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Autorotation scenario - Load disk with additional RPM?


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This from:

 

http://helicopterflight.net/autorotation.htm

 

Here is a scenario for you to answer - While flying along by yourself at an altitude of 1,200 feet AGL and at 80 KIAS, suddenly and without warning, your lose your engine. Thanks to good training, you promptly lower the collective to the stop, but much to your surprise the rotor rpm remains in a decreasing trend which is now less than the lower limits of the green arc. What will you do to regain that lost rpm? Most will answer flare, will this work? What do you think?

 

My answer is: Make some turns if able to (slow down from 80 KIAS.... will that load the disk with additional RPM's?

 

Anyone give me some input?

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This from:

 

http://helicopterflight.net/autorotation.htm

 

Here is a scenario for you to answer - While flying along by yourself at an altitude of 1,200 feet AGL and at 80 KIAS, suddenly and without warning, your lose your engine. Thanks to good training, you promptly lower the collective to the stop, but much to your surprise the rotor rpm remains in a decreasing trend which is now less than the lower limits of the green arc. What will you do to regain that lost rpm? Most will answer flare, will this work? What do you think?

 

My answer is: Make some turns if able to (slow down from 80 KIAS.... will that load the disk with additional RPM's?

 

Anyone give me some input?

Collective is very important, however to maintain rotor rpm there must be an initial aft cyclic input to begin the air flowing upward through the disc to maintain rotor rpm. A good memory aid is DOWN AFT RIGHT.

Down collective Aft cyclic and Right pedal for trim. Yes S-turns will build rotor rpm but their has to be an initial aft cyclic input to get that relative wind flowing upward. JUst my two cents. There is a great article in autorotate magazine cant recall the month but the title to it was a chat with Pete Gilles on autorotations!

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What about stuffing the nose over? It might be the voice of inexperience talking, but were my RRPM decaying to or past the point of no return I'd dive for airspeed. Turn that remaining altitude into blade inertia. You might lose your ability to make it to your landing spot, but it beats the risk of falling like an anvil.

 

Any thoughts on this idea? Good or stupid?

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West Coaster,

 

What about stuffing the nose over? It might be the voice of inexperience talking, but were my RRPM decaying to or past the point of no return I'd dive for airspeed

 

Based on the scenario, this would be the last thing I would do. You run the risk of completely stalling the blades.

 

Read TLJ's post, and you'll see why. There's a reason why he mentions aft cyclic - to restore the decaying RRPM. Cyclic forward - RPM drops out!

 

At 80kts aft cyclic and then slowly forward. If your speed is less than 80kt at that altitude , the best solution I can think of would be a quickstop in the air (aggresiveness relative to the amount of decay). Run the RPM back up to red line without losing altitude. Then regain the airspeed once you have RPM stable. Ensure to begin to restore airspeed no less than 500 feet!

 

If your speed or altitude is already really low, then a hard wingover.

 

Remember that with after any action to restore RPM, you need to settle out slowly. For example, if you use hard aft cyclic, you need to ease it forward slowly, or else you'll be right back where you started. Same with loading the disc with a hard right turn. Level out slowly or you'll unload the disc just as quick as you loaded.

 

Bristol,

 

While flying along by yourself at an altitude of 1,200 feet AGL and at 80 KIAS, suddenly and without warning, your lose your engine. Thanks to good training, you promptly lower the collective to the stop, but much to your surprise the rotor rpm remains in a decreasing trend which is now less than the lower limits of the green arc.

 

If you are correctly weighted, and quick enough, lowering the collective should be enough....that's what the minimum weight in the R22 is for! If your RRPM is still decaying, then you need to use some cyclic input to restore it until your RoD is high enough. Check your collective is fully bottomed out too. If RRPM continues to decay, well....I don't know!

 

 

Joker

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dive for airspeed on engine failure??? methinks we have a fixed winger amonst us..(jk)

 

Based on this scenario, if aft cyclic failed to restore RPM, even to the piont of having zero airspeed, your goose is prolly cooked. This usually happens if the aircraft doesnt weigh enough to develop the RoD, and thus induced airflow from below. I know some r22 instructors who wouldnt do an auto solo w/o 30lbs under the pax seat.

It seems to me though that improper pitch settings on the blades, where full down collective does not get the blades flat enough to maintain the correct driven/driving/stalled regions could be the culprit. I usually see this as a poorly autorotating ship, but never to the extreme where rpm was unrecoverable.

 

Here's a fun question: whats the lowest rpm you've had?

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Forward cyclic, rotor RPM goes down. Aft cyclic RPM goes up.

I would flare the aircraft after I fully lowered collective. If that didn’t work I would start a turn. Remember your 180 autos? Rotor RPM shoots up during the turn so you usually have to pull more collective. :blink:

 

RW

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If you start a sharp turn to bring the RRPM back up, you're just going to lose it when you level out and use extra energy to arrest the increased ROD that you just created.

 

If you're helicopter is rigged correctly, you should have no problems with RRPM at minimum weight. It will be low, but not low enough to cause a problem if you're flying at the recommended autorotation speed.

 

From what I've heard at RHC safety school, you're not going to stall out the rotor system from low RRPM if the collective is all the way down. This isn't something I would want to test, but it kind of makes sense.

 

The answer to the question is slow to the recommended autorotation speed (usually ~60 knots.) Don't turn, don't push the nose over, don't flare to below 50 knots, because in each one you're going to end up losing RRPM after a brief increase.

 

And remember, up until the flare stage, it's much better to have low RRPM than high RRPM. In the R22, you can reduce your ROD by 500 FPM descending at 90% rather than 110% with a constant 60 knots. That's about 6 MPH closer rate you won't need to burn energy for at the end of the auto.......6 MPH doesn't sound like much, but sprint into a brick wall at see what 6 MPH can do to you.

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I stand corrected, and the fixed-wing jab was well deserved looking back haha.

 

I thought I could remember my instructor showing me the recovery I mentioned earlier, but that being 2 years ago my memory lied to me. After deeper thought he didn't drop the nose, just the collective to increase ROD only. We fell like a stone for what felt like an eternity before the RRPM recovered.

 

Good thread!

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As Delorean is saying, low RRPM on auto will increase your glide and decrease your rate of descent for a given airspeed . This is because the extra lift produced by increasing the angle of attack is greater than the amount of lift lost by slowing down the rotors. Many schools teach this method under "advanced autos" for commercial level. Many schools probably doesn't teach it at all.

However if you ever go to the MD factory school you will do this on every auto. Initially down collective, then raise collective until RRPM is stabilized in the bottom of the power off green arc.

 

If you haven't already used this tecnique, i suggest asking your instructor, check airman or whichever proficient person you fly with to demonstrate it, as it is very useful. Doing it in a 300 actually makes it feel like you are "gliding", instead of the normal "controlled falling".

 

And as a couple of guys here already mentioned, if collective is all the way down and RRPM is still decaying.......talk to the mechanic about adjusting the pitch of the blades, if you walk away from the auto that is.

 

Safe autos.

Flyby

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West Coaster!

 

Taken like a man! B)

 

Everyone,

 

A common theme emerging is that if the helicopter is within weight limits, and collective is right down, and reaction is quick enough, then at autorotational speed, RPM should recover sufficiently.

 

Delorean is exactly right in his post, where he comments on the loss of RPM after any aggressive maneuvers. I tried to hint at this same point. It is possible (I have done them) to use advanced maneuvers to restore RPM, but it the execution is critical, based on good judgement of weight, height, speed etc..etc...something that takes practice.

 

I think I was also imagining a much more drastic situation than the question meant!

 

So I would agree that Delorean's answer is better than mine in this flight training forum. The techniques I described require quite an intimate understanding of your aircraft, requiring literally hundreds of 'advanced' autos under your belt in order to pull them off.

 

Better advice is 'keep it simple'. Stay within limits and stick to the textbook methods and you should be alright.

 

Regards,

 

Joker

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Well, I guess that my initial "slowdown" was right (maybe not using turns, but my answer was in the right place). Aft on the cyclic.

 

Thanks to everyone for the input...

 

Regards,

 

Bristol

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On a side note, and a touch off topic, to those who fly the R-22.... what do you think of the auto charactistics with the new -4 blades? Been doing some flying lately in a newer BII and it autos like a dream compared to the older Beta with the old blades. Seemed a lot more forgiving, or maybe it was just me?

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On a side note, and a touch off topic, to those who fly the R-22.... what do you think of the auto charactistics with the new -4 blades? Been doing some flying lately in a newer BII and it autos like a dream compared to the older Beta with the old blades. Seemed a lot more forgiving, or maybe it was just me?

 

 

Yeah, its a dream all right, more like a nightmare ! Hard Yaw, coupled with that roller coaster feeling of dropping 1500 feet per minute or more as your rotor RPM slips away with even 1/4 of an inch too much collective ....is real forgiving !

 

Try a Bell 47 or an R44 and then the R22 wont seem so forgiving ! Seriously, I didnt really notice a difference in autos, and I flew the same bird before and after the new blades. The new rotor has a different sweet spot than the old, and a different forward speed at which you get minimum vibration..but the auto is still a roller coaster ride.

 

 

*** UPDATED**

 

Ok, perhaps I was being a bit harsh on the old 22 with my comments....It is a great machine, I just think it could be sooooo much better, and all the R&D are on other projects, seems they dont want to improve the 22 anymore. I thought the new blades would give Robinson a chance to solve some of the low inertia issues..if it helped any, I really didnt notice.

 

 

BTW Bristol- Great topic of discussion

 

Smooth full down auto's to all of you !!

 

Goldy

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Something to think about - why is there a maximum autorotation airspeed published in the RFM? If you exceed this airspeed, you will get DECREASED RRPM and greatly increased ROD. Flaring isn't always the best thing to do, either. What you want, in most cases, is an attitude that gives you minimum rate of descent, or maximum rate of climb, which is synonymous, for all practical purposes. Here, you'll get the best performance in autorotation. Get much below that airspeed, and you'll quickly get increased rate of descent, which can be difficult to stop. In practice, what you have to do depends on the situation - getting to the open area where you can land, whatever it takes. If you're doing practice autos to a prepared surface, then the best way to do it is to get to Vy and stay there until flare altitude, using the collective to keep the RPM in the green. I guarantee you that if you're at Vy, the RPM won't decay out of the green, in any helicopter, unless there is a serious mechanical malfunction, in which case you're in trouble anyway. Whatever model you're flying, know Vy by heart, and use it for takeoff, approach, and all sorts of things. If you're going to have an engine failure, the absolute best airspeed at which to have it is Vy, regardless of how many engines you have, and if you're not at Vy when it happens, you want to get there as quickly as possible.

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