What is the speed of the tip of a rotor of a helicopter with a rotor width of 36 feet and a rotor spin rate of 383 rpm?

4/29/2011 7:58:50 PM

Blade Tip Speed = RPM X Pi X Rotor Diameter / 60

4/29/2011 8:01:42 PM

I got 491.850 mph, right, wrong?

4/29/2011 8:07:10 PM

It's been a long time since I've tried to do something like this.

4/29/2011 8:08:46 PM

The pilot claims 496 mph

4/29/2011 8:11:23 PM

Yeah its 491ish MPH.

4/29/2011 8:15:05 PM

Quote :

" Blade Tip Speed = RPM X Pi X Rotor Diameter / 60"

Id note that This will give you feet per second,

4/29/2011 8:19:25 PM

Nowadays, the only math I do is MPG and IV drip rates for drugs ie. 15 mcg/kg/min

4/29/2011 8:20:12 PM

Doesn't that strike anyone as a little low? This is for a BK 117. The newer EC 135's are much quieter but have less lifting capacity. I thought that the reason the UH-1's were so loud and distinctive was because the blade tips were supersonic. I wish i had figures for them and the UH-60

4/29/2011 8:24:58 PM

I don't like to memorize formulas. Just go to the basics of circles and speed.

Speed = Distance/Time

Basically, we need to find the distance covered by the tip in unit time.

Speed is usually given per second or per hour, so let's first find the speed in ft/s since the diameter is given in feet and per minute can be easily converted to per second.

383 RPM = 383/60 RPS

Each revolution covers a distance equal to the circumference of a circle with diameter 36 feet. And circumference of a circle is pi x diameter.

Distance covered in 1 revolution = 36pi feet

Distance covered in 383/60 revolutions = 36(383/60)pi feet

Since the above distance is covered in 1 sec, the speed:

Speed = 36(383/60)pi feet/sec ~ 721.94 feet/sec

Speed = 721.94(60)(60)/5280 mph ~ 492.23 mph

4/30/2011 5:29:22 AM

^

Quote :

"I don't like to memorize formulas. Just go to the basics"

Exposed?

4/30/2011 8:10:55 AM

Quote :

"Doesn't that strike anyone as a little low? This is for a BK 117. The newer EC 135's are much quieter but have less lifting capacity. I thought that the reason the UH-1's were so loud and distinctive was because the blade tips were supersonic. I wish i had figures for them and the UH-60"

While I don't know much about aerodynamics, I thought prop efficiency was more inefficient and conditions unstable if the rotor tip was in a supersonic condition?

Also though, I will note that the tip of the rotor itself doesn't have to be greater than the speed of sound in the local air in order for the rotor to exist in supersonic conditions. The shockwave can form before the rotor blades actually reach 750mph or whatever the air speed is.

4/30/2011 8:38:25 AM

Holy shit. I opened this thread and "Mathematics" by Little Boots came on my winamp lol

[Edited on April 30, 2011 at 3:24 PM. Reason : .]

4/30/2011 3:24:31 PM

Quote :

"[quote]I don't like to memorize formulas. Just go to the basics"

Exposed?[/quote]


I lol'd.

4/30/2011 3:32:04 PM

Quote :

"OMEGA CROSS R"

[Edited on May 1, 2011 at 2:36 PM. Reason : by the way, it's 984.5 mph]

5/1/2011 2:27:00 PM

That's double of my answer.

Please to explain why.

5/1/2011 7:41:58 PM

radius vs diameter disagreement

is 36' diameter or radius?

[Edited on May 1, 2011 at 7:48 PM. Reason : .]

5/1/2011 7:45:51 PM

yeah i figured that would be the cause...

to me 'rotor width' to me means diameter to me.

5/1/2011 7:52:23 PM

This is most definitely 36 ft diameter for a BK 117. Even the 53 only has a 79 ft diameter and that thing is MASSIVE with MAD LIFT compared to a BK 117.

5/1/2011 9:58:50 PM

bk117 is a 36' radius






[Edited on May 2, 2011 at 5:20 AM. Reason : google it if you must]

5/2/2011 5:19:27 AM

the whole aircraft is 42.65 ft long from nose to tail, but hey, the rotor is 72' wide

see that symbol beside "36.09 ft"? the circle with a line through it? that stands for diameter.

the dimensioning is somewhat misleading, but the symbol clarifies. if it's radius, then the rotor is way out of scale in the pic, but that isn't the case.

[Edited on May 2, 2011 at 6:35 AM. Reason : asdf]

5/2/2011 6:31:08 AM

What is the diameter of tilapia? Is it more or less than its body length?

5/2/2011 6:36:32 AM

Quote :

"the whole aircraft is 42.65 ft long from nose to tail, but hey, the rotor is 72' wide "

ouch

5/2/2011 9:51:11 AM

you're right

i was drunk

twice

beardawg got it 1st

5/2/2011 10:21:47 AM

that post came across a lot harsher than i intended it to. sorry broski.

5/2/2011 10:25:36 AM

The plane helicopter takes off

5/2/2011 10:26:46 AM

regarding speed and sonic boom:

how fast do they travel? because that speed you just found is with the helicopter being stationary.

5/2/2011 10:40:48 AM

Someone in the middle of taking a PY205 final?

5/2/2011 11:06:52 AM

Oh no worries I ain't that sensitive

I didn't even notice the diameter symbol, I thought that was long but that's what I get for trying to do math drunk

My 9th grade drafting teacher would slap the shit out of me

5/2/2011 4:54:45 PM

Tired of seeing people blame fucking up on an internet message board for being drunk.

You weren't drunk. You just fucked up.
















In the odd event that you actually were in fact drunk...the fuck are you doing on tdub in the lounge...not once but fucking twice...getting some basic math wrong?

5/2/2011 5:22:46 PM

math fight!

5/2/2011 6:05:07 PM

^^ to be fair, he didn't get the math wrong

5/2/2011 6:37:06 PM

Definitely was drunk. sorry you're mad about it


wud itttt mkae you feele bettr if i tpyed liek this

5/2/2011 7:14:40 PM

The "math" is the fucking length of the blade. He got it wrong.

5/2/2011 7:21:12 PM

i was the only one to point out that one side is moving faster than the other, so you are all wrong

5/2/2011 7:32:01 PM

Wikipedia says the never-exceed speed for a BK117 is approx 173mph, putting our max rotor tip speed at 492+173=665mph. Climb speed relative to rotor speed is insignificant, so I ignored that, and the average speed of sound at this particular helicopter's service ceiling (15,000ft) is 720mph, putting us at Mach 0.92 assuming the helicopter is at it's never-exceed speed at it's service ceiling.

According to another wiki article (Dissymmetry of Lift), it's detrimental to achieve supersonic speeds anywhere along the rotor, as it produces radically different airflow patterns and can result in a catastrophic failure/loss of control.

From the article:

Quote :

"...never-exceed speed. This speed is the speed beyond which the aerodynamic conditions at the rotor tips would enter unstable régimes - if v was sufficiently fast, the rotor tip at position A would be travelling fast enough through the air for the airflow to change radically as the rotor tip became supersonic, while the rotor tip at position B might have insufficient net linear speed through the air to generate meaningful lift (the stall condition - known as retreating blade stall. Needless to say, entry of the rotor tip into either of these aerodynamic régimes is catastrophic from the point of view of the pilot, and the maintenance of stable forward flight..."

Pretty neat read for anyone interested.

5/2/2011 8:19:57 PM

Yeah, iirc, you'll end up with some fucked up boundary layers from the shockwave.

5/3/2011 12:46:46 AM

yeah that's what I was saying earlier. And I believe you can start seeing the problems when the rotor blades are > .8 Mach.

5/3/2011 8:20:48 AM

what is the airspeed velocity of a well-laden swallow

5/3/2011 1:32:56 PM

african or european

5/3/2011 1:53:24 PM