Test Time II

[RockyMountainPilot]

Here is some helicopter trivia for you.

 

Which of the following conditions are most conducive to experiencing VRS at the lowest rate of descent.

 

High gross weight

Low gross weight

High density altitude

Low density altitude

High disc loading

low disc loading

[kodoz]

Here is some helicopter trivia for you.

 

Which of the following conditions are most conducive to experiencing VRS at the lowest rate of descent.

 

High gross weight

Low gross weight

High density altitude

Low density altitude

High disc loading

low disc loading

 

Ahh...i'll take a shot at this since VRS has got to be coming up in ground school next week. Anything that increases induced flow should put you at higher risk for VRS at lower rates of descent. So, that would be high GW, high DA, and high disk loading. Why...none of these things affect the behavior of the vortices, it's just that the vortices are stronger (or more well-formed?) at high collective settings, right?

 

so back to the books or beer on the porch before that lesson?

[RockyMountainPilot]

Ahh...i'll take a shot at this since VRS has got to be coming up in ground school next week. Anything that increases induced flow should put you at higher risk for VRS at lower rates of descent. So, that would be high GW, high DA, and high disk loading. Why...none of these things affect the behavior of the vortices, it's just that the vortices are stronger (or more well-formed?) at high collective settings, right?

 

so back to the books or beer on the porch before that lesson?

 

Kodoz, I would say back to the books, but I am not sure you will get an answer from any published book. Let me give you a clue. Focus first on what causes VRS, then go from there.

 

Only one brave person willing to guess? Anyone else?

[Goldy]

Here is some helicopter trivia for you.

 

Which of the following conditions are most conducive to experiencing VRS at the lowest rate of descent.

 

High gross weight

Low gross weight

High density altitude

Low density altitude

High disc loading

low disc loading

 

 

Oh OK...I really had hoped that some student would jump in while this stuff is fresh in his mind and not rely on some old guy with too few synapses.

 

High Gross weight, High DA, and low disc loading is the worst scenario.

 

Why? Well, the first two are easy...they result in the greatest vortex creation. Disc loading directly effects the speed of the downwash, which is what you want to avoid. Heavy disc loading results in a "faster" downwash. so to get trapped in it requires a much higher sink rate...like 2000 FPM in a really big bird. A really light disc loading means the downwash is traveling slower...and you get stuck in it at 4 or 500 fpm descent rates...which is why they teach keep it at less than 300fpm/.....this is a pretty quick overview that Wagdentonk probably wrote 2 chapters on ! ( I couldnt find my book, or his correct name spelling!!)

 

Whew..I'm exhausted !

 

Fly safe.. Goldy

Edited July 11, 2007 by Goldy

[Goldy]

Kodoz, I would say back to the books, but I am not sure you will get an answer from any published book. Let me give you a clue. Focus first on what causes VRS, then go from there.

 

Only one brave person willing to guess? Anyone else?

 

 

I replied on the other related post...see there for details. High DA, High weight and low disc loading is what you are looking for.

 

Now, since I answered I can ask the next one.

 

Why does the R44 have less VRS tail rotor issues than the R 22?

 

Hint. It has everything to do with Bell Helicopter, and nothing to do with the size of the blade.

 

 

Good luck, Goldy

[joker]

Quick stab in the dark, but I'd go with all the 'lows'.

 

Quite simply, downwash velocity is lower, thus descent rate required to get VRS is lower.

 

Joker

 

P.S. Goldy, what you write is correct, but doesn't answer the question. School boy error!

 

"Which of the following conditions are most conducive to experiencing VRS at the lowest rate of descent?"

P.P.S. "4 or 500 fpm" - I think R22 is more like 700-900 fpm, based on downwash velocity of 1400fpm, which is based on disk loading of 2.75*210. Just a hunch.

Edited July 11, 2007 by joker

[C of G]

I think this may be a matter of reading the whole question. The last bit asks about VRS with the "least rate of decent" not the conditions that may be more conducive. So I would be looking at the less obvious, ie, if you're settling in your own down wash, what would match the two quickest? A high down wash velocity, or a low? Is it easier to get into VRS with a high pitch setting, or a low?

[RockyMountainPilot]

Quick stab in the dark, but I'd go with all the 'lows'.

 

Quite simply, downwash velocity is lower, thus descent rate required to get VRS is lower.

 

 

Joker is correct. The lower your DA, gross weight, and lower disc loading mean you have a lower velocity downwash. VRS is all about catching up with your downwash. So, the worse case scenario is one pilot, low on fuel, at sea level. An R22 with experience VRS at a rate of descent as low as 569 FPM with a GW of 1100 pounds. However, at gross weight and 10,000 feet, it is 739 FPM.

 

With one person in an R44, it will experience tail rotor VRS more than an R22 at gross weight. However, it is somewhat more efficient because of its design which I won't go into because that is what Goldy is questioning.

[Tenacious T]

An R22 with experience VRS at a rate of descent as low as 569 FPM with a GW of 1100 pounds. However, at gross weight and 10,000 feet, it is 739 FPM.

 

What is the formula for calculating that?

[kodoz]

Joker is correct. The lower your DA, gross weight, and lower disc loading mean you have a lower velocity downwash. VRS is all about catching up with your downwash. So, the worse case scenario is one pilot, low on fuel, at sea level. An R22 with experience VRS at a rate of descent as low as 569 FPM with a GW of 1100 pounds. However, at gross weight and 10,000 feet, it is 739 FPM.

 

Doh. I get it now. So even tho you are more likely to enter your downwash under conditions of low GW/DA/loading, are the effects the same? Under these conditions, you'll be using a lower pitch angle, which translates to weaker vortices.

thx,

--c