Nr conservation

[t'aint natural]

I got this problem. Maybe some of the technically-literate could help me out.. .For autorotation you need a rate of descent airflow. After a practise engine failure (in an R22) I get my students to dump the lever, then wait a couple of seconds before raising it about an inch to contain Nr as the rate of descent airflow begins to drive the rotor.. .That couple of seconds interests me. A student who’s slow on the lever (and on coming back with the stick) will hear a lot of horn before the rotor revs start to build. Consider this scenario:. .I’m light… me and five gallons. I’m climbing at max power – well north of 1000 fpm. (Or I’m in a zoom climb, eating up airspeed.) The engine quits cold.. .I’m now in a relatively prolonged bunt, am I not? I’ve got a lot of blade pitch to get rid of. Nr is dropping like a stone, and I’ve got no rate of descent airflow – quite the opposite.. .Is it possible that even with my cat-like reflexes on the lever, I could attain such a rate of climb that Nr would reduce beyond recovery in a bunt before a sufficient rate of descent airflow had been established to save my ass? Should I limit my rate of climb, to 500fpm max? Or what?. . . . <small>[ 21 March 2002, 19:28: Message edited by: t'aint natural ]</small>

[captaincrack]

Interesting and not terribly comforting thought. . .. .I'm not an expert in POF, but had one or two interesting moments teaching autos in Robbos, including an instance of having to get the Nr needle back from 82%.. .. .My thoughts: Bare in mind that 1000fpm is only 10knots. (1800fpm in auto is only 18knots, but doesnt seem that slow at 40feet!). If you are that light, I would imagine the aircraft would stop climbing very quickly (low upward momentum) if it went deathly quiet from behind. But like you say, I'm not convinced it would be quickly enough.. .. .Yes the Nr would drop like a stone, hence lever down to around your ankles and then flare like hell. In the flare the air flow will have a large component through the rotor disc from below so in theory at least you could get some autorotative force going on whilst still climbing. That all assumes that you are climbing with an IAS of 60knots as per the flight manual. Hopefully though, you would end up with a sustainable Nr, with albeit a very low IAS. Providing you have some of that altitude stuff you should get down safely.. .. .Another reason for a large flare would be to keep the disc loaded. Otherwise a tail might get docked.. .. .Those are my thoughts, what does anyone else think? I am not going to try to prove my theory! Something about old and bold! So may be there is something to be said for keeping a low ROC.

[rigid_rotor]

You asked "Is it possible that even with my cat-like reflexes on the lever, I could attain such a rate of climb that Nr would reduce beyond recovery in a bunt before a sufficient rate of descent airflow had been established to save my ass? . .Yes is the answer to this bit.. .. .Should I limit my rate of climb, to 500fpm max? Or what?" It would be difficult to put a figure on it without specific data from a model or flight test. However, if you could get into such a situation where you could climb at a ROC where if the donk quit you could not lower the lever in time, this should (hopefully)come out during certification with an appropriate ROC limitation in your tech manual. . .. .Anyone involved in certification flying please comment??

[3 D]

Just an idle thought on this subject I have had in the past:. .. .Why not include a bottom section of collective travel perhaps protected by a spring like the detent on the throttle. This portion of the collective travel could allow the pitch on the main rotor blades to go to a negative setting. Whilst I appreciate that this is an extremely undesirable situation in powered flight, which would probably result in a boom strike, it would at least allow rotor rpm recovery from very low values provided that enough height is available. . .. .I have just wondered why this isn't the case in the past maybe someone could enlighten me.

[The Nr Fairy]

It's not the positive or negative pitch which is the issue, 3D, I think it's the fact there's anything other than flat pitch on.. .. .Pitch = drag, and whether pitch is negative or positive wouldn't really help, the rotor system will slow down. It seems to me to be a case of having as little drag as possible to aid RPM recovery.. .. .As an aside, I understand naval Lynxes have a negative pitch setting to help holding onto the deck.. .. .( I've just thought - given my name I bloody well hope I'm right ! )

[RW-1]

Taint,. .. .Can't say I haven't had similar thoughts, but. .when we discussed this before, can't remember when but we did;. .. .Let's say you are slow on the collective, the thing as I see it though is that as soon as you begin lowering the lever, you begin the reverse trend in rotor NR droop.. .. .Now you are in the climb, Well ... I don't think you really can get going up so quickly that ... <img border="0" title="" alt="[Confused]" src="confused.gif" /> . .well I think CaptainCrack said it nicely:. .. . </font><blockquote><font size="1" face="Verdana, Arial, Helvetica">quote:</font><hr /><font size="2" face="Verdana, Arial, Helvetica"> That all assumes that you are climbing with an IAS of 60knots as per the flight manual. Hopefully though, you would end up with a sustainable Nr, with albeit a very low IAS. Providing you have some of that altitude stuff you should get down safely. </font><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica"><img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

[heedm]

Nr Fairy, if there is upwards flow through the rotor system, then the angle of attack of flat pitch blades is positive. Negative blade pitch would help reduce the blade angle of attack. . .. .I fly an H46 and just recently oversped the rotor in an auto because I had collective was in the spring loaded detent. Won't bother going into the way, lets just say I fouled up with a little help from my friends.. .. .As to the original question, yes. Types are all different, but unrecoverably low Nr is a possible outcome.. . . . <small>[ 22 March 2002, 00:51: Message edited by: heedm ]</small>

[the coyote]

I was taught and advocate a takeoff profile that uses only enough power to accelerate the aircraft smoothly through to BROC and establish a comfortable rate of climb, this usually is no more than 0.5 to 1" MP above hover power.. .. .This is specifically to configure the aircraft in the best possible way should the noise stop during the critical stage of takeoff. Power=pitch and pitch=bigger Nr decay if the power goes (and a more violent Yaw), nothing new there. Same goes with ROC. The longer it takes the hunk of metal and its intertia to stop climbing and begin descent to establish a ROD airflow, the bigger the Nr decay. I think there is many pilots out there that don't think enough about that.. .. .I have seen the R22 tacho at 80% numerous times and obviously don't recommend it (my fault for letting the student go too far before taking corrective action). One thing that puzzles me though is the view of many pilots to initially (or instinctively) flare after an engine failure to restore RRPM. Let me pose this scenario: You are at 100'AGL, straight and level at 55Kts, close enough to BROC in the R22 and therefore not using a whole lot of power. The noise stops, you lower the collective immediately (which would still be 1-2 sec if you weren't expecting it) and then flare to restore RRPM (which at that power setting would have only decayed to around 90% unless you are way too slow lowering the lever). You are now at a guess 50-75', yes with good RRPM but not much airspeed. ROD is high and nothing to flare with for the touchdown. In my opinion you are going to hit the deck very hard indeed, yes?. .. .Same scenario, 100' S&L at 55Kts. Noise stops, lower collective, RRPM around 90% but DONT flare. Bite your tongue, hold your precious airspeed. At the bottom delay the flare just a fraction and flare harder to restore the RRPM then.. .. .Give me the second scenario any day.. .. .You need RRPM for the touchdown, AFTER you have flared, not at 50-100'. You need AIRSPEED more than RRPM leading up to the flare, in order to arrest your ROD and give you some RRPM.. .. .Think about the other consequences of immediatley flaring after the engine fails, what about range? Maybe if you held your speed it would carry you over the edge of the trees and not in them?. .. .When you stretch the glide, your RRPM is at 90% anyway. What if you are stretching all the way, clipping the tops of the trees as you reach the edge of the clearing and then you're into the flare at 90%. Same thing if you ask me.. .. .As a last resort to arrest a CRITICAL RRPM situation I would then flare or more probably a quick tight turn so as not to risk hitting the tail boom and hopefully preserve a bit more airspeed as well.. .. .In reality in the R22 I think an engine failure anywhere on takeoff up to 100' or maybe more will not be pretty. Given that it takes around 75-100' to enter autorotation anyway, you are not going to have much RRPM left when you started with a ROC and high pitch setting.. .. .I would appreciate any feedback from you guys on what I think.

[Steve76]

Airspeed over RRPM .....I don't think so. It's a dangerous attitude to consider Airspeed over RRPM.. .To preserve your future in rotary wing aviation you should always make RRPM your greatest consideration. The airspeed being #1 priority thinking is something I heard a lot of while in Aussie ..... mainly from R22 drivers. It normal retoric heard from mustering guys all the time. I think it is a fallacy and all wannabes and low time guys should know that the order of priority is: . .. .A)Rotor RPM . .B)Airspeed . .C)Height. . .. .Sacrifice each to save the other, starting from C to A. Remember you will not always be driving a Robbie with low inertia blades (hopefully...) and having light blades means you can spool them up quickly with a flare. Medium to Large helo's will not respond as quickly.. .As Coyote said, RRPM can be sacrificed for distance ... but that is always in the training environment. In real life the good driver will bottom the pole, sort out the situation, access the options and then lower RRPM to make the best landing site if needed. . .Just a few thoughts. No offence intended blokes <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

[the coyote]

I certainly agree with your priorities, Steve 76.. .. .All of my discussion applied to the R22. What I was discussing was largely to do with the scenario I put forward. If the RRPM isn't within the acceptable range, by all means it is absolutely imperative to get it back there (90-110% in the R22).. .. .What I was posing was this: Low level, in auto and with an imminent flare, would you rather RRPM in the mid range and not a lot of airspeed or RRPM at the bottom of the acceptable range with more airspeed. Any situation where there is height to spare of course there is no worries establishing good RRPM and airspeed in auto.. .. .When you do engine failures in the R22 from 40-50'AGL you are not really entering autorotation, you are conserving what RRPM you have left and relying on the flare to give you some more. Anyone that has done them from this height to the ground knows its possible. However if you flared at 50' and got your RRPM back at the sacrifice of airspeed, then I believe you will hit hard.. .. .Once you get below about 40Kts in the R22, the flare doesn't really do much for you in the way of arresting your ROD prior to touchdown.

[tgrendl]

Good thread,. .. .Similarly if a failure mode of the transmission was causing the rrpm to head towards.. say 75% and you were at 2,000 feet.... .. .Right 45 degree spiralling downward turns until flare time?

[Hughes500]

t'aint. .Not that I know anything about R22's but I would have thought that flaring to regain RRPM is not a good idea if you are at a low altitude. Speed is rather important at low level so you can flare at the bottom ( which does a couple of things reduces to virtually o your ROD and spools the disc up ). Having had an engine failure at 100 ft on the approach to a confined area in a 300 ( slowing down passed 30 kts just bringing in the lever passed 18 " manifold pressure everything went silent). All I could do was dump the lever to maintain RRPM and stick it on its nose to try and regain some more fwd speed. I did not have enough speed to flare but had to run on at about 30 kts with the lever up round my ear. If I had flared at the point of engine failure I would have washed my speed off to 10 kts and it would have hurt from 100 ft. I think the answer to your question depends on how high you are v your airspeed. I would suggest <img border="0" title="" alt="[Wink]" src="wink.gif" /> you need to look at your height velocity curve and add to the height a bit if you are climbing at best rate of climb using full power to allow you to make a good eol. Look at it another way - when you can you treat the helicopter like a plane. I teach my students not to rotate until 50 kts if they have the length of field. This will mean in the event of an engine failure they have forward speed to play with. The worst take off is a vertical, the donkey goes its going to hurt !

[Grey Area]

One point that is rarely considered is that during an autorotation the biggest factor affecting Nr is 'g'. This means that you can, in fact, lower the lever TOO quickly as the trade off between ROD aiorflow and 'g' must be maintained - if you achieve much less than .5 g you are going to have lots more problems so a negative pitch detente is not going to help.. .. .On the subject of regaining Nr at low altitude, ie. to flare or not to flare. If you regain Nr by initially flaring and have at least 30 kts most SE helos can be quite safely landed using the no flare technique (constant attitude to those CFS trained chaps). I have not done this in an R22. .. .GA

[RW-1]

Coyote,. .. .I'd certainly go for #2 myself. And still hold out hope, as I'm still inside the V-H knee for a successful demonstrated outcome.. .. .I only consider a flare or slight aft cyclic in my entry determinant upon the overall conditions of when/where it occurred (or at least I hope I will! <img border="0" title="" alt="[Smile]" src="smile.gif" /> ) At the least I'll lower collective and hold off on the flare and evaluate Nr, then make adjustments as necessary.. .. .I believe most of this AS over RPM comes abount for the Robbies own manual states that most of the energy required for the auto is in the forward momentum of the AC, not in the rotor, therefore a well timed flare, etc etc etc.. .. .Depending upon the overall location and just how low RPM is (lets say above 85%, horn going, but not dropping further), I may let it sit there with lowered collective and keep the airspeed up to get a good flare later, which I can use to bring Nr up into the green range, and slow/stop my descent and forward airspeed.. .. .(Did I make sense there? Don't get me wrong, RPM=Life in my book). . . . <small>[ 22 March 2002, 16:25: Message edited by: RW-1 ]</small>

[t'aint natural]

All… many thanks for this food for thought.. .The fix differs according to the circumstances, but in most of those circumstances – anything other than low and slow – I’m persuaded of the value of RRPM above airspeed.. .That being so - I’ve got a thing about spending so much time teaching people to land engine-off, and so little time teaching them to get into autorotation in the first place after an engine failure… ie, no sudden throttle chops in an R22 etc. I don’t care much about bouncing it, rolling it, whatever, in a real emergency – you’ll mostly walk away. But the fatal accidents I can think of resulted from not reacting quickly enough to engine failure. Terminal every time.. .Should we not be teaching students that if it goes quiet in the cruise (at circuit height or above), they should always flare like a crazy man, a real nose-in-the-air job, while the lever is going down? And especially (in order to keep the thread vaguely glued to the original premise) when climbing.

[[email protected]]

Tain't, you imply by the use of the word bunt that you are pushing the cyclic forward as well as lowering the lever which is a very bad thing to do in a robbo. Without positive G applied the rotor can do it's own thing due to lack of control power and the only thrust produced is by the tail rotor which wants to roll the aircraft. The usual consequence of this manoeuvre is to introduce the main rotor to the tail boom with obviously unpleasant results. The other reason for not bunting is that it will decay the Nr - just the opposite of flaring to increase the Nr - a low level auto from 100 kts is a good way to show this - lower the lever and flare to wash off the speed, possibly gaining a little altitude then at 60 kts push the stick forward briskly to stabilise the speed and watch the Nr decay.. .My advice in the high power climb would always be to lower the lever as quickly as possible to conserve your rapidly decaying Nr - then consider turning and or flaring some speed off to recover the Nr to limits - pushing the nose down to gain speed just increases your RoD and if you dont have much height left will just make the EOL more interesting. Out of interest I think the constant attitude EOL would be survivable in an R22, just not very pretty.

[t'aint natural]

Sorry, Crab... I don't mean bunt, that's just lose terminology on my part. I mean the parabola the machine would describe in those conditions.

[the coyote]

t'aint. .. .I don't think you should be teaching students to flare like mad as they are lowering the lever. As we know, depending on the situation, every auto is different. When I used to teach autos I initially focused on the entry only:. .. .1. Get the collective down as positively/quickly as you can (but do it smoothly) and make sure it is all the way down. Otherwise game over.. .2. Control the yaw and don't let the nose drop, keep it where it was.. .. .These two must become INSTINCTIVE actions. I wouldn't confuse them with anything else until they had a good grip on doing that. Whether I am at 50' or 5000' those two initial actions are the same to me.. .. .Then follow on and get the basic auto (500' or higher) right, RRPM management, correct airspeed, into wind if possible, timing and amount of flare, touchdown etc etc. Once you get the technique right and later in their training, I think it is vitally important to take steps beyond that as an instructor.. .. .Food for thought:. .1. One of the best things you can teach any student is to UNDERSTAND WHY they are doing what your getting them to do. Never to blindly do something just because that's what they were taught.. .2. Develop their RRPM awareness and understanding: the importance of RRPM conservation, what factors affect RRPM in auto (ROD, loading/unloading the disc, DA, airspeed), what side effects does a high or low RRPM have on ROD. Eg If they think "Great my RRPM is at the top of the range" do they realise that also gives them a higher ROD to try to arrest later?. .3. Same thing with what you originally mentioned regarding ROC and power settings.. .. .Way too much to mention but lets all remember this: When we first got our licences, it was the absolute bare basics wasn't it? How much did we really learn (mostly by ourselves) over the next couple of thousand hours, and continue to learn? They are many times more likely to do damage to themselves due to their own lack of knowledge, skills, experience and decision making (pilot error) than due to the aircraft malfunctioning.. .. .We can teach them how to fly but how much do we teach them how to think?. . . . <small>[ 23 March 2002, 02:04: Message edited by: the coyote ]</small>

[paulgibson]

I am fairly sure the Bell 407 has a maximum ROC limit of 2000fpm for the very reason of Nr not recovering in a auto situation, not exactly sure of the details of this limitation but worthy of interest by this thread...

[Aesir]

When I attended the R-22 safety seminar 1991 I remember Frank Robinson saying that at cruise power you had 2.9 seconds to lower collective in case of complete and sudden engine failure. Doesnt sound like much time and its even less during climb so better not take your hand off that collective.
But seriously, Frank never said anything about any restriction on ROC if flying within the H/V envelope and Limit MP. I think if you are really interested in getting the most accuarate information about the R-22 is right from the horses mouth i.e. send e-mail to Frank.

Walter