Aero Nerdery Thread (rotary-curious welcome)

[Gatordev]

I thought there was another aero thread, but I can't find it. And because I am so fucking tired of dealing with former or current Army IPs who don't seem to understand what they're teaching, I'm hoping the TPS and Aero-smart people can help...

I plan to peruse Aerodynamics for Naval Aviators this week while I'm at work, but I'm thinking that may be a bit too technical for what I'm trying to figure out. This video is a great break down of Vspeeds for helicopters (it's 10 minutes, but you can skip through it), but he's using a chart that I haven't really dealt with (or at least, not what he's calling it). No big deal, as it's clear it's a power chart, mirrored and then rotated 90 degrees.

I'm completely comfortable with what Vy and Vx (or Vtoss if you sprechen EuroDeutch) are practically. What I'm trying to understand is the math and why is Vx not at the bucket and why is Vy not slightly "higher" than the bucket on his chart. Yeah, it's a crude drawing, so it could just be the way it's laid out, but on the Ps diagrams, Ps max was always at the bucket (if I'm remembering correctly). And you never really had to figure out where Vy was, so other than knowing it's "more" than Vx, you didn't have to do anything else.

I may not be asking the question clearly enough, so I can elaborate as needed, but at the end of the day, I'm trying to figure out an easy way to explain Vy to someone that seems to think "best rate" means the highest VSI, which isn't the case. And as a result, what they're teaching is going to put people into trees if someone loses an engine.

 

[ChuckMK23]

A good starting point!

The Power Struggle

The helicopter does not care what kind of engine powers it, any more than the engine cares what it supplies power to.

digitaledition.rotorandwing.com

 

[ChuckMK23]

Another good discussion reference to the work of Ray Prouty, the godfather of Helo aerodynamics

https://aviation.stackexchange.com/questions/80860/what-forward-airspeed-produces-maximum-rate-of-climb-in-helicopters

 

[ChuckMK23]

 

[JTS11]

Get 3 engines. Bucket airspeed is around 60 kts. Max range is around 125 kts. No need to nerd out on parasitic drag. or retreating blade stall..just get 3 engines. 😁

 

[Griz882]

Aren’t the aerodynamic (mathematical) fundamentals basically the same wherein Lift = 0.5 * ρ * V^2 * CL * A; when "L" represents lift force, "ρ" is air density, "V" is the airspeed (Vy), "CL" is the coefficient of lift, and "A" is the rotor blade area.

It’s all that rotor stuff that confuses me.

 

[JTS11]

Aren’t the aerodynamic (mathematical) fundamentals basically the same wherein Lift = 0.5 * ρ * V^2 * CL * A; when "L" represents lift force, "ρ" is air density, "V" is the airspeed (Vy), "CL" is the coefficient of lift, and "A" is the rotor blade area.

It’s all that rotor stuff that confuses me.

It's easier stated as PFM, where the P=Pure and the M=Magic. 😄

 

[hscs]

Calling @IKE

 

[Griz882]

Calling @IKE

You’re asking for full nerd….you never go full nerd.

 

[IKE]

Vy is bucket*. The greatest rate of climb is found with the greatest margin between power available and power required.

* Okay, so the helicopter has an induced flow vector that doesn't point straight down (thrust vector points up and slightly forward when in forward flight), but it's pretty much bucket airspeed, because our bucket is wide (small change in power req'd across 10-20ish knots).

For Vx, I'm not sure he drew that line correctly. He started from max torque required. I think it might be from max torque available, but I'll have to ask one of our real helo whisperers or go back to my notes. IF you have power to HOGE, then clearly best climb angle is 90° at 0 KIAS. If you're high/hot/heavy, you may not have the power to go straight up, so Vx would be non-zero, but no faster than bucket.

/end nerdery

 

[Random8145]

I thought there was another aero thread, but I can't find it. And because I am so fucking tired of dealing with former or current Army IPs who don't seem to understand what they're teaching, I'm hoping the TPS and Aero-smart people can help...

Curious, but why do the Army IPs (I am guessing that's Instructor Pilots) not know what they're teaching? I mean the Army flies loads of helicopters...?

 

[IKE]

Curious, but why do the Army IPs (I am guessing that's Instructor Pilots) not know what they're teaching? I mean the Army flies loads of helicopters...?

There's a big difference between "pull collective = go up" and understanding just how crazy helicopter aerodynamics are or how performance charts are made. TPS mostly taught me I don't understand it and to go ask someone who does.

I remember HT aero circa 2007 was pretty terrible. IIRC, they let some random O-4 with a BSAE rewrite the aero book, and it was a combination of (1) dumbing down the science to the point of being misleading, and (2) just being wrong.

 

[JTS11]

There's a big difference between "pull collective = go up" and understanding just how crazy helicopter aerodynamics are or how performance charts are made. TPS mostly taught me I don't understand it and to go ask someone who does.

I remember HT aero circa 2007 was pretty terrible. IIRC, they let some random O-4 with a BSAE rewrite the aero book, and it was a combination of (1) dumbing down the science to the point of being misleading, and (2) just being wrong.

One thing that comes to mind is Vortex Ring State. The mindless caveat of < 40 kts and greater than 800 fpm rate of descent was regurgitated across every helo's NATOPS. From my understanding it was based on a single engine Huey from way back when.

Actual TPS dudes published charts that showed those numbers were not even close for a 53-E...but alas it never translated into our NATOPS while I was in.

 

[Gatordev]

Guys, remember, I need to Army it down to something digestable.

For Vx, I'm not sure he drew that line correctly. He started from max torque required. I think it might be from max torque available, but I'll have to ask one of our real helo whisperers or go back to my notes.

I can accept his diagram has errors since he's free-drawing it, so taking that into account...I had a similar reaction to you...he's got Vx and Vy at two different power settings, which shouldn't matter, which is why the video isn't quite as "simple" as I'd like.

Vy is bucket*. The greatest rate of climb is found with the greatest margin between power available and power required.

* Okay, so the helicopter has an induced flow vector that doesn't point straight down (thrust vector points up and slightly forward when in forward flight), but it's pretty much bucket airspeed, because our bucket is wide (small change in power req'd across 10-20ish knots).

Vy =/= Vx. The point on a power chart may be the same for certain aircraft, but rate/time is very different than rate/distance.

Now, for the -60, I think you're right, they were pretty much the same. But not for other helos. For the -135, Vtoss is 40. When you put your thinking hat on* (and actually look a the VSI), Vtoss has to be Vx. Vy in the -135 is 65 knots, and the VSI concurs that it's a slower rate/time, but better rate/distance.

Or to put it another way, there's times (like daily in the summer) when I can't climb and clear an obstacle at Vy, but I can at Vtoss (which matches what Vx would be).

IF you have power to HOGE, then clearly best climb angle is 90° at 0 KIAS. If you're high/hot/heavy, you may not have the power to go straight up, so Vx would be non-zero, but no faster than bucket.

And this is the corollary to what I just said. Regardless of high/hot/heavy, You'll eventually stop climbing in a HOGE, but will still climb at Vx. There will be some margin between Vx and Vy (depending on airframe) where you also won't be able to climb at Vy but can climb at Vx (if they're not the same). And this happens regularly in the -135 during OEI with a weak engine.

My takeaway from the video I posted is to maybe approach it with that baseline discussion point of rate/time vs rate/distance.

*Thinking hat quip was directed at the IPs, not you, IKE.

 

[hscs]

There's a big difference between "pull collective = go up" and understanding just how crazy helicopter aerodynamics are or how performance charts are made. TPS mostly taught me I don't understand it and to go ask someone who does.

I remember HT aero circa 2007 was pretty terrible. IIRC, they let some random O-4 with a BSAE rewrite the aero book, and it was a combination of (1) dumbing down the science to the point of being misleading, and (2) just being wrong.

Didn’t the H60 Chapter 11 get rewritten around the same time by the same guy?