BO105 fatal accident back in 2006(?)

[DennisK]

B 105

Having studied Hoffman's B105 manoeuvre, it occurs to me that he was experiencing a fairly well known handling difficulty that can result during the steep 'wing-over' recovery, especially from an into wind low level, low speed entry.

If the speed is allowed to drop below the transition lift point at the top of the manoeuvre, or even to a zero speed, I have found it is vital to regain translational airflow to obtain full disc authority for the normal recovery.

I hate to admit, that in two instances, (albeit in 1213 displays) I have experienced this 'loss of lift' condition, but fortunately on both occasions I had sufficient height to regain translational lift speed in the descent. It seemed to me that Hoffman did not have that height and during the following steep descent where there was not sufficient forward airspeed to regain the required lift, the 105 simply 'fell' into the ground.

I know that on one of the occasions I experienced the problem was when I failed to achieve the correct 'gate' speed for a 270 degree 'wing-over' following a 'toward the crowdline' level approach into a stiff breeze.

In the second half of the 270 degree heading change - the resultant tailwind significantly slowed the recapture of translation lift. The unexpected loss of immediate cyclic response and loss of height gave me more than a second of concern.

I now don't do 'into wind' display wing overs without an extra margin of 'gate' height and speed for the manoeuvre entry.

Not sure this is totally relevent to the Hoffman accident, but I do recall having a conversation with him on the subject at the 1978 Farnborough display, where he performed the first Bolkow 195 Loop I had seen. He certainly knew of the possible condition then.

Safe flying to all,

Dennis Kenyon.

[FH1100 Pilot]

Dennis, as I have 2,500 hundred hours of 105 time and have watched this video clip and over and over and over, I'm inclined to go with your assessment while rejecting the "infamous Bolkow running out of left cyclic" thesis. I'll bet that Ziggy dearly wishes now that he'd started that maneuver with 10 or 20 knots more airspeed...or alternatively had started it on the downwind run instead of the up.

[Gaseous]

I dont get it. The downwind element. I understand landing downwind is a problem due to transition through hover to backward flight to get groundspeed to zero but in a manoeuvre such as this, how is the groundspeed relevant?

If Ziggy entered the manoeuvre at say 70 knots airspeed and that was enough to safely complete, 70 knots IAS upwind is the same as 70 IAS downwind. What difference does the groundspeed make? The rotor system responds only to the airmass it is travelling through. The groundspeed is irrelevant to the actual physics of the manoeuvre.

It seems to me Ziggy's problem was too low, too slow (airspeed), too steep, too mad. RIP.

Can you explain why the rotorsystem responds differently downwind, if the airpeed is the same in any replies? Thanks.

PS. Ive re-read Dennis's post several times but I still dont get it.

[DennisK]

B105 handling

Okay ... the differences is 'inertia.' Something I routinely demonstrate in PPL steep turn and quick stop training.

In display manoeuvres, and for that matter any manouevre involving a 180 degree change of direction parallel to the current airflow, inertia of the airframe IS directly related to 'groundspeed.' The effect is more significant on the heavier helicopters.

Given a 25 knot breeze and a rapid 180 degree turn, the airframe IS NOT going to accelerate instantly with the reversed airflow. Indeed in demonstrating the effect, I've noted 3/4 seconds delay before the ASI indicates a positive reading even though the ground below is passing by at 20knots. That time is more than enough to lose the little height Hoffman appeared to have had based on my video viewing.

The difficult thing to get ones head around, is that in a steep nose down attitude with 'nothing on the clock' the cyclic has to be pushed further forward to regain translation lift as quickly as possible to enable the subsequent aft cyclic to be fully effective!

At Rochester in the mid 1990s, I saw a similar situation, when a Hughes TH55 sank into the ground turning off a strong wind in a low level steep turn. It does happen.

We can all observe the 'inertia' effect by practicing at height. Commence a 30 knot rapid turn on to a downwind heading and monitor the ASI. The reading will often decrease to zero or less than translational lift speed for a second or so.

I'm suggesting that may have happened in the Hoffman case. As I noted in the earlier post ... it has certainly happened to me, albeit it won't again.

Other views very welcome please. I'll bet my bottom $ Nick Lappos has the real answer!

Take care all,

Dennis Kenyon.

[InducedDrag]

Dennis

I am definitely not trying to argue, but I just must protest.

There is NO inertia involved in turning downwind or upwind. The helicopter does not know any different. It is flying within the "sea of air".

Your idea of inertia would be analogous to saying that a boat on the current of a river, turning from traveling against the current to going with it, takes a moment to gain speed.

Or even a more extreme example is....that it makes a difference turning east to west.....because the earths rotation will either add or subtract from our speed.

The only way there could be a factor, which is quite possible I will add, is if a steep wind gradient exists between the ground and the moving air mass. This can most definitely sap energy from an aircraft descending through it turning in the manner shown in the video.

[DennisK]

Hi Induced Drag,

I'm trying to think up another way of explaining the condition. Not sure where I'll be leading myself even!

Our dear friend Isaac Newton tells us that every body in a 'uniform state of motion' continues in that uniform motion until acted upon by an external force. That, as we all know, is inertia, ie movement or lack of movement relative to space. Much the same as the pendulum of a clock swings relative to space, not the clock it sits in. Ditto a gyroscope. In the case of the pendulum it has a name. Cycloidal error. (Sinsinoidal)

So to try and relate this to our aircraft - its inertia is relative to space, which in its simplest term is relative to the bit of ground sitting underneath.

So let's say I am at the top of a wingover, ASI showing 15 knots in a fifteen knot wind. My position relative to space/earth underneath is static, and the inertia I possess is zero. The only energy I possess is the potential to fall to earth under the influence of 1G. ie ... 'Potential Energy.'

Now to achieve some speed I need an external force applied to accelerate me. Well in the instance I have given, I only have the wind and gravity.
Yes, as I reciprocate 180 degrees, the 15 knot wind is applied to my airframe, but I am not going to accelerate in a micro second to 15 knots in the opposite direction, and depending on the aerodynamic drag of my hull, that may take a few seconds. It is interesting here that the more streamlined my airframe, the slower the acceleration. A nice big draggy helicopter will speed up more quickly! (until it equals 15 knots groundspeed, therafter it becomes less efficient)

I hope I am making some progress here!

So we have a situation where, my airframe is at zero or low inertia for a short period and the only external force now affecting me is the 1G towards mother earth!

Now IF the time period before striking the ground is sufficient to allow the 15 knot wind to take full effect, and give me translational lift, I'm home and dry. But IF NOT, the 1G acceleration to earth takes precedent and you can see what happens. So we are talking here about extra height and extra speed for a safe wing-over manoeuvre ... something we all know.

All this of course assumes I don't increase power at some early stage, but since I am performing a low entry speed 'wing-over' manoeuvre followed by a steep descent, I wasn't planning on doing this until I need to level off close to the ground.

So in summary, I believe it is that short period of zero inertia when the aircraft is at risk. Plenty of height (ie time) to convert to speed is good. A streamlined and heavier ship has a corresponding higher period at low inertia and is correspondingly more at risk.

I can tell you that in display flying, the transition effect in a steep descent is as noticeable as the extra lift we all feel on a standard take off profile. The same thing happening in a steep descent takes some getting used to until one works out what is going on.

I haven't used any technical jargon or lift/gravity formulae here, since there are others better versed in those areas than me. But here's a plea. Nick if you are reading this, we'd all like your pennorth!

Best wishes,

Dennis Kenyon.

[Gaseous]

Thanks Dennis,

My head is still hurting over this. To paraphrase you, if I have understood correctly, the inertia is relative to the ground, not the air flow caused by the wind. This explains the proposed phenomenon neatly if correct.

My problem is based on my (infantile) grasp of Newtonian laws of motion. I feel the inertia should be related to the air flow, and that the ground is simply not relevant being in a different inertial frame. Am I wrong?

Its a long time since I did A level physics. I could well be wrong.

Edit: you just posted above as I was writing this. Apologies if you have just explained.

[Shawn Coyle]

Induced drag:
The boat analogy is not valid, as the boat doesn't get any of it's bouyancy from motion through the water, only by displacing water.

[ShyTorque]

Many folk forget that a helicopter isn't always in a balanced turn. Once yaw pedal is used, the rules change and inertia has much more effect. A wingover / torque turn is one of those occasions.

[InducedDrag]

Sorry, and with all due respect, the helicopter does not care about the ground as a reference point. (unless you hit it)

Inertia measured relative to the ground is not valid....just as it is not valid to say that a helicopter hovering at the equator has more energy then one hovering at the pole.

Sure it does from the frame of reference to an interstellar object....but that does not have any effect on the dynamic of flight.....Just as a frame of reference to the ground has no effect.

AIRSPEED is the the only measurement that matters to a flying object. It does not care if you turn upwind or downwind....so long as airspeed stays the same.

Like I said earlier.....it is possible that climbing or descending through a gradient CAN have a effect or flying in gusty conditions. This can add or decrease energy from the ship.....relative to it's airspeed.....which is the only one that matters.

[Gaseous]

Having spent some time with fount of all knowledge, Google, it seems to confirm my gut feeling that the inertial frame of reference relevant to the aircraft is the airmass it is moving through, not the ground. Indeed, the ground appears no more relevant than the centre of the solar system to the helicopter. If this is the case then being 'downwind' is also irrelevant in this context.

Load of examples such as the juggler here:http://id.mind.net/~zona/mstm/physic...tialFrame.html

I realise this is at odds with Dennis's explanation and I hesitate to take a contrary point of view. NICK, where are you when we need you. Induced Drag is clearly thinking on the same lines as me.

I'd be happy to be proved wrong so as not to be at odds with Shawn, Dennis, FH1100, and just about everyone else but I can find no convincing physical reason to support the assertion that Ziggys accident was related his track relative to the wind direction other than ID's gradient explanation.

Come on guys, Ive got to be missing something here.

Edit: My interest is more than academic. I experienced a similar drop during an LPC a few years ago doing steep turns, fortunately at 1500ft. Lost a couple of hundred ft in a few seconds.

[brett s]

The way I see it, if you weren't looking at the ground & kept the same airspeeds throughout the maneuver when turning either upwind or downwind it wouldn't matter at all except for any different wind gradients you're passing through as another poster mentioned.

But because you are referencing the ground track & not airspeed when doing that sort of maneuver at low altitude the wind direction matters.

Biggest scare I ever got ag flying was the same sort of situation - heavy as I'd just taken off with a full load, field rows were directly aligned with the wind, turning downwind. Couldn't widen the turn to keep airspeed up because of obstructions on both sides, so end up with basically zero airspeed at the top. Dropping back down into the field with plenty of ground speed but very little airspeed yet, aft cyclic isn't going to help there. Ended up about 6" above the dirt with rpm bleeding off before stopping the descent... Bad choice on my part getting into that situation, it was definitely a learning experience!

[jab]

This example works for me since I have experienced it myself. During fire fighting in a reasonable wind, you try to take off into wind and get at least 40 knots on the clock before turning out. There are times when this is not possible and as you turn downwind you really have to be careful because you lose IAS and the helicopter descends with the loss of IAS and sometimes translational lift in strong wind conditions. The helicopter also accelerates very slowly with a few tons of water hanging below and even though I have been aware of this, it has still scared me silly more than a few times.

The principle remains the same as Dennis has explained, the helicopter has inertia and when turning downwind, especially if the turn is very tight, it takes time to overcome the wind gradient and therefore the IAS decreases, possibly to below transition. Without the translational lift there is insufficient pitch / power available to overcome the rate of descent established in those few critical seconds and a prang is very likely as in this example.

I agree that you are flying in an airmass, there should not be a change in IAS and there wont be if the turn is gentle. With a really tight turn the inertia is much more obvious and there is definitely a drop in IAS when turning downwind. It wont make a difference if you started the turn with sufficient IAS, as Dennis said, but it certainly will if you start the turn with the speed too low.

[Brian Abraham]

Oh dear, the downwind turn raises its ugly head again. People have trouble coming to grips with the aircrafts relationship with respect to (a) the ground and (b) the air. Extensive discussion here, but as usual be careful what you take on board from the posts, some poor understanding of both aerodynamics and physics evident, but if you visit the military forum often you will be aware of who to give credance. http://www.pprune.org/forums/showthread.php?t=275575Well we can but try and educate. Please read the following and note the last paragraph. http://www.flyingmag.com/article.asp...7&print_page=y

[jab]

Hi Brian

Peter Garrison's comment is only valid for "normal" flying. Ziggy's turn was not what you would see in the circuit being practiced by students and involved a rapid change of heading out of the wind. Dennis mentioned IAS dropping close to zero during his shows, or at least below transition, resulting in insufficient lift to overcome the rate of descent. At least he had sufficient height to recover. It appears this happened to Ziggy but he was unfortunately too low to escape. My example has similarities and is based on experience. I believe Dennis' theory as to the cause of the accident to be reasonable, insufficient airspeed and height for a high rate of turn out of the wind.

"There, but for the grace of God, go I." RIP

[Gaseous]

Thanks Brian, all is now clear. I shall persue this no further.

[Shawn Coyle]

Ask any model aircraft flyer or ultralight pilot about the effects of turning downwind, and they'll all agree there is a significant issue.
My rule of thumb is that if the windspeed is more than 25% of your airspeed, turning downwind will have a noticeable effect on flightpath.
And doing wingovers from into wind to downwind catches everybody out the first time. anytime You are maneuvering vertically, energy with respect to the earth has to be considered.

[InducedDrag]

I am sorry but I still dont buy it.

With regard to asking a model aircraft pilot, their frame of reference is from the ground...which the aircraft does not care about. Now if the guy on the ground tries to maneuver with reference to the ground, then there will be issues.

I agree that if the helicopter pilot was maneuvering by ground track, he could get sucked into this. But if he flies by the ASI, the ship will still fly the same. (although ground track may look slightly different due to wind drift)

Again....I still hold that climbing or descending through gradients CAN have an effect on energy where it matters (airspeed). It is a technique that many sea birds use to stay aloft for hours at a time without flapping. This is why you observe the birds diving and climbing steeply. They are extracting energy from the gradient.

Here is a link:

http://en.wikipedia.org/wiki/Dynamic_soaring

http://www.wfu.edu/biology/albatross...ic_soaring.htm


Just to illustrate my point another way..... Imagine if during the wingover, the earths rate of rotation suddenly sped up twice as fast underneath it. The ground would accelerate and the pilot would suddenly see the ground passing under him at about 1000mph (near the equator). Ignoring friction between the air and the earth (meaning the air would not begin to rotate), ....would the helicopter suddenly have more "energy" to fly? Of course not....the helicopter only cares about the surrounding air it is flying in......the earth is of no consequence.......unless you hit it.

[DennisK]

Wing-over manoeuvres

This is bubbling away nicely. We still need NL's view.

Let's look at this condition from another viewpoint.

Imagine a 20 stone rugger forward standing stationary at Twickenham. You charge him for the tackle but you are 10 stone. At impact he will barely move but you'll bounce off and have a headache. It will matter not a tot whether there was a 20 knot breeze blowing at the time.

The reason is he possessed no positive inertia relative to the surface he was standing on. But you did.

Now visualise the same 20 stone guy heading for the touch down line at 15 knots. You are standing still this time. To throw him off balance you'd only need a gentle shove as he sped by ... 20 knot wind or no wind, the above example is sound.

The reason is, relative to the surface the rugger forward now possesses 15 knots x 20 stone of positive inertia. (I won't put it in ergs.) How well I recall the situation from my playing days. Get the big man when he is moving fast!

Al I can say lads is that you won't ever get me performing a rapid 180 degree turn into a downwind situation at low level. A few hours of experience taught me that a while ago.

Can we return to the 'earth moving' condition. Contrary to what has been said. If the globe were to instantly rotate from stationary to 1,000 mph, then the poor guy standing WILL for a few seconds similarly find himself launched across the surface at the same 1,000 mph. Once the 1,000 mph wind took effect however, he would slowly find himself returning to a stationary position relative to the earth once more.

BUT ... IF he was in a vacuum he'd remain forever at 1,000 mph somewhat like a satellite!

The condition is there and perhaps a physicist can explain better it than me.

Over to you lads, and especially Nick Lappos.

Dennis Kenyon.

[Overdrive]

Correct this is is getting interesting!

The reason is he possessed no positive inertia relative to the surface he was standing on. But you did.

No, the reason is he has enough inertia to resist your momentum.

Now visualise the same 20 stone guy heading for the touch down line at 15 knots. You are standing still this time. To throw him off balance you'd only need a gentle shove as he sped by ... 20 knot wind or no wind, the above example is sound.

Now he still has inertia, but plenty of momentum, which you can "vector" with a glancing blow, maybe deflect him into touch. You would feel this inertia well enough if you stood directly in front of him. This inertia is why you can't stop him instantly, or even push him sideways at 90 degrees (unless you are bleedin' enormous, i.e., have much more inertia than him)


Inertia is resistance to change of velocity/direction, and is present regardless of whether moving or stationary.