best ROC and min ROD speeds

[lmlanphere]

is the best rate of climb speed always the same as the minimum rate of decent speed (autorotation), or is that just something I assumed because the two happen to be the same on a certain small piston engine helicopter I am most familiar with? and if they are always the same, please explain why!

[JoePilot]

The power-curve can be interpreted to show the rate of energy consumption required by a helicopter to produce as much lift as it weighs (not maintain height).
That energy can be supplied by burning any energy available to you; fuel, (Air)speed, height, or rotor RPM(very temporary).
For simplicity keep Airspeed and height constant. Leaving just height and fuel as energy sources.
Burn fuel more slowly than the curve requires and the surplus comes from height consumption. Burn fuel more quickly and the surplus is converted to height.
In a 'first order approximation' it is fair to assume that the curve does not change shape.

To neither climb nor descend you must be supplying fuel at a rate demanded by the curve - so that height is not required (as either a source or sink)

Another (really) simple way of looking at it is: a ROD is just a negative value of ROC. So in the case of descending you have a ROC which is negative. If the biggest ROC occurs at a given speed while you happen to be descending it is because it is the ROC is the least negative (min ROD).

'Second order'
The curve moves: and could be thought of as a family of curves for different weights, Densities etc. you'd have to plot a family of curves for different rates of height change (ROC: positive or negative) to see how min power (rate of energy) requirement changed with ROC. I don't think anyone bothers to do this... although it will certainly change ('shadow' of fuselage on disc in descent, changing Cd and Cl of fuselage for different Relative Air Flows etc)

OR
If you are always in debt then you are wealthiest when your overdraft is smallest.
Most weathy IS least poor!

Sorry about the gobbldey-gook...

[offshoreigor]

Imlanphere:

VBroc must be calculated for each AC you fly. For example, the S76 has a VBroc of 74 KIAS (at Sea Level). This is not to be confused with the best angle of climb speed of 45Kts.

As for Min ROD, in an auto, the 76 is about 70 KIAS. However, min ROD in an Auto may not be the best option as glide may be your preference. In this case, you would want to go with a combination of speed vs. Nr. In the case of an S76, 90 KIAS and 90% Nr.

I'm sure you will find many variations based on each aircrafts performance charts.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

[lmlanphere]

so in other words, the two speeds aren't necessarily the same - just a coincidence if they are?

[Multp]

Whilst often VBROC and VminROD may coincide, when an EOL is the aim at the bottom of descent flare effect and rotor inertia become important factors. Thus, for example, in the R22 whilst 53 KIAS may give you the min ROD, there is little flare effect: a 60 KIAS auto makes a world of difference, for only a slight change in ROD. On the other hand you get the best of both worlds in a B206...plenty of rotor inertia will enable you to descend at min ROD and have a low groundspeed touchdown well before the Nr goes off the bottom of the clock.
In every case RTFM is the best advice: both the manufacturer's Flight Manual and type conversion notes from your TRTO or flying school.

[CTD]

If the question is purely "are Best ROC and Min ROD speeds the same" then the answer is, in theory, yes. The speed at which the ratio between power required and power available is the greatest. Otherwise known as the 'bucket' speed, because it is found at the bottom of the curve which Joe Pilot referred to. Sometimes, that portion of the curve is shallow, almost flat, so the sweet spot may not be as easy to define.

Another speed which is derived from this curve is the 'best glide' offshoreigor talked about. This is the speed at which a tangent from zero through Vbroc reintersects the curve.

[lmlanphere]

one of the better books I've been reading says that the power required curve and the rate of decent curve for autorotation are two different things (although they do resemble one another). the posts above give conflicting answers to the original question, and I wonder if there might be more to add. if the two curves don't necessarily coincide, then I find it difficult to understand how speeds for best ROC and min ROD (auto) can always be the same, especially since the airflow during power on flight is very different from that of power off flight. I do understand that best range is a different subject....its the bottom of the curve(s) that I'm wondering about

[This message has been edited by lmlanphere (edited 07 February 2001).]

[CTD]

The curves manufacturers develop for Power Req'd vs. Power Available are done in level flight, not climb or descent. Yes, in reality there will be small differences in Vbroc and Min ROD, but they will vary from type to type and a blanket statement is impossible.

One of the things which in the real world will contribute to this difference is TAS vs. CAS vs. IAS. In a typical Bell light helicopter, in a climb IAS will be greater than CAS. In descent the opposite is true and IAS is less than CAS. Therefore, even if Vbroc and Min ROD are the same, the difference in airspeed information to the pilot will have a small effect.

We're talking about a couple of knots here, nothing of any real consequence. Differences between pilots would likely have as much or greater influence on it.