NerdGuy Friday #26: The Fastest Helicopter in the World

Helicopter n.
1. Ten thousand parts rotating rapidly around an oil leak waiting for metal fatigue to set in. (https://aircraftmechanicshirts.com/products/funny-helicopter-definition-shirt )
2. Ten thousand parts that just happen to be flying in the same direction. (That was the popular helo t-shirt when I was flying airplanes in the 1980s. I like this one a little better even though it appears to be out of fashion.)
One thing that helicopters aren't is...FAST! (At least not until recently.)

Fast n.
1. Moving or able to move rapidly (per Merriam-Webster)

What is "Fast"?

Let's look at a few examples from the flying world. (We'll just look at "Cruise" speed. Cruise is the speed used during normal flight when going from here to there.)

• Very Basic helicopter (Robinson R22) - 96 knots / 110 mph
• Very Basic airplane (Cessna 172) - 122 knots / 140 mph (uh-oh, not looking good for the helos)
• Fastest military helicopter in the world (CH-47 Chinook [which will be featured in Miranda Chase #6, Chinook in March 2021]) - 160 knots / 180mph (Max: 174 knots / 200 mph)
• Fastest helicopter in the world [until 2019] (Westland Lynx) - 175 knots / 202 mph (Max: 216 knots / 249 mph [That's specially configured for record setting])
• Fast Basic Airplane - (The Mooney M20V is the fastest single engine propeller-driven, conventionally aspirated production plane) [what all that means is that it isn't a jet turbine engine with a propeller stuck on the front--a turboprop, and that it isn't a one-off. It's just a plane with a normal engine that Mike flies throughout the Miranda Chase series.] - 242 knots / 278 mph (That's beat all of the helos...until recently.)
• Basic Jet (Miranda's Citation M2 twin-jet [she gets it in Raider] - 404 knots / 465 mph
• Most airliners and bizjets fit here
• Miranda's 1958 F-86 Sabrejet - Max: 597 knots / 687 mph (Mach 1)
• Fighter jets and other supersonic craziness go here

The "New" Fast in Helicopters

Then along came the S-97 Raider.

• Cruise Speed: 220 knots / 250 mph
• Max Speed (called Vne - Velocity, never exceed--basically things start falling off...things you really need, like rotors): 240 knots / 280 mph! That's fast for a bunch of parts rotating around an oil leak!

So, how did they do this?

Let's start with the basics. Take a close look at the helo on the cover of my and Miranda's next book.

There are two very strange things going on here.

First, the coaxial (stacked) rotors. There's a lot of physics behind this, but the key point is that because there are two of them, the rotor blades are shorter for the same amount of lift. Why does this matter?

A lot of very strange things happen when you break the sound barrier. That's why what Chuck Yeager did in the Bell X-1 was so amazing. However, he had it easy in comparison to a helicopter rotor--all of his ten thousand parts were moving at the same speed.

Now imagine a rotor spinning fast enough that most of the blade is subsonic, but the tip is moving at supersonic speeds. That is when parts start falling off at inopportune moments as I mentioned above.

Then it gets even more exciting as the helicopter moves faster. The rotor on a Chinook helicopter is 60' across and is spinning 4 times/second. The tip of the rotor blade is moving at 514 mph. No problem.

EXCEPT: The forward motion of the helicopter moving at 200 mph plus the tip speed of 514 mph = 714 mph. That is getting very close to the speed of sound (761 mph at sea level). But the speed of sound drops to 731 mph at 10,000'. At its service ceiling of 20,000', the speed of sound is only 702 mph due to atmospheric thinning. Now things are getting very dicey out at the tip of the rotor.

HOWEVER: The retreating rotor blade is actually going backward at 514 mph tip speed - 200 mph speed of flight = 314 mph (400 mph slower than the advancing blade tip just 60' away)... Like I said, it gets complicated.

So, the coaxial rotor helps (because of the possibility of using shorter blades for the same amount of lift), but it doesn't solve everything. The Russian Mi-28 Havoc gunship helicopter is not some super-fast performer: (Cruise: 150 knots / 170 mph)

 

Why the Raider is Really Fast?

Notice the propeller stuck onto the tail. This 7' diameter rear-facing propeller drives the Raider ahead. The shorter coaxial rotors mean that the tips of the blades are: a) not turning so quickly and b) aren't plagued with retreating blade vs. advancing blade issues because each side had both a retreating and an advancing blade stacked, so it can go fast!

At least until it crashes, but for that, you'll have to read the book!