Power required & translational lift

[Isallobaric]

Here's a question that's puzzled me since my first transition.

All the curves I've seen for power required v. airspeed are smooth and continuous, with the only 'complication' being the difference between power required for HIGE and HOGE.

But, when I transition the R22, it feels as though something different is happening. The onset of translational lift feels almost like a step change - the aircraft shudders and then appears to need an inch or two less MAP to maintain height.

So, does the power required curve have a step or non-linearity around the TL airspeed, which isn't normally shown on the curves ?

Or is it just that at the onset of of TL the Power v. airspeed curve is steep, which gives the impression of a step change when coupled with the shuddering, yaw etc that occurs at TL ?

Comments, anyone ?

[IHL]

As you go through translation there is more airflow (volume) through the rotor system. More airflow through the rotor = more lift therefore ; to maintain the same performance a reduction in power ( reducing volume) is required to compensate for the increased volume induced through the system by the forward speed.

There:I think I've even confused myself.

[Flingwing207]

I think it's as you said, Isallobaric - (the second part). The ETL transition feels dramatic, what with the yaw and roll. The HPR curve is pretty steep at that point, but still progressive.

[Isallobaric]

IHL

That's pretty much what I've been taught, and read in the textbooks. I think my question comes down to how quickly the transition process occurs.

What you've described seems a fairly progressive introduction of transverse flow to the disk, reducing the inflow angle and therefore increasing the efficency of the system. And the smooth form of the power required curves would seem to reflect this.

So why then does it seem like a sudden transition ?

Flingwing

That does seems the most likely explanation. I've certainly noticed that the drama has reduced as my rotary handling skills have improved :)

[Gaseous]

Usually at this point in the flight the nose is low the pole is up and you are accelerating like a dragster. Most pilots I know seem to do this, (probably 'cos it looks cool). Try gentle acceleration and the transition is much less dramatic. As flingwing says, it is a steep curve which feels like a step if you do it fast.

[jayteeto]

IHL, careful with the wording... more airflow through the rotor is more induced flow which equals less lift. It is the horizontal component that is good. Technically that is less airflow through the disc.

[vorticey]

if you look closely at the power required / air speed diagram it has a lump at the spot your talking about, about 5 or 10 knots. before this power was increasing because your using some to move forward. but at this piont uncut air is starting to come into the front of the disc and producing heps of lift at the front(transverse flow/ inflow roll happent at this point which make it more dramatic). as you get faster more of the disc is in clean air, this happens fairly quickly.:ok:

[NickLappos]

Isallobaric,

Your question is a good one. I can assure you that we do not measure the performance at each knot or airspeed. The curves you see are derived from data taken at only a few points - steady zero wind hover, steady flight in the Vy "bucket," a few points at Vtoss and lots near best range and Vmax. Nobody takes careful performance quality data elsewhere, so the curve is probably just a pretty big smoothing of what you feel.

If we nailed the rotor down and controlled the flow around it, we might find a much sharper character in the power to thrust relationship. I will look at some wind tunnel data to see how correct you are. My money is on you, BTW.

OK, I found some wind tunnel data, and it shows a cliff at about 8 to 10 knots. Not a smooth curve!

Look at page 41 of this NASA TM:

http://ntrs.nasa.gov/archive/nasa/atrs.arc.nasa.gov/960974_shinoda/960974_shinoda.pdf

Here is a brief discussion and one plot from the report that I just posted:

http://www.s-92heliport.com/translation.htm

Way to go, Isallobaric (whatever THAT means!)

[Arm out the window]

Gut-feeling wise, I reckon that the shuddery step-like feel of transitioning from the hover to forward flight feels that way because the airflow pattern around the rotor is changing fairly abrubtly from one fairly stable flow pattern to another; ie in the hover, it's down through the disc in a kind of tubular flow, and in forward flight it's probably a bit more like the flow over a fixed-wing type aerofoil (obviously not exactly the same). I haven't seen wind-tunnel flow patterns to corrobborate this, but it makes sense to me that the disc would act a bit like a solid disc-shaped wing in forward flight.
Does that make any sense? Thought not!!

[Giovanni Cento Nove]

The shudder you feel when translating is a good explanation, but in reality how much of it is actually caused by flying through your downwash? Try translating IGE and then OGE - big difference. This also explains why the aircraft sometimes sinks a little as you translate - you are flying into the downside of the wake you are rolling in front of you. The Prouty Bibles have a good picture.

[thecloud]

A very interesting little area of flight! My last 'discussion' over coffee with the boys also mentioned the shedding of the tip vortices - hence the 'suddenness' of it, and the burble.

[IHL]

Jayteeto :
I don’t agree; if we express volume through the rotor as cubic metres per second (CMS), the more CMS through the rotor the more performance i.e. lift. This is true in aerodynamics, fluid dynamics, thermodynamics, anydynamics.

The more CMS through a jet engine = more thrust, the more CMS through a rotor system = more thrust.

With > 10-knot wind you will get the same performance as after passing translation on a calm day. With a strong headwind while hovering the increased flow through the rotor greatly increases performance. Compare the difference in power required for external load operations with no wind and with a 10-knot wind

Isallobaric:
The shudder through translation seems to be more prevalent in a
teetering rotor system and less noticeable in a fully articulated system. The rigid rotor system of the BKs is also quite harsh through translation.

[overpitched]

Look out. Here we go again!! :D :D :D

[Shawn Coyle]

The manifold pressure is just an indirect measure of the drag on the rotor blades - the power required to turn the blades.
The reason for the reduction in power required is that the large change in inflow as you go through translational lift decreases the drag, and hence the power required to turn the blades.
There is also the ground vortex being overrun that causes the large change in handling at about the same time.
Do the transition to forward flight from a higher height, and you'll notice less effect on handling.

[Head Turner]

This question should have been adequately covered by your instructor.
However, just to reiterate the points posted above

In a nil wind situation, in the hover the vector diagram will theoretically show that you have an amount of INDUCED FLOW which positions the RELATIVE AIR FLOW to produce an ANGLE OF ATTACK. This IF will be modified by GROUND EFFECT. Basically the closer the hover height is to the surface the more the GE modifies the IF. The greater the GE the lower the ROTOR DRAG.

Now as you make a forward cyclic imput, the disc is tilted, disc area reduces, helicopter sinks by a minute amount. A further forwrd cyclic imput accelerates the craft and the craft sinks again.
Collective will be applied to maintain height. This results in an increase in power and RD.
As airspeed increases less of the tunnel of vertically down flowing air will be sucked into the rotor disc area and more horizonal air will be touched by the front of the disc. This reduces the IF and the power requirement. INFLOW ROLL now occurs as TRANSLATIONAL LIFT is achieved.

This whole process happens at the rate of acceleration applied.

As described above, a very gentle acceleration will allow you to experience these effects more gradually.

You must surely have been shown and practiced downwind 'cushion creep' limited power take-offs.

This technique quite clearly demonstrates the take-off sequence.

[vorticey]

your right, more flow caused by the disc = more lift, but Jayteeto is talking about the restriction to the flow caused by flying into stationary air (undesending air).:ok:

[Isallobaric]

Thanks to everyone for their comments (especially those that addressed the question :D ), and especially Nick Lappos. I'll read up on those references when I've had some sleep.

(And for those that are wondering: http://www.merriam-webster.com/cgi-bin/dictionary?va=isallobaric)

[NickLappos]

Isallobaric,

Sorry, but I think you have it wrong - you READ those references to GET some sleep!!

Frankly, helping to answer your question taught me a bunch.. PPRUNE strikes again!