Retreating blade AoA.

Aviator168

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In general cruise, what is the AoA of the retreat blade? At what RRPM or fly speed will I see the retreating blade stall?
 
The AOA of the retreating blade varies across the span of the blade. The inmost foot or two is stalled most of the time. This stalled region spreads outward from the root toward the tip as the aircraft's airspeed increases. A sudden, full stall of the entire blade in steady flight is unknown.

A full-blade stall can occur if (1) sufficient RRPM is lost during a low-G maneuver, followed by a sharp increase in disk AOA; or (2) if a violent nose-down rotation of the airframe, with spindle fixed, occurs, suddenly increasing the retreating blade's AOA.

What gyro pilots call "ground flapping" is a special case of Scenario #1 above, except that the low RRPM is not a result of a low-G maneuver, but instead results from insufficient pre-rotation during the takeoff sequence.
 
For example, the A.o.A calculated for an ELA 07 gyroplane at 75 mph, 950 lbs. The red area is stalled (more 12 degrees)
Sans titre.png
 
Of course ! These results obtains the autorotation steady rpm, with the disk A.o.A required. Here 375 rpm, with disk A.o.A: 8.8 degrees and a1: 2.2 degrees
 
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Of course ! These results obtains the autorotation steady rpm, with the disk A.o.A required. Here 375 rpm, with disk A.o.A: 8.8 degrees and a1: 2.2 degrees
Looking at the chart. If the blade pitch (static) is lower, it can reduce the stall region of the retrieving blade? Also has delay stall devices (Vortex Generators) been put on blades?
 
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Using the collective trim control fitted to some A&S18As, you can slightly reduce collective pitch, (which also leads to an increase in rotor rpm) and thus delay the onset of retreating blade stall to higher speeds. It's useful when flying fast and lightly loaded where the rpm is at the low end of the desired range and the ride gets rough with increasing speed.
 
Interesting information... It would be also interesting to know the formula for the instantaneous angle of attack of any blade section, as a function of rotor revs, horizontal & vertical airspeeds, disk tilt, blade azimuth and section distance to the rotor axis... Sure JC could tell us...
 
Looking at the chart. If the blade pitch (static) is lower, it can reduce the stall region of the retrieving blade? Also has delay stall devices (Vortex Generators) been put on blades?
As said Waspair, lower pitch setting reduces the stalled area and increases the rrpm. However, it also increases rotor drag.

In my opinion, the blades have a cyclic oblique attack that would interfere with the operation of vortex generators.
 
Using the collective trim control fitted to some A&S18As, you can slightly reduce collective pitch, (which also leads to an increase in rotor rpm) and thus delay the onset of retreating blade stall to higher speeds. It's useful when flying fast and lightly loaded where the rpm is at the low end of the desired range and the ride gets rough with increasing speed.
There is a limit of how much collective pitch can be reduced.
Just curious, is this done with a trim on a A&S 18A?
 
There is a limit of how much collective pitch can be reduced.
Just curious, is this done with a trim on a A&S 18A?
The original manufacturer's design did not include it, but an STC was developed by Don Farrington to add it. It used a small electric motor, activated by a miniature switch on top of the cyclic stick, to drive the swash plate up and down (response with fine gearing was too slow to use it for a collective flare at touchdown unless your timing was absolutely perfect). You could easily delay retreating blade stall onset by about 10 knots, at the cost of a big reduction in the available climb rate at full fine-pitch/high rrpm travel, but it was very helpful at low load and high speed.
 
Quote: It would be also interesting to know the formula for the instantaneous angle of attack of any blade section /Quote

It is probably not helpful to calculate the angle of attack near the blade root. Here a reverse flow region occurs on the retreating side, but since it is close to the blade root it can safely be neglected (as can be seen in Jean-Claude's example here https://www.rotaryforum.com/threads/retreating-blade-aoa.1145366/post-1153273

According to naca-716, page 12, the angle of attack for any position psi around the disk is:

alfR = thtaN + thta1*uT - thta1*mu*sin(psi) + b1*cos(psi) - a1*sin(psi) + 2*b2*cos(2*psi)...
- 2*a2*sin(2*psi) + 1/uT*(lamNf + mu*a1 +(-mu*aN +1.5*mu*a2)*cos(psi)...
+ 1.5*mu*b2*sin(psi) - 0.5*mu*a2*cos(3*psi) - 0.5*mu*b2*sin(3*psi));

To evaluate this formula you obviously need the inflow ratio lamNf, from which you can calculate the first and second harmonic flapping angles aN (coning angle) a1, a2, b1, b2. The operating state of any rotor in auto rotation depends on just two parameters: advance ratio mu and collective pitch angle theta.

Formulae #9 and #11 from naca-716 give the de- and accelerating torque. Equating the two ( on page 9 of 716) you can solve for the inflow ratio lamda and get a second order equation in lamda which can easily be solved by the well known p-q formula. This is done in clcRotState2 of the package (see below)

with inflow ratio known the above formula can be evaluated around the disk for different angles psi. For the example case the results are given below. I have cobbled together the whole thing in about an hour, the only sanity check is, that the maximum angle of attack is at 305° (talking of retreating blade stall).

You can download the whole package, containing the octave m-files and the two reports naca-716 and naca-591, from here:
https://www.magentacloud.de/lnk/xLBJEBG0

password is

alfamax

all lower case


You need octave to run it.
https://www.gnu.org/software/octave/download

results for a Bensen style gyro are:

n= 459.86 [1/min]
alfaNfD = 9.7945
1 alfR 2.72° 0.00
2 alfR 2.49° 5.00
3 alfR 2.25° 10.00
4 alfR 2.01° 15.00
5 alfR 1.78° 20.00
6 alfR 1.54° 25.00
7 alfR 1.31° 30.00
8 alfR 1.09° 35.00
9 alfR 0.87° 40.00
10 alfR 0.67° 45.00
11 alfR 0.48° 50.00
12 alfR 0.31° 55.00
13 alfR 0.15° 60.00
14 alfR 0.00° 65.00
15 alfR -0.12° 70.00
16 alfR -0.23° 75.00
17 alfR -0.33° 80.00
18 alfR -0.40° 85.00
19 alfR -0.46° 90.00
20 alfR -0.51° 95.00
21 alfR -0.54° 100.00
22 alfR -0.56° 105.00
23 alfR -0.56° 110.00
24 alfR -0.55° 115.00
25 alfR -0.53° 120.00
26 alfR -0.50° 125.00
27 alfR -0.46° 130.00
28 alfR -0.41° 135.00
29 alfR -0.35° 140.00
30 alfR -0.28° 145.00
31 alfR -0.21° 150.00
32 alfR -0.12° 155.00
33 alfR -0.03° 160.00
34 alfR 0.07° 165.00
35 alfR 0.18° 170.00
36 alfR 0.30° 175.00
37 alfR 0.42° 180.00
38 alfR 0.56° 185.00
39 alfR 0.70° 190.00
40 alfR 0.85° 195.00
41 alfR 1.01° 200.00
42 alfR 1.18° 205.00
43 alfR 1.36° 210.00
44 alfR 1.54° 215.00
45 alfR 1.73° 220.00
46 alfR 1.93° 225.00
47 alfR 2.13° 230.00
48 alfR 2.33° 235.00
49 alfR 2.53° 240.00
50 alfR 2.73° 245.00
51 alfR 2.92° 250.00
52 alfR 3.11° 255.00
53 alfR 3.29° 260.00
54 alfR 3.46° 265.00
55 alfR 3.62° 270.00
56 alfR 3.76° 275.00
57 alfR 3.88° 280.00
58 alfR 3.98° 285.00
59 alfR 4.06° 290.00
60 alfR 4.12° 295.00
61 alfR 4.15° 300.00
62 alfR 4.16° 305.00
63 alfR 4.14° 310.00
64 alfR 4.10° 315.00
65 alfR 4.03° 320.00
66 alfR 3.94° 325.00
67 alfR 3.82° 330.00
68 alfR 3.68° 335.00
69 alfR 3.52° 340.00
70 alfR 3.34° 345.00
71 alfR 3.15° 350.00
72 alfR 2.94° 355.00
 
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They say the best way to learn is to try and teach others, well, the first thing I learned is that you shouldn't program rotary wing stuff late at night, the result may be rubbish...;-(
The part calculating inflow ratio, disk angle and rotor speed is ok, but it seems I misinterpreted uT in naca-716. In version MkI of my program that one is not dimensionless and so the formula for alfaR is obviously wrong. Rereading the report my current understanding is that uT is a ratio that basically allows to investigate the angle of attack along the blade, with uT=0.3 being the value at the 30% radius station and uT=1.0 at the rotor tip. I have revised the program accordingly, the MkII version is available here:

https://www.magentacloud.de/lnk/PnhJkQoZ

password is again

alfamax.

Quote:
I am wondering how much of the rotor drag is due to simply increase in rrpm and how much is induced by getting to a higher rrpm.
/Quote

I don't understand the question, what is the difference between "increase in rrpm" and "higher rrpm"?



======================
Sample program output:
======================
I have added the formula for alfaRmax given for the case of infinite Lock number.

mu = 0.15 R= 3.05 sigma= 0.0313
V = 84.3 [km/h] n= 467.3 [1/min] uT= 0.30
cTs= 0.075456 cLs= 5.863832 lam= 0.03829
.#......psi..........alfaR
.1......0.00°.......8.39°
.2.....45.00°......7.67°
.3.....90.00°......8.15°
.4....135.00°......9.40°
.5....180.00°....10.56°
.6....225.00°....11.05°
.7....270.00°....10.81°
.8....315.00°......9.79°
.9....360.00°......8.39°
alfRm 10.81

Below is the output for the example for alfaR max, page 11/12 in the pdf (220, 221 using the document page numbers)

Example 716
lamNf 0.768
lamNf/uT 1.269
thtaN 2.028
thtaN/uT 0.359
thta1 1.096
thta1/uT 0.261
alfRm 10.04
 
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Hello, Juergen. I have successfully downloaded your files from 'Magentacloud'. Thank you very much...

I fear it's all probably too complex for me, but I will try...

Thanks again for your interest...

Javier
 
I am wondering how much of the rotor drag is due to simply increase in rrpm and how much is induced by getting to a higher rrpm.
For example, a rotor like ELA 07 carrying 370 kg with an aerodynamic pitch setting of 3°, the rrpm is 342 steady (at sea level). The drag of the rotor alone is 566 N at 110 km/h
Now, with an aerodynamic pitch setting of 0°, the rrpm would be 450 steady and the rotor drag alone would be 890 N at 110 km/h.
 
Hello Javier,

the basic idea was that the program allows you to modify the variables you are interested in, e.g. advance ratio mu or collective pitch thtaN and see what each modification does. Just treat the rest, especially the inflow calculation, which is necessary to make it work, as a black box. If at one point you'd like to dig a bit deeper just ask, I will be glad to explain the things you are interested in.

Have fun!

Juergen
 
For example, a rotor like ELA 07 carrying 370 kg with an aerodynamic pitch setting of 3°, the rrpm is 342 steady (at sea level). The drag of the rotor alone is 566 N at 110 km/h
Now, with an aerodynamic pitch setting of 0°, the rrpm would be 450 steady and the rotor drag alone would be 890 N at 110 km/h.
So it looks like for very speed a gyroplane flies, there is a most efficient RRPM that goes with it. For a given rotor of cause.

Wait. With the gyro flying at the same speed, the drag on the fuse supposed to be about the same. If the rotor drag is increased, shouldn't an attitude change be apparent on the fuse?
 
Rpm faster allows a Vne faster. Not means better efficiency. The efficiency appears in the ratio L/D, clearly lower.
And with this greater disc A.o.A, the attitude of airframe should be corrected, of few degrees nose down.
 
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