Speed tests crash

[marcosp]

After returning from IRCHA and seeing the German devils fly at crazy headspeeds I have been spending the last month improving my setup. I went 14S and started revving up the head speed. My last test was using 2300 at 92%, this time I moved two teeth up to a 21t pinion and was supposed to have 2300 governed at 80%.

One little detail: DO REMEMBER TO CHANGE RATIO IN THE JLOG

I came from a 19t and changed to 21t, so what telemetry read as 2300 was... 2560!!!

My first flight I took it carefully and was a bit surprised that in Idle 2 at 70% I had only 2100 rpm (real 2300). I used Idle 3 at 83% (that's the % I had from the 19t setup) and the heli was howling! No wonder... For some reason of destiny, it never occurred to me the telemetry was calculating lower rpm.

Second flight I said ok let's push this thing, went into Idle 3 and started doing speed passes. I was hearing a strange motor reduction, very abrupt, every time I raised pitch to over 3/4 stick. Very marked immediate step down of about 300 rpm. On the last pass I went a bit over 3/4, had that sound of rpm decay and after 2 or 3 seconds the heli violently pitched down to the ground. The effect seems to be the abrupt rpm fall caused the pitch down, but I am not 100% sure.

Breakage was extensive, even the intermediate gear was bent. Full front of the heli byebye, Kosmik totaled, motor damaged, 10 flight old TP LiPos one dead other damaged, intermediate gear bent, HV2 BEC lost a piece, HV2 BEC screen broken, Voltron GPS (brand new unused) screen broken, main shaft bent, tail hub bent, and of course blades, canopy, landing gear, etc. Who knows what else I'll find.

Failure candidates are:

- Heavy duty belt: it had some 30 flights on it including practice and competition for Speed Cup. It is stripped in two places which indicated a possible strip in flight and later strip in the crash.
- Motor magnets coming loose: After the crash about 4 magnets were out of place. Most probable cause was the violent hit which ripped the complete front of the heli and sent the motor flying 30 feets away. However, the strange lower step in rpm could be magnets ungluing by the brute force above certain power demand. I am investigating this. Lowest probability.
- Thrust bearings giving in: When I found the real rpm were 2560 there is a chance the thrust bearing exploded locking one blade and causing the pitch down.

[marcosp]

[wblombard]

Holly molly!!! Im so sorry to see this! Damn! 2600rpm...that thing must have been screaming! Good luck with the re-build! Where im from we say "BLIKSEM"

[mkovalcson]

Damn!!!!! I'm sorry!

I feel like I need to put all the wind socks at my local fields to half mast.

[marcosp]

Motor "hiding" 10 mts / 30 ft away from the crash:



R2 screen broken (auch!)



Motor magnets broken by the big impact!



Belt stripped in two places:

[marcosp]

Upon investigation of he motor pics and logs it seems 99.999999% impossible that the magnets came loose in flight. It should have caused havoc inside the motor which it clearly hasn't. The glue used is extremely strong to forces and heat but is like crystal, very sensitive to strong shocks like this crash.

[abanihani]

Wondering if anything to do with the vbar. I remeber i told you my scarry story when i lost complete control of the Banshee at 2300 HS and had complete control when i dropped the HS to 1800. no one could explain what happened and thats why i ditched the vbar in favor of the helicommand. If you go outside the vbar loops with your setting, only bad things can happen. the belt is not broken so it can't be explained that way. The speed itself and thrust bearing don't explain that either, the Banshee should take this HS without problems. I have sneaky suspicion it maybe your vbar sensor went bad.

[RyanW]

I am definitely not a 'speed' expert, but have experienced the violent pitch up at extreme speed on a 14s machine. It isn't the V-Bar, but I believe an aerodynamic issue. The machine that violently pitched up had the CGY-750 on in.

I am convinced there is another phenomenon happening on the violent pitch down actions, such as Stefan Segerer's at the German World Record Speed event a few weeks back.

Many years ago, while learning aerobatic autos, my mentor and arguably one of the best aerobatic auto pilots ever taught me a lesson in having the 'air come over the disc' during an auto. You can dip the nose to a certain point, then all of a sudden, when things are just right, the air will go from coming up through the disc to going over it. If you could measure it I bet the difference between safe and crash is down to the tenth of a degree.

What happens is the split second the air goes over the disc instead of through it, it will tuck the nose faster than you can possibly react and accelerate straight into the ground. I have done this twice. Once it was a push-over auto and I tried to bring it closer to the field before pushing over. Result was immediate acceleration into the ground from 30~40' with no chance of recovery. The second time was last month with my Logo 550SX during a windy day. High autorotation, tilted the disc a tiny bit too much (right side up) trying to get it to penetrate the head wind and it tucked violently, stopping the blades almost instantly. Luckily I was high enough to patiently get the headspeed built up and safely landed it.

I am pretty sure this can also happen under power when the perfect angle is reached with many other factors playing a role.

[Vinger]

Vbar log should enlighten you a little bit. Ouch it is sad to see a Banshee looking so sad! Rebuild her and try again.... Doubt it is the VBar, but then I'm biased. Sounds like the TDR crash at the speed event in Germany, they also don't know the cause, might be a failed link?

Your telemetry should also tell you a bit more, surely.

[jcflyer]

Marcos,
Sorry to see this happen brother, take your time and try and sort it out. That will go a long way in it maybe not happening again. So sorry brother!!!

[jamin_00]

Noooooooooooooooooo!!!!!!!!!!

So sorry to see this Marcos but as you know when doing speed runs and getting near the limit anything can happen.

I accepted that when I started ramping up the rpms that my next flight could be my last. Im currently on a break with the speed stuff, only a short one


But man Im so sorry to see this

[marcosp]

Originally Posted by abanihani

Wondering if anything to do with the vbar. I remeber i told you my scarry story when i lost complete control of the Banshee at 2300 HS and had complete control when i dropped the HS to 1800. no one could explain what happened and thats why i ditched the vbar in favor of the helicommand. If you go outside the vbar loops with your setting, only bad things can happen. the belt is not broken so it can't be explained that way. The speed itself and thrust bearing don't explain that either, the Banshee should take this HS without problems. I have sneaky suspicion it maybe your vbar sensor went bad.

I highly doubt it Ahmad. I tested every rpm in a hover, flew with this rpm for over a minute. No problems here. I'll check the Vbar log but I bet it's not that. The sound was happening and on that last pass as the sound occurred I kept adding pitch and boom. I pushed too far.

[marcosp]

Exactly like the TDV crash only from 45 feet instead of 15. Took 0.5s instead of 0.1s. And I had the strange sound and rpm loss. It is most definitely an aerodynamic effect that doesn't happen at 2500 and does at 2000 if you are going 230 km/h. If I had been flying at 2000 the effect would have been gradual and controllable. Have any of you been flying at a high rpm going fast and when you change to a very low rpm curve the heli pitches up? This seems like the same the but the limit reached very quickly so the swash is pushing elevator and all of a sudden rpm falls 500 and the heli goes straight down. That's how I see it.

The question still remains, what caused the sudden loss of rpm. Could be the belt, could be a link and then a blade going nuts though I didn't see anything weird in the disk while falling. A broken thrust bearing causing a stuck blade holder when the FBL needs absolute flight control. Don't know. A deep understanding of blade precession and recession, damping and flapping, plus FBL control at the limit, and how a stuck blade or sudden change in rpm makes all of that react would be useful. The crash was aerodynamic and we'll find an answer eventually. Still the mechanical issue is a mystery.

[rossk]

So what do you do to repair this?
Normally I would just rekit it, but since you can't get a kit, what do you do?

Order every part from banshee and rebuild it yourself?
Send it back in pieces and get back a new built one?

I personally would be going over every single part and replacing anything that I have any doubts on, which would most likely be the majority of the heli.
It would no longer be built by banshee after a rebuild like this.

[Vinger]

Marco, also could be blade failure, difficult to see after the crash, but the forces on those blades are huge and they could crack and flex weirdly causing the sudden dive.... Are the thrust bearings damaged?

[wblombard]

I would send it back to Banshee to get it fixed...going to cost an arm and a leg but definitely worth it...

Man it kills me to see this.

[helicraze]

Sorry to see this!

I think its aerodynamic.

My experience in speed has been a frw years now and slowly ramping it up.

I have found after 230kmh thinks start to happen, above 250kmh stranger things can happen!

My vbar settings and pitch are not what youd expect, but it really is locked in!

How much pitch did you run?? Retreading blade stall heli pitches up not down. Was the tdv crash cause determined? The links are small compared to minicopter

[helicraze]

Banshee sent me some really hd belts, harder than normal hd that they supply. Belt has about 3mm movement

[marcosp]

I was using 16º. The path I was following was changing to 14S, raising rpm, governing and only then I planned on raising pitch a bit more. Could very well be a blade blowing up but I should have seen it in the rotor disk which looked normal while falling. When it pitched down it offered me a good look at the whole disk. Vbar settings are fine tuned already, I've been using Vbar for speed for over a year and two competitions and all the preparations.

I am seriously thinking about sending it to Banshee since the damage was extensive and there seems to be something worn or not working properly. Intermediate gear is bent as is main shaft so this repair implies a serious check on all drive train and main rotor components. I've already contacted Customs to find out how to declare the outward shipping so as not to be charged on the way back. I have quite a few spares that I would send for them to use.

[mkovalcson]

This is a real tough one because even if you could turn back time and have your Banshee back the way it was two days ago, if you have no idea what failed or needed to be modified it would most likely happen again.

The best you could do is replace all the parts that you thought were possible failure points and just hope you got it.

Out of curiousity what kind of belt tension do you want from the motor pinion to the intermediate?

Does Banshee have a recommended replacement interval for the belts?

For example, do you all replace your tail belts once a year ?

How many of you have logged a LOT of flights on your Banshee? > 100