First proper 450 crash today

[rudi]

I've had minor incidents before (blade scrapes, tail scrapes on landing) but no major fall-out-of-the-sky crashes. Until today.

I'm not entirely sure what happened.


I came out of a slow forward flight coordinated banked turn onto downwind. When trying to return to straight and level, the heli responded like a plane that has stalled - large amounts of cyclic inputs were needed to keep it level and it started losing height. It was very wobbly and it seemed impossible to keep it straight and level. I remember thinking one of the cyclic servos must have stopped responding.

I hit throttle hold and managed to crash it onto the skids. What is strange is there was no problem controlling the heli in the last couple of seconds and I was pretty pleased my "crash" was upright and straight down (like a heavy landing). Luckily the only damage was to skids, blades and mainshaft.

I suspect it was pilot error because everything checked out afterwards, and the cyclic responded normally on the way down. Looking back I wonder if I could have saved this by applying lots of collective rather than giving up and hitting throttle hold? I think my brain didn't know why the heli was behaving as it was, and assumed a fault rather than pilot error, hence choosing to hit throttle hold rather than try and save it with collective.

Oh, and while it happened quickly, I do remember thinking "Thank this is an Align 450 and not a [insert expensive heli here] 700".


What would cause a heli to respond like an aeroplane that has stalled?
What effect does a strong tail wind have on exit of a turn?
Should I have continued trying to save it and risk greater damage, or was it the right thing to accept a crash and hit Throttle Hold?
Is it possible there was an intermittent fault with one of the servos/FBL? I did have some control, so it wasn't a receiver problem. What should I check?
Any tips for next time?

Thanks

[scallybert]

I read something similar to this in the Art of The Helicopter (which refers to full-scale helis)

Imagine you're maintaing a 70MPH air speed into a 30 MPH headwind. Your groundspeed is 40 MPH.

You performm the 180 degree turn, maintaining a constant airspeed. However, your groundspeed must now be 100 MPH.

Where this gets awakward, is that the aircraft's kinetic energy is 0.5 * mass * [groundspeed squared].

So a lot of energy is required to go from 40 MPH to 100 MPH. This energy comes your motor, or a loss of altitude.

Or, if you keep the groundspeed constant, the airspeed will drop a lot. Which maybe what you saw - or a combination.

[cjcj1949]

It is difficult to understand what happens with wind and changes of direction. Newton's laws give the whole story and kinetic energy comes from those laws but with planks it was easier to push the nose down when it comes into wind. Basically anything will keep going at the same speed unless acted on by an external force. That force will accelerate the object. As you turn into wind the force will change immediately but your speed will change gradually.(It may seem instant if the force is very large). All the forces on the rotors will change as well and this could be difficult to predict with a heli, especially if very large. Usually it's a good idea to try anything new far enough up to recover. I haven't noticed any instabilty in winds of up to 15mph although I am used to flying planks in wind and I may be adjusting without realising it. Perhaps you managed to produce more force than something could handle and lost control due to that. Perhaps servo slippage or worn ball joints? If you give everything a good tug you might find something. The other possibility is that you went through some turbulence. This could be transient with the wind or due to local things like trees or fences. There is also a difference in wind speed with altitude and this could have caused a problem.

Helis are difficult enough when flying ok. When things go wrong it is very difficult as we don't know if it is a one off or a repeatable fault.

[Cagey]

May or may not be significant, but when I do a slow co-ordinated banked turn with my 450, it always needs a bit of a push with throttle to keep penetration and altitude same as entry out of wind. Unless you are coming around fast then inertia keeps her going nicely. It took a while before I realised I was automatically compensating for wind speed and direction. Personally I reckons it's throttle control you are needing to concentrate on, and three mistakes high too.

[milly0812]

First proper 450 crash today

I read something similar to this in the Art of The Helicopter (which refers to full-scale helis)

Imagine you're maintaing a 70MPH air speed into a 30 MPH headwind. Your groundspeed is 40 MPH.

You performm the 180 degree turn, maintaining a constant airspeed. However, your groundspeed must now be 100 MPH.

Where this gets awakward, is that the aircraft's kinetic energy is 0.5 * mass * [groundspeed squared].

So a lot of energy is required to go from 40 MPH to 100 MPH. This energy comes your motor, or a loss of altitude.

Or, if you keep the groundspeed constant, the airspeed will drop a lot. Which maybe what you saw - or a combination.

Arghhhhh me brain hurts lol

[cjcj1949]

Arghhhhh me brain hurts lol

That's your fault for reading it. We automatically compensate for lots of things, our brains haven't evolved for thinking, that's overrated. Useful for finding out what went wrong, but usually a bit too slow to stop it happening. We can replace chess grandmasters with computers, but not footballers. They can fly helis better than us though, that's computers not footballers. All you need to know is Force = mass x acceleration. You can get to the moon with that, simple...in principle, until you try to apply it.

[rudi]

I think I understand what happened now.

I had a fairly low headspeed as I was flying for duration. Flying upwind my airspeed would have been high compared to ground speed resulting in a lot of translational lift. As I made the 180 degree turn, the ground speed of the heli increased quickly as it turned downwind. I wanted to maintain constant ground speed so I pulled back on the cyclic to slow the heli. This resulted in the heli climbing, so I reduced collective to maintain altitude. I'm now exiting the turn downwind with a high nose up attitude, low collective and low (or zero) air speed. Translational lift is lost and the heli sinks rapidly. Increasing collective has no immediate effect due to the low head speed.

I had recently rebuilt the heli and damaged one of the servo wires. I had to resolder the wire inside the servo, and it has been at the back of my mind that it may fail and I should replace the servo. I think this worry led me to incorrectly conclude the situation was an unrecoverable failure rather than disorientation.

I think the following led to the crash:

1. Instinct to hit Throttle Hold at the first sign of trouble (built up from flying my mCPx). It would have been better to keep flying rather than give up.
2. Headspeed too low for the wind conditions. Dealing with strong winds and gusts needs more power.
3. Failure to understand translational lift, how it relates to air speed and not ground speed, and the large effect it has on the heli.
4. Inexperienced in windy conditions in general and especially managing ground speed when turning downwind - never specifically practiced in Phoenix.
5. Failure to increase collective to reduce impact - never practiced auto-rotations.
6. Flying the helicopter while having concerns about a repair to a servo.
7. Not knowing how a single servo failure would cause the heli to respond and misidentifying that as a cause.

I have managed to recreate the above scenario in Phoenix - wind 15mph, T-rex 450 with low headspeed (2200rpm).

I've learnt a lot from the above. I hope the analysis helps somebody else to avoid a similar crash.

Hope it's windy tomorrow so I can try again

[nobbycopter]

First proper 450 crash today

Easy way to stop it I found is to fly in Idle up, therefore with constant headspeed, not necessarily maxed out but at 75% or above

[rudi]

I was in idle up, running at 80% throttle using the stock Align ESC in governor mode. I don't think the ESC governor is that accurate, jumping from 80% throttle to 85% throttle makes a massive change in headspeed, so 80% is actually a low headspeed on my heli giving near 8 minutes flight time.

[scallybert]

Crikey, that's a thorough analysis...

My 450 Pro has the headspeed governed to 3050 RPM. Possibly high for the flying I do; but gives good power and 5:00 to 5:30 minute flights.

[cjcj1949]

Getting 8 minutes is good. Having the head speed too low does cause nastys very quickly. Perhaps you could post the Phoenix recording.

[rudi]

Crikey, that's a thorough analysis...

My 450 Pro has the headspeed governed to 3050 RPM. Possibly high for the flying I do; but gives good power and 5:00 to 5:30 minute flights.

My YouTube history consists mainly of "Air Crash Investigation"

[tomatwalden]

Setting with power maybe ?

Helicopter Aviation

T

[rudi]

Setting with power maybe ?

Helicopter Aviation

T

Interesting. Yes, I think that had something to do with it. I'm pretty certain I'd have entered the downwind leg with zero airspeed even though groundspeed was constant throughout the turn.