Height/Velocity Curves

[Ian_J]

Thought this was interesting. Here is the R22 HV curve:

And here is the Enstrom F28 HV curve.

Notice the difference! Has to do with the low inertia rotor of the R22 vs. the high inertia system in the Enstrom.

And now for a laugh, check out the CH-47 HV diagram.

Now that's a lot of avoid area!!

 

[BobDDuck]

Just to clarify this for a guy who doesn't think it whirlybird terms, these "avoid" areas are places where your tail rotor is likely to strike the ground due to pitch? If so, why do the charts go up to 1000+ feet? Or am I missing the point entierly? And thinking about it more, as a helo accelerates doesn't it pitch nose down? This would negate the tail strike problem... although I guess you would have to worry about the main rotor at that point.

 

[RynoB]

Height/Velocity curves deal with the ability to auto rotate after an engine failure, tail rotor failure, etc. If operating in the shaded areas, the chances of making a successful auto rotation are very slim. If a pilot stays in the non shaded areas and encounters a problem, he/she should be able to auto rotate successfully. Auto rotating is all about managing the energy of the helicopter (altitude/speed) and the energy of the main rotor (rotor rpm). Hopefully one of the active rotor guys will explain it in more detail.

 

[scottyboy75]

Basically you want to stay away from high/ slow or low and fast. It allows you to land in the event of an engine failure. The guys who fly utility stay in those slow/ high curves all day. They are getting ready to build new lifts up at Snowbird and place the poles by helo I will try to get some video of it.
I did notice the Enstroms chart is at sea level. Is there a supplement for higher elevations? With it being hot here we are milking the Robbies all we can to get power out of them. Most of our takeoffs feel almost like running takeoffs. Got to love the heat.

 

[n57flyguy]

I don't get it

 

[Ian_J]

Alright flyguy... we'll make you a helo guy yet... now look at the chart. The numbers to the right are indicated airspeed. The numbers going up are altitude AGL. Got it? Good... now forget the chart for a second.

What is an autorotation? In the most simple terms, it is the equivilent of an airplane gliding with out power. But in order for a helicopter to glide, its wings (rotor blades) have to be spinning. How do you make them spin without engine power?? Air has to pass through them from below... and that part isn't difficult since you'll be going down towards the earth once your engine fails anyway.

Another thing to get those rotors spinning on their own is to remove all the pitch from the blades. Imagine the blades pitched (angled) into the wind... this creates lift when power is applied because each pitched blade is like a mini airplane wing in a climb attitude. But when no power is applied, imagine all that surface area trying to bite through the air. It's not going to happen. Eventually (quickly actually) the rotor blades will slow down to a point where they're incapable of lift and you will tumble from the sky. But at flat pitch, the blades meet the least amount of resistance and can spin freely.

So, once your engine fails and you successfully reduce the pitch of the rotor blades (by quickly reducing collective) your next step (besides looking for a place to land) is to get to your best glide airspeed (like an airplane!)

But helicopters can hover and fly verrrrry slowly, so unlike planes where you'd usually slow down to reach best glide, in a helicopter you might have to speed up. How? Only one way - pitch the nose down and rapidly trade altitude for airspeed. Now back to the charts. Look at the left side of each chart. Its telling you that going slow at low altitudes is hazaardous for your health. You won't be able to build up airspeed fast enough before you smack into the ground.

Okay, the right side of the chart is the fast and low chart (Or as I like to call it, the Army side.:bandit: ) This one is pretty simple: upon an engine failure you might not have enough reaction time to lower the collective and enter an autorotative profile before the helicopter hits the ground.

Whew! And that's my typing quota for the day! Hope that explains it better. That will be $35.

Disclaimer: For the helo guys I am just ignoring hovering autos with this explanation.

 

[Ian_J]

Basically you want to stay away from high/ slow or low and fast. It allows you to land in the event of an engine failure. The guys who fly utility stay in those slow/ high curves all day. They are getting ready to build new lifts up at Snowbird and place the poles by helo I will try to get some video of it.
I did notice the Enstroms chart is at sea level. Is there a supplement for higher elevations? With it being hot here we are milking the Robbies all we can to get power out of them. Most of our takeoffs feel almost like running takeoffs. Got to love the heat.

I'm sure there's more charts for the enstrom... that's just a sample.

 

[BobDDuck]

I might have guessed that it had to do with autos... Seems like just about everything does. It's sort of like VMC being the defining factor in twin operations. Thanks for the explaination. That said, some of those 180 auto rotations are crazy. I sat in the back of a BellJet Ranger (I think) and went through a few of them. They certainly make a short approach look normal. As far as the temp/density altitude thing. If I recall, the helo school that we shared a ramp with (Silverstate) grounded their fleet when the temps got about 41. I always thought it was just that their instructors didn't want to be out in the heat:insane:

 

[Ian_J]

In flight school we couldn't do autos when the DA was really high.

41 is pretty hot.

We actually had to get Boeing to ship us extra performance charts in Iraq because ours "only" went up to 50 degrees.

 

[n57flyguy]

Alright flyguy... we'll make you a helo guy yet... now look at the chart. The numbers to the right are indicated airspeed. The numbers going up are altitude AGL. Got it? Good... now forget the chart for a second.

What is an autorotation? In the most simple terms, it is the equivilent of an airplane gliding with out power. But in order for a helicopter to glide, its wings (rotor blades) have to be spinning. How do you make them spin without engine power?? Air has to pass through them from below... and that part isn't difficult since you'll be going down towards the earth once your engine fails anyway.

Another thing to get those rotors spinning on their own is to remove all the pitch from the blades. Imagine the blades pitched (angled) into the wind... this creates lift when power is applied because each pitched blade is like a mini airplane wing in a climb attitude. But when no power is applied, imagine all that surface area trying to bite through the air. It's not going to happen. Eventually (quickly actually) the rotor blades will slow down to a point where they're incapable of lift and you will tumble from the sky. But at flat pitch, the blades meet the least amount of resistance and can spin freely.

So, once your engine fails and you successfully reduce the pitch of the rotor blades (by quickly reducing collective) your next step (besides looking for a place to land) is to get to your best glide airspeed (like an airplane!)

But helicopters can hover and fly verrrrry slowly, so unlike planes where you'd usually slow down to reach best glide, in a helicopter you might have to speed up. How? Only one way - pitch the nose down and rapidly trade altitude for airspeed. Now back to the charts. Look at the left side of each chart. Its telling you that going slow at low altitudes is hazaardous for your health. You won't be able to build up airspeed fast enough before you smack into the ground.

Okay, the right side of the chart is the fast and low chart (Or as I like to call it, the Army side.:bandit: ) This one is pretty simple: upon an engine failure you might not have enough reaction time to lower the collective and enter an autorotative profile before the helicopter hits the ground.

Whew! And that's my typing quota for the day! Hope that explains it better. That will be $35.

Disclaimer: For the helo guys I am just ignoring hovering autos with this explanation.

Ohhh, thanks chinook. Seriously makes more sense. If you dop have engine faliure, how long do you have till the rotor blades stop moving and say you prayers?

 

[UH60driver]

Ohhh, thanks chinook. Seriously makes more sense. If you dop have engine faliure, how long do you have till the rotor blades stop moving and say you prayers?

Depends on the rotor system. Low inertia rotors systems drop in RPM faster than high inertia. However, if you get below a certain RPM, you might not be able to recover it. Ideally, you never want to let the RPM go below the minimum RPM for power-off operation.

Luis

 

[scottyboy75]

I believe the time to start an auto in the 22 is about 1.4 seconds. After that can we say sack-o-crap.

 

[Ian_J]

After a second engine failure in a Chinook it's quick... have only done it in the sim. All I can say is thank goodness for crashworthy seats!

 

[n57flyguy]

I believe the time to start an auto in the 22 is about 1.4 seconds. After that can we say sack-o-crap.

1.4 seconds, thats not enough!!! Thats crazy.
But I still want to fly helos:nana2:

Speaking of those seats chinook, what are they made of ?(off topic but I want to know)

 

[Ian_J]

I dunno... regular stuff I suppose. What makes them crash-worthy though, is that on a vertical G impact they telescope into themselves, and thus absorb the impact forces. Thankfully never had to test them in that capacity.

 

[UH60driver]

I dunno... regular stuff I suppose. What makes them crash-worthy though, is that on a vertical G impact they telescope into themselves, and thus absorb the impact forces. Thankfully never had to test them in that capacity.

Yeah, that's a lot of G's.

 

[RynoB]

I dunno... regular stuff I suppose. What makes them crash-worthy though, is that on a vertical G impact they telescope into themselves, and thus absorb the impact forces. Thankfully never had to test them in that capacity.

Come on, take one for the team. I want a full report on Monday.

 

[scottyboy75]

I hope to god I never have to test my emergency skills for real. I know the day is coming though. Down collective, Right pedal, Aft cyclic,. Eyes outside. Airspeed , RPM.Outside ,airspeed, RPM. Flare, level. Thats as far as I have ever taken an auto. NOw with the change in the PTS thats probably it.

 

[Ian_J]

Come on, take one for the team. I want a full report on Monday.

Well for you guys... anything. Hopefully my spine telescopes as well...

 

[Ian_J]

I hope to god I never have to test my emergency skills for real. I know the day is coming though. Down collective, Right pedal, Aft cyclic,. Eyes outside. Airspeed , RPM.Outside ,airspeed, RPM. Flare, level. Thats as far as I have ever taken an auto. NOw with the change in the PTS thats probably it.

I've had to shut down an engine a couple of times because of various problems... thank goodness for the other one.