Autorotation capabilities accurate?

[TurboHog]

Hey,

 

I'm having some second thoughts about autorotations.

 

I have never been able to do a full down auto without sliding forward at the end. Putting it on the ground from a hover should be possible right? RPM loss seems to be far too rapid when not in forward flight. Someone here told this story about an instructor that claimed he could power up the heli, cut the throttle, lift off, rotate 360 degrees and land safely. If I raise the collective to max after cutting the throttle it wouldn't even let me do a bunny hop. I stay where I am. Something is not really accurate here or am I missing something?

[Zentaos]

That was a reference from the book "Chickenhawk" by Robert Mason, and that discussion already came to the conclusion that the US Vietnam era Huey's had metal blades, and the DCS huey is modeled from an early 80's version with composite blades. Composite blades are lighter and have less inertia due to lower mass, but produce better fuel economy. Anyway, during an auto, blade inertia from blade mass is not all you have to rely on. You need to learn to balance decent versus speed to keep rotor RPM up. As you descend the air moving up through the rotor will increase RPM if you are on the correct glide path. Too steep of a decent can actually over speed the rotor.

[TurboHog]

That was a reference from the book "Chickenhawk" by Robert Mason, and that discussion already came to the conclusion that the US Vietnam era Huey's had metal blades, and the DCS huey is modeled from an early 80's version with composite blades. Composite blades are lighter and have less inertia due to lower mass, but produce better fuel economy. Anyway, during an auto, blade inertia from blade mass is not all you have to rely on. You need to learn to balance decent versus speed to keep rotor RPM up. As you descend the air moving up through the rotor will increase RPM if you are on the correct glide path. Too steep of a decent can actually over speed the rotor.

 

So it is normal that the Huey is not capable of doing a full down auto with landing from hover? I don't think so... Or am I the limitation? Anyway I would like to see a track of someone doing this type of auto. No sliding, but landing from hover.

[lesnyborsuk]

I have never been able to do a full down auto without sliding forward at the end. Putting it on the ground from a hover should be possible right?

 

What is your altitude when you start?

Are you keeping it out of dead man's curve in Height-Velocity Diagram ? From manual it seems that it is between 50ft to 600ft for Huey.

[26-J39]

What is your altitude when you start?

 

Good Question.

 

Generally I find auto-rotation fairly easy in the huey provided sufficient airspeed or altitude. Full "full-downs' are really hard, a little too hard imo aswell. In the track attached I tried 3 attempts, the first 2 I managed to successfully auto wo damage but nothing im happy with, the 3rd was a fairly low alt attempt :P

 

Food for thought

0auto.trk

[Sven2157]

...

If I raise the collective to max after cutting the throttle it wouldn't even let me do a bunny hop. I stay where I am. Something is not really accurate here or am I missing something?

If you raise the collective to 'full max' with the engines running full, you will lose power too. Watch your engine RPM gauge, and listen to the engines when you do so. They bog down; and they will even lower to the point of audible 'Low RPM' warning sounds. Keep in mid that the collective adjusts the blade pitch while collectively controlling the engine speed. You want to keep the 67/320( i.e. 6700 RPM on the main rotor, and 320 RPM on the tail rotor ).

 

Auto-rotation is not an easy skill to master; not by far! The key to performing what you are asking is: TIMING!

 

Think of this as the ramp used by skateboarders( the half-pipe ). There is a point in your descent that will be too shallow, resulting in too slow rotor rpm, with little forward speed. Consequently, there will a point in your descent where it is too great, and you will have really high RPM, with too much forward speed.

 

To start - your best approach is to try to get a 45°-50° angle from your current height to the ground, between 20-40knts; maybe even a little steeper. You will have to practice, but will eventually find that 'sweet spot'.

 

The really trick to Auto-rotation, is when to precisely add the collective. If you add it to early, you will lose the energy to maintain flight. So, if you have a high forward speed, leave the collective down, and use that forward momentum to stop descent while also slowing the aircraft to a hover - pulling back on the cyclic. As the helicopter starts to want to drop, apply your collective - BUT REMEMBER! You only have a couple seconds to set it down before the blades run out of inertia!

 

Open your Mission Editor, start a new map, click the little helicopter icon on the left. On the right there is a drop down, select the UH-1H( should be yellow ), under that is the 'Skill' dropdown, select 'Player' - also should be yellow. Now click on the map several feet from the end of a runway at any airport. Again on the right, you will find a box where you can adjust speed and height. Set your height( alt ) to say 1500 ft MSL. Save and load that flight. You can then just keep starting in a ready to die - I mean auto-rotate position.

 

Practice - Practice - Practice!

 

Good luck! ;)

[TurboHog]

 

Practice - Practice - Practice!

 

 

I can perform normal auto's all day now without losing parts. It's just the sliding when I settle it down that I cannot prevent from happening. If I aim for a full stop a few feet above the ground to settle it down in a hover (which should be possible) I need more collective than I have available to make it somewhat soft...

[Zentaos]

The first priority in an automation is to get the ship to the ground at a survivable speed. The second priority is to cause the least amount off damage to the ship as possible. If you don't have enough rotor rpm to hover, set it down. Otherwise, keep practicing until you can get the correct rate of decent to have enough rpm left to set it down from hover.

 

 

 

Also, payload & weight need to be taken into account for auto's. I would not try to hover in an auto near gross weight.

Edited July 2, 2013 by Zentaos

[Hueyman]

... You want to keep the 67/320( i.e. 6700 RPM on the main rotor, and 320 RPM on the tail rotor ).

 

 

Dude, I can assure you with that, you won't go much further ;-)

 

No worries, I'm sure you would say 320 MR and 6700 TR ... but this is still problematic ...

 

I think the correct RPMs are 324 for the Main Rotor, and considering the 1:5,5 gear ratio between MR and TR, 1782 RPM for the Tail Rotor

 

Cheers,

Hueyman

[HuggyBear]

6600 is ideal N2 and yep 324 is ideal NR.

 

There's no gauge for the tail rotor so don't worry about it. :)

 

- Bear

[TurboHog]

It still feels as if RPM is decreasing too fast. I follow the official recommendations from the official UH-1H / UH-1V manual. Tried both optimal glide ratio (80 kts IAS) and optimal vertical speed (50 kts IAS). Optimal rotor RPM for both is 314 RPM. I start leveling at 200ft and start adding collective at 50ft. All by the book and the result is a perfect landing with some forward sliding. If I try to achieve a stop in mid-air (few ft above the ground) the helicopter goes brick mode as soon as I try to settle it down from a hover. Even my RC heli's with carbon fiber blades have more reserves...

[Sven2157]

Correct me if I am wrong here, but I remember my old CAP Captain saying something about the ability for the pilot to 'cut' the blades for auto-rotation. Meaning that there is some way for the pilot to disengage the rotor from the rotor shaft, effectively making the main rotor free-spinning.

 

Is this actually possible, and if so is it modeled? My thought on it is that the main rotor is still receiving friction from the main rotor shaft, and causing it to slow too quickly ... I will dig the net and see what I can find on that( I know Belsimtek has properly researched this, so please don't think I am questioning that ). :book:

 

...

Even my RC heli's with carbon fiber blades have more reserves...

Awesome, another six-stick! I thought about this too, but quickly brushed it off with the whole difference in mass 'thing'. Though, there may still be a valid argument there.

 

It could just boil down to, Belsimtek still 'tweaking' the flight model, perhaps? :huh:

[Zentaos]

viscous centrifugal clutch?

[HuggyBear]

There's no 'clutch' to disengage the rotors from the mast or main transmission.

 

There is a sprague (freewheeling) clutch which allow the main transmission disengage from the engine and allow for autorotation if the engine fails.

 

The auto and rate of blade deceleration in DCS is pretty close to the real thing. RC aircraft behaviour has little relevance to the full size aircraft as an RC doesn't simply have a scaled down version of the transmission/engine/drivetrain and the associated forces and frictions involved.

 

There are several variations of autorotation technique, a zero/zero touchdown is the hardest but certainly safe and achievable in the H.

 

After your decelerative flare, apply a sharp pull on the collective (only about 2 inches on the real collective) to 'pause' the descent and quickly level the aircraft with cyclic, then use the remaining collective to cushion to the ground. If you are aiming for zero/zero you won't have much more than a single chance to cushion without the blades slowing to a dangerous level.

 

For something different which may help you become used to the collective response, try a 'constant attitude' auto, maintain 40 kias all the way down and then cushion with collective in the last 10 feet. Maintain the same attitude all the way down.

 

If you keep some forward airspeed then the whole process becomes a lot easier/slower, but a zero/zero is a very important tool to have if you don't have the available space for a run-on.

 

...and remember, whatever gets you on the ground safely is a good option. :)

 

- Bear

[JoeDiamond]

In the latest version the sprague clutch does not seem to disengage, at least not the RPM indication anyway.

 

See this thread:

http://forums.eagle.ru/showthread.php?t=109497

 

The autorotation seems normal so I'm guessing it's a RPM indication problem.

[HuggyBear]

Thanks JD, that bug was submitted previously and the devs are working through it.

 

The N2 should fall away to somewhere around 5500.

 

An N2/NR split is the clearest indication that the aircraft is in autorotation and the rotors are being driven by RoD airflow and not the N2.

 

- Bear

[BSS_Sniper]

Hey,

 

I'm having some second thoughts about autorotations.

 

I have never been able to do a full down auto without sliding forward at the end. Putting it on the ground from a hover should be possible right? RPM loss seems to be far too rapid when not in forward flight. Someone here told this story about an instructor that claimed he could power up the heli, cut the throttle, lift off, rotate 360 degrees and land safely. If I raise the collective to max after cutting the throttle it wouldn't even let me do a bunny hop. I stay where I am. Something is not really accurate here or am I missing something?

 

I'm a professional commercial pilot now that I'm out of the army. I fly fixed wing but according to my friends that fly helos in the army you need some forward speed to autorotate correctly. You can't do it from a hover.

[JoeDiamond]

I'm a professional commercial pilot now that I'm out of the army. I fly fixed wing but according to my friends that fly helos in the army you need some forward speed to autorotate correctly. You can't do it from a hover.

 

You can do it from a hover close to the ground, where the rotor has enough inertia to land safely. You can do it from a hover at a high enough altitude where you can convert your altitude into forward airspeed. You can't do it from a hover in between those extremes. That is where the height/velocity chart comes into play. Flying in the prohibited areas of the H/V chart puts you in a position where a successful autorotation is unlikely.

[Zentaos]

The sprague clutch then, is why the rotors don't start spinning until the turbine hits a high enough rpm to engage the clutch?

[Bushmanni]

Rotor and engine have only an aerodynamic coupling, ie. engine exhaust turns a turbine that then turns the rotor. You need a certain amount of "wind" from exhaust for the turbine to generate enough torque to overcome friction in the drivetrain.

[Zentaos]

There is an output shaft on a turbshaft (ie: helicopter turbine). The gas exhaust goes out the tailpipe. Bear was explaining the clutch operation. I thought it was centrifugal, like a weed wacker, but this must not be how it works.

[HuggyBear]

There's both kinds of clutch/link in play.

 

The 'aerodynamic' link refers to the fact there is no direct mechanical link between the N1 (Gas Producing) and N2 (Power Turbine) parts of the engine, aside from bearings and supporting structure of course.

 

The N1 part of the engine works to provide combustion (draws in air, pressurises it, adds fuel, etc). This combustion is then exhausted, passing through both N1 and N2 turbine blades on its way out.

 

The force transmitted by exhaust passing through, and driving, the N2 turbine is the 'aerodynamic link' which occurs entirely within the engine.

 

The N2 drive is then passed out the front of the engine, through a drive shaft, into the main transmission. The connection between engine drive shaft and main transmission is via the sprag clutch, which was mentioned before.

 

Here's a video which is boring at the beginning, but has a great animation showing the action of a sprag clutch. Skip to 0:45.

 

Fsp3fm4KHs0

 

- Bear

[Zentaos]

Very insightful, thanks bear

[Flanker52]

I've just started messing with autorotations.

 

Once throttle is totally cut - should I immediately push collective 100 percent down? Or should I just decrease collective but not all the way?

[sondo214]

Anyway I would like to see a track of someone doing this type of auto. No sliding, but landing from hover.

 

I have uploaded many videos on youtube on spotlanding autorotations. It is possible (thought not easy)

 

 

 

 

 

 

Edited August 29, 2014 by sondo214