Rotor Rpm Vs Vne

[Outwest]

Wondering if anyone out there with a background in Physics/Math can tell me what the relationship is between Rotor Rpm and VNE. Specifically on the S61, the VNE chart is based on 100% (203rpm) rotor speed, we routinely operate at 102%. What would the VNE increase by for every 1% of RPM. Thanks for any input.

[cyclic monkey]

The rotor diameter of a S61 is 62 feet, if this is multiplied by Pi i.e. 3.14159265358979323846, the circumference of the rotor disc is 194.77874452256718078452 feet.

If the tips are rotating at 203 RPM they will travel 39540.08513808113769925756 feet in one minute.

This equates to 449.31914929637656476429045454545 miles per hour tip speed.

 

Adding 2% to this tip speed would increase it by 8.9863829859275312952858090909091 miles per hour.

 

You may wish to reduce your VNE by the above amount if operating at 102%.

Unfortunately I have no "background in Physics/Math" as you requested, so these numbers are only approximate.

What I am is a helicopter pilot that is paid by the flight hour, so I suggest you slow down alot more than that !!!

Also, avoiding the VNE limit makes the flight alot more comfortable, and requires alot less thinking. Now where's my beer ??

[cyclic monkey]

It's OK, I found it !!

[Bladestrike]

All I know is going the other way is very bad. Excessively low RPM yields very low Vne...retreating blade stall and all that. Got to see it first hand in the sim as things got out of hand in very short order!

 

Its in the AFM limits and placarded in the 61 for varying Vne as to AUW and DA, but as there is no RPM notations (in the AFM), I would say they would hold for the entire normal range, 100% to 106%. I would imagine the worst case would be at the low end, 100% RPM, and thats whats charted. Anything beyond that, voila, you're a Sikorsky Test Pilot!

[Outwest]

Cyclic M......thanks for the info, you may not be a Math major but you are light years ahead of my cyphering. Following your numbers, would you not be able to INCREASE your vne. The retreating blade would be going that much faster no? Hence stall later? Great stuff, thanks again

[Winnie]

Wasn't there an accident somewhere with a 212, where they beeped down the NR a little, to have better economy, and after multiple other small items, ended up in the ####? ,

May not have my story straight, but I think there was something like that, but I can't remember where I read it...

[cyclic monkey]

Outwest, yes you may be right about the faster speed of the retreating blade; however the advancing blade is also going faster and will be getting CLOSER to it's design limits at speeds near VNE.

Because this subject is beyond my level of knowledge of aerodynamics, I just lower the collective a bit and leave a margin between myself and the VNE.

 

As for the 212 in flight.....I don't know of that event.......but for takeoff and landing the 212 rotor system hates anything below 100%. The older 205 systems didn't mind being beeped down a little, but don't try this with the 212.

 

Also remember that when you beep down the main rotor on any aircraft you are also lowering the speed of the tail rotor. The tail rotor is very small and relies on high rotation speeds to provide anti-torque. It is therefore more sensitive to losing a few RPM and you will encounter a tail rotor stall sooner (also known as Loss of Tail Rotor Effectiveness). This will, of course, be even more critical at higher altitudes and/or temperatures.

 

The limits in the flight manual are there for a reason and have been fully tested.

If you can't remember all the numbers, just look at the red and green areas on your gauges.

Keep it in the green, avoid the red, and never run out of Blue !!

[Winnie]

Actually I just recalled where I read it, it was in Flight Safety Foundation's Helicopter Safety, If I get the time later, I'll post a link directly to it, so you can all read it! :shock:

[Bellguy]

Good memory Winnie. Here's the link that you need

 

http://www.flightsafety.org/members/servem...s_sep-oct90.pdf

[Winnie]

Righton Bellguy!! :up:

 

To all, I just wondered, is the purpose of the N2 Beeper, just to reduce the RPM to prevent an overspeed, or is there anoterh "hidden function"?

Obviously it will reduce the Rotor RPM as well as the N2/Nf?

 

any takers?