerful enough for a terminal dive. Plus foam can still bend and twist under those sort of dives.

There are servo torque calculators online that can take into account the control surface area, width, and speed.
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Old Apr 02, 2012, 09:52 AM
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So basically both of you are saying that I just pushed it too far behind its limits? That's an easy fix then, I can try to keep from doing that in the future At least I got a sweet video out of it!

As a side question, how important is it to mount the motor angled to prevent prop torque that messes up the flight of the plane? Since this crash blew the motor out of the housing, I just kind of had to set it back in there and glue it in place. I'm not sure if it was angled properly or not. Will that come back and bite me when I try to re-maiden it?
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Old Apr 02, 2012, 10:06 AM
'tis nothing
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United States, CA, Napa
Joined Dec 2011
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Thrust angle, you'll find out when you fly it.

The plane was porpoising a lot. Either it's a control issue on your part, or the ele was beginning to fail. The stock servos are cheapie units. Consider installing new ones. HXT900 might be a good choice.
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Old Apr 02, 2012, 10:13 AM
who put that tree there
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Originally Posted by shunyata View Post
Thrust angle, you'll find out when you fly it.

The plane was porpoising a lot. Either it's a control issue on your part, or the ele was beginning to fail. The stock servos are cheapie units. Consider installing new ones. HXT900 might be a good choice.
+1 change out the cheapo servos to decent ones..
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Old Apr 02, 2012, 10:21 AM
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Hmm, good question as to whether it was my fault, or a plane issue, with respect to the porpoising. The wind was a bit higher yesterday (around 8-10mph), so it was one of the higher wind days that I've ever flown it in. Would that affect the porpoising at all? Plus, I think the COG might be a bit too far behind the wing spar. I've read that a lot of people put some additional weight into the front of it. I've never attempted that yet.

As far as servos, I think I'll wait and see if the plane still flies after it's last ordeal! I agree they should probably get swapped out. But I don't want to swap them out of the plane doesn't have much life left in it!

If it does, and it still flies, I have toyed around with the idea of upgrading the electronics all around the plane. The servos will definitely be included in that upgarde!
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Old Apr 02, 2012, 10:29 AM
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The control horn is on the left side, like this?

The joiner for both halves of the elevator looks thin and flexible. At high speed, the left side moved up, but the right not so much. The 2 halves were working more like ailerons and you get a hard roll left like in the video.

Also, the servo has to "push" the elevator up, with the long thin wire. As flight forces increase, this will give uneven response. You can glue guide tubes insode, or even thread the wire thru the end of a servo horn in the foam like this. The thin wires still "pull" perfectly, so you can dive hard, but recovery becomes a problem. one thing i like about wings with top mounted servos is they "pull" the elevons up, so dive recovery is more reliable.
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Old Apr 02, 2012, 10:39 AM
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Originally Posted by RedLine19k View Post
The control horn is on the left side, like this?

The joiner for both halves of the elevator looks thin and flexible. At high speed, the left side moved up, but the right not so much. The 2 halves were working more like ailerons and you get a hard roll left like in the video.

Also, the servo has to "push" the elevator up, with the long thin wire. As flight forces increase, this will give uneven response. You can glue guide tubes insode, or even thread the wire thru the end of a servo horn in the foam like this. The thin wires still "pull" perfectly, so you can dive hard, but recovery becomes a problem. one thing i like about wings with top mounted servos is they "pull" the elevons up, so dive recovery is more reliable.
I think you hit the nail on the head! Yep, it's exactly like the picture you posted. Your explanation makes perfect sense too. Since that connector is somewhat weak connecting the two halves of the elevator, it appeared to cause what was apparent in the video. Genius! I've always wondered if that connector was weak, but never really had any problems with it. But now after giving it that hard pull up, it makes perfect sense.

Couple your explanation along with a video that I made last week, where I pulled a similar (but less speedy and intensive) dive, when I pulled out of the dive, it came up, but did veer off to the left as well. Again, that would appear to coincide with your explanation of the two halves, and where only the left half pushed up, as compared to the right half, causing it to angle left.

Looks like I either need to strengthen that connector between the two halves of the elevator, or go a bit easier on the plane. I think I'll start with going a bit easier. It's a beginner plane, not a trickster! I'm just having too much fun with it and loving the hobby!
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Old Apr 02, 2012, 12:19 PM
'tis nothing
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United States, CA, Napa
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Originally Posted by kenzietech View Post
Hmm, good question as to whether it was my fault, or a plane issue, with respect to the porpoising. !
The weak connector on the ele does make total sense. It would certainly cause control issues, too, (like porpoising) even when flying within the proper envelope.

Glad to see you may have found the culprit.
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Old Apr 02, 2012, 12:39 PM
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Now I'm just itching to get back out there and see if it still flies! Although I just checked the weather, and no winds under 15mph until Thursday.
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Old Apr 02, 2012, 01:54 PM
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Originally Posted by kenzietech View Post
I decided to take it into a steep nose dive, and then pull up near the ground.
Why?
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Old Apr 02, 2012, 02:20 PM
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Why not?

In hind site, it wasn't necessarily terribly smart. But I was having a bit of fun seeing what sort of maneuverability it had. And apparently I exceeded it's maneuverability
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Old Apr 02, 2012, 02:28 PM
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it's all about fun. Planes are easy to fix and you learn something in the process. My game was how many vertical rolls can i get? Made 6 before the battery ejected, and maybe 1/2 of one after
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Old Apr 02, 2012, 02:34 PM
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Lol!

I would like to make one thing clear, however - while I was messing around, and risked damaging the plane, and taking full responsibility for the consequences of the dive, I made 100% sure that nobody was around anywhere in the vicinity, in the case of a mishap with the plane. The only other person in the nearby 1/2 mile range was my wife, who was with our dog, watching it the whole time. I am EXTREMELY cautious when flying anywhere near anybody else, and maintain my distance as necessary.

This was just the case of me wanting to have a little bit of fun, and to test the plane's limits
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