R44 200ftAGL engine out Autorotation video
Surely for a serviceable mag and P lead, the rpm drop should be instantaneous
I get a quick and obvious drop on the right mag, albeit nowhere near 7%, and almost no detectable drop on the left mag
By way of example, on my R44 the right mag has got 100's of hours on it total and about 70 hours since the last 500 hour mag inspection (approx., I don't have the logs in front of me) and the left mag is brand new, less than 10 hours on it. When I do a mag check right now, in the prescribed "2 seconds", I get a quick and obvious drop on the right mag, albeit nowhere near 7%, and almost no detectable drop on the left mag.
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I can 100% guarantee you that I don't have that issue. I am meticulous about my mag checks, as is everyone else who flies my helicopter, and I have a meticulous maintainer. A bad P lead is actually what lead to the replacement of the left mag. It was a bad connection inside the mag, and the mag failed inspection in other ways when we opened it up. So a new mag it was. In the past I've also experienced the same issue due to a bad ignition switch. Sadly, both were expensive fixes, of course. What I wouldn't give for just a plain old busted wire terminal!
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It takes time to observe the loss of power in the engine because this is an indirect measurement which requires time for the lower power to have an effect on what you are measuring.
Each cylinder has 2 spark plugs, each of which is fed from a single separate mag. Working correctly, each initiates a flame front in the fuel / air mixture within the cylinder which travel outwards from the spark and eventually (in fractions of a second) meet to provide full combustion. Before the test the engine manifold pressure is set to provide power sufficient to maintain the rotor at a steady 75% ROTOR RPM. That level of power is required because of the resistance at zero blade pitch (air on the rotors causing drag and friction in the mechanical systems).
Switching to one functional mag at the same manifold pressure (same volume of fuel / air in the cylinder) means only a single spark and flame front within the piston, and that is all that you have changed. The burn from a single flame front now has to cover the whole of the cylinder volume and hence the fuel / air mixture is slower to achieve full combustion. This is less efficient, the burn having to be at the correct point in the cycle of the engine (timing) for peak efficiency, and less power is now produced by each cylinder even though the manifold pressure has not changed. Resistance in the system is initially unchanged (ROTOR RPM is 75% when you start the test) but so less power will then be less than needed to sustain the ROTOR RPM at that same speed and it will slow to a point where the resistance forces balance the new (lower) power output.
The drop in engine performance is instantaneous but what you are measuring is not. You are watching the ROTOR RPM, and it takes finite time for resistance to slow the blades to the new equilibrium. You notice the loss of power straight away - not as smooth and RRPM begins to drop. With a "good single mag" the new ROTOR RPM steady state will be achieved well within the test time.
A weak (failing) single mag will provide a weaker spark than intended and provide a smaller ignition patch, or bad timing on that mag will provide a mis-timed spark; in either case these additional issues will result in further power drop than expected. The ROTOR RPM then falls further to achieve the new equilibrium between resistance and power. The bigger the power drop, the longer it takes for the ROTOR RPM to decay to the new equilibrium - there is quite some momentum in the system, after all. Hence the duration of the test is what Robinson have determined is enough to to check that the mag is within serviceable range, i.e. long enough for the ROTOR RPM to drop out of range and be spotted if it is not within serviceable range..
I was told that the difference in drops between L and R mag is due to the position of the two spark plugs in the cylinder, which results in different flame progression in the cylinder - though I can't vouch for that.
Each cylinder has 2 spark plugs, each of which is fed from a single separate mag. Working correctly, each initiates a flame front in the fuel / air mixture within the cylinder which travel outwards from the spark and eventually (in fractions of a second) meet to provide full combustion. Before the test the engine manifold pressure is set to provide power sufficient to maintain the rotor at a steady 75% ROTOR RPM. That level of power is required because of the resistance at zero blade pitch (air on the rotors causing drag and friction in the mechanical systems).
Switching to one functional mag at the same manifold pressure (same volume of fuel / air in the cylinder) means only a single spark and flame front within the piston, and that is all that you have changed. The burn from a single flame front now has to cover the whole of the cylinder volume and hence the fuel / air mixture is slower to achieve full combustion. This is less efficient, the burn having to be at the correct point in the cycle of the engine (timing) for peak efficiency, and less power is now produced by each cylinder even though the manifold pressure has not changed. Resistance in the system is initially unchanged (ROTOR RPM is 75% when you start the test) but so less power will then be less than needed to sustain the ROTOR RPM at that same speed and it will slow to a point where the resistance forces balance the new (lower) power output.
The drop in engine performance is instantaneous but what you are measuring is not. You are watching the ROTOR RPM, and it takes finite time for resistance to slow the blades to the new equilibrium. You notice the loss of power straight away - not as smooth and RRPM begins to drop. With a "good single mag" the new ROTOR RPM steady state will be achieved well within the test time.
A weak (failing) single mag will provide a weaker spark than intended and provide a smaller ignition patch, or bad timing on that mag will provide a mis-timed spark; in either case these additional issues will result in further power drop than expected. The ROTOR RPM then falls further to achieve the new equilibrium between resistance and power. The bigger the power drop, the longer it takes for the ROTOR RPM to decay to the new equilibrium - there is quite some momentum in the system, after all. Hence the duration of the test is what Robinson have determined is enough to to check that the mag is within serviceable range, i.e. long enough for the ROTOR RPM to drop out of range and be spotted if it is not within serviceable range..
I was told that the difference in drops between L and R mag is due to the position of the two spark plugs in the cylinder, which results in different flame progression in the cylinder - though I can't vouch for that.
Last edited by John R81; 15th Mar 2021 at 09:47. Reason: Missing "each" when referring to leads - thannks for spotting that RB14
Good post John R81 - what would it have taken for Robinson to have included such very useful information in the handling section of their POH?
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GulliBell - i don't have trouble with piston engines as have been playing with them since I was a kid (first rebuild aged 8 - under supervision of Dad) but with communicating clearly and simply - as rb14 spotted and kindly pointed out. Best build effort so far - turning a crate LS1 engine into a 1,000 HP twin RB38 turbo monstor for.... actually, it was road legal.
rb14 - thanks for this, I think it's important to avoid potential confusion, so I have edited accordingly
rb14 - thanks for this, I think it's important to avoid potential confusion, so I have edited accordingly
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I’m only a lowly PPL (H) but the majority of my hours have been in a Raven II, and I think a crucial mistake was not noticing MP before lifting. A few people have mentioned this already but I was always taught to check MP as part of my final pre take off procedure before lifting in to the hover. A reading of 20” would indicate a problem when the machine is still on the ground. As far as I am aware it be should be at 15” or below. We can debate all we like about his mag checks or lack of, and the key position and possible reasons for it. On here and on social media there have been plenty of comments about both. If he had checked MP before lifting, he may have realised he had a potential issue. To be fair, the pilot did well to get the machine on the ground and everyone walked away. But it looks like it should have been totally avoidable, and it will be interesting to see how the authorities view it. Best Regards.
I’m only a lowly PPL (H) but the majority of my hours have been in a Raven II, and I think a crucial mistake was not noticing MP before lifting. A few people have mentioned this already but I was always taught to check MP as part of my final pre take off procedure before lifting in to the hover. A reading of 20” would indicate a problem when the machine is still on the ground. As far as I am aware it be should be at 15” or below. We can debate all we like about his mag checks or lack of, and the key position and possible reasons for it. On here and on social media there have been plenty of comments about both. If he had checked MP before lifting, he may have realised he had a potential issue. To be fair, the pilot did well to get the machine on the ground and everyone walked away. But it looks like it should have been totally avoidable, and it will be interesting to see how the authorities view it. Best Regards.
Thing is, you can teach pilots the proper way of doing things, give them all the knowledge they could ever need, put every conceivable possibility onto the checklist, or in the POH (as some keep griping on about) in the end though,...you cannot save the distracted/inattentive/in a hurry/overcome by "get the job done itis", pilot.
That's why I went IIMC (wearing my "Land and Live" cap) and why this guy didn't notice his key was in the wrong position,...and trust me he wouldn't have noticed the MAP either even if it was on the checklist!
We are not perfect,...we're just human.
Avoid imitations
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Originally Posted by [email protected]
aa777888 - I'd pick a Barrett 0.5 inch as my choice - a bit big for helicopter use maybe but if you hit it you will kill it unlike the 5.56mm AR - I understand the need to eradicate the hogs but there is no need to be cruel about it, wounding them to leave them dying in pain.
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Thanks megan for the photos.
From a previous poster's observation
Then how could the key be turned further left from the OFF position? Perhaps the lock barrel from the other type of helicopter was fitted to this one, or the barrel is loose and turns around in the panel.
Having two types of orientation for the same switch that performs the same function is in my opinion bad engineering.
From a previous poster's observation
Some observation from the video as posted.
00.07 Pre Take Off, key at 1 O'Clock.
00.34 Take Off Roll, key still at 1 O'Clock.
01.53 Post Arrival, key still at 1 O'Clock.
02.01 Shut Down, key turned to 12 O'Clock.
02.06 Post Shut Down, key now at 12 O'Clock.
00.07 Pre Take Off, key at 1 O'Clock.
00.34 Take Off Roll, key still at 1 O'Clock.
01.53 Post Arrival, key still at 1 O'Clock.
02.01 Shut Down, key turned to 12 O'Clock.
02.06 Post Shut Down, key now at 12 O'Clock.
Having two types of orientation for the same switch that performs the same function is in my opinion bad engineering.
I personally think that automotive ignition type switches have no place in the aviation industry. There are too many opportunities for murphy, eg if the key is 180 degrees out, it will still look to be in the correct position - even on close inspection. Switches with important function should have positive position indications, like an old fashioned clunky rotary switch.
teej013 - the problem is that things are designed and built by engineers - I don't think any aircraft manufacturers have an ergonomics dept for Human Factors input
Originally Posted by [email protected]
teej013 - the problem is that things are designed and built by engineers - I don't think any aircraft manufacturers have an ergonomics dept for Human Factors input
Yes, tongue-in-cheek but sometimes you have to wonder with some aircraft designs - the Lynx Tq matching worked counter-intuitively, you were re-datuming the no 2 ECU governor but turning the knob clockwise increased the number 1 Tq instead of the no 2.
The close positioning of the mixture knob to the fuel shut off ISTR on the R22 was another one.
The close positioning of the mixture knob to the fuel shut off ISTR on the R22 was another one.
Originally Posted by [email protected]
Yes, tongue-in-cheek but sometimes you have to wonder with some aircraft designs - the Lynx Tq matching worked counter-intuitively, you were re-datuming the no 2 ECU governor but turning the knob clockwise increased the number 1 Tq instead of the no 2.
The close positioning of the mixture knob to the fuel shut off ISTR on the R22 was another one.
The close positioning of the mixture knob to the fuel shut off ISTR on the R22 was another one.
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We should start a separate thread on our pet design peeves. I bet every helicopter model has at least a dozen. What I wouldn't give for an extended oil filler tube and adjustable co-pilot side pedals on the R44. Can't get them. Never will have them. But I can get air conditioning, Lithium batteries and all manner of other esoterica. It just boggles the mind, sometimes.
Fit it with a `proper big round RRPM/ERPM GAUGE......