Military helicopters with high Vne

[PA Pilot]

A fixed-wing CFI friend posed the question how does a Cobra attain a Vne of 222 kt without running into retreating blade stall. Others include:

AH-64 197 kt

Ka-50 210 kt

UH-60 194 kt

 

The numbers are from Jane's.

 

The only thing I could guess is that they have lower rotor tip speeds than GA helicopters, but I don't have a clue as to what the side effects of that might be. Anybody know?

[Linc]

Lower flat plate drag combined with a high inertia rotor at a lower rotor rpm.

 

Just a guess.

[mechanic]

Hey,

The 64 is a little slower than the Cobra. Most Cobras are 170-190 kts max vne. While the 64 is 145-150 kts.

 

I remember reading about this somewhere???? I am thinking it is a specific airfoil design, blade twist, construction combination designed for a specific application, that yields a higher vne.

 

The F model Cobra has a 1,800 shp turbine, the G has a 1,100 shp, the J has twin 1,800 shp, and the W model has twin 1,600 shp range!

 

 

 

Carson Helicopters has a retrofit blade for a Sick Horskey S61 that increases load, cruise speed, and doubles the life limit over the factory Sick Horskey blades.

 

Have you ever researched airfoils? I am thinking there are thousands of them both good designs and not so good. I had seen a book on nothing but airfoils and it was thick with many cross views of airfoils. I found it so boring I never finished reading it.

 

Later

[BillyBob]

It's been a while but if memory serves:

 

AH-64A Vne standard day is ~180's, Vh with wing stores etc is ~140's

 

Vh is where the power available value meets the high speed end of the total drag curve. i.e. 100% Tq straight and level gives you Vh.

You can get a whole bunch more speed in a dive with power applied.

 

 

 

For the higher Vne's, my guess would be the swept blade tips / advanced designs and airfoil designs. Also, civil manufacturers may want to publish larger safety margins = lower Vne values on the charts. All guesses though, it's been awhile since I read about compressibility and RBS

 

R.W. Prouty probably has an unreadable article out there about this.

[iflytandemrotor]

222 for the Cobra!!!! That's crazy? Sounds like the Super Secret Squerrel Model was let out of the bag.

[mechanic]

BillyBob,

I know I have read or watched, anyways, I knew about diving attack speeds but I can't remember where I got the info from???? Makes me mad when I forget.

[iflytandemrotor]

The CH-47 Vne is about 170 knots. I can tell you that the only time I have ever come close to that was with a mega tail wind. (and that was at 1500' MSL)

[mechanic]

One of the best times in the USAF was when I was allowed to sit in the RH front seat of a CH47!

First best time was 6 hours of refueling F15/16's and F111's.

 

Later

[iflytandemrotor]

Mech,

 

Sounds like you had a blast. I have had the opportunity to take my family into the CH-47 Simulator. The kids loved it. My teenage stepson was able to hove no problem....damn kids and there video games. My teenage cousin was the same way!

[mechanic]

Is the Centurions CH-47's still around? Thats the group that let me tour their aircraft. They were in Italy.

 

Later

[iflytandemrotor]

I don't know for sure. The Army restructured all of their divisions so Army Aviation has changed a lot over the last couple of years. It closed some units and opened others.

 

Best bet would be to Google it.

[mechanic]

Here is a sample, like the book I had.

 

Airfoils-1550 types

 

I am thinking back to my RC plane days that the Clark Y airfoil was popular in RC airplanes.

 

Clark Y airfoil

[iflytandemrotor]

That's some serious egg-head stuff!

[Gomer Pylot]

To avoid retreating blade stall, you want higher Nr, not lower. More blades also helps. Having a tailwind or a headwind has absolutely no effect on airspeed. The helicopter has no idea what the wind is doing, if anything, when in forward flight. I've seen >200 knots groundspeed, but the airspeed was <150. Vne is for airspeed, not groundspeed.

[iflytandemrotor]

http://www.helis.com/howflies/maxspeed.php#4

 

 

Above is a pretty good link for some basics.

 

Gomer, I stand corrected. My ground speed was 186 knots on that flight.

 

IFLY

[Linc]

To avoid retreating blade stall, you want higher Nr, not lower...

I stand corrected. Thank you, sir. Lord knows I've heard enough stories about how the Cobra could get to RBS. Just came along a tad bit late in the game...