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Rotor downwash

Old 4th Feb 2015, 17:22
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Rotor downwash

Dear (Rotary) PPruners...

Does anyone know (or understand how to calculate) the rotor downwash figure for Puma 2, AW 159 (Wildcat), Chinook and Bell 212?

Hope this is not too taxing!
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Old 5th Feb 2015, 04:59
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You might do better here: Rotorheads - PPRuNe Forums
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Old 5th Feb 2015, 06:06
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Something to perhaps be respected rather than calculated.

There is a wide scope of variables. The rotor span and all-up-weight of the aircraft would be fundermental as an empty aircraft would create less downwash then a laden aircraft. I am assuming that the answer you seek is goven in windspeed and probably mph rather then knots. The nature of the terrain will influence an accurate answer. The only people who may have sought such information would be the Rotary Wing Test Sqn at Boscombe Down. I doubt even JADTEU who own the HHI course would have access to formal data such as this.

Cranfield considered 'wake' - downwash by another name in performance calculations. Some serious night time reading here:
http://naca.central.cranfield.ac.uk/...rc/cp/1341.pdf
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Old 5th Feb 2015, 06:27
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From elswhere:

Question:
How would you calculate the rotor downwash velocity (in mph), specifically for the Hughes 500?

Answer:
Following the advice of Mr. Newton, we know that our rotor must produce an upward force, or thrust, equal to our helicopter’s weight if we are to hover. Since Force = (Mass) x (acceleration), that upward thrust must come from continually accelerating a stagnant mass of air downwards through the plane of our rotor disk to some final downwash velocity. This final velocity depends on the weight of the helicopter, the size of the rotor disk area, and the density of the air the helicopter is trying to hover in. Physics says that the energy transfer between the rotor and the air must happen at an equal rate. Equating these energy expressions and manipulating them with a little algebra results in the velocity of the downwash at the rotor disk being equal to the square root of: Weight divided by 2 x (Air density) x (Disk Area). Considering the example of a Hughes 500: Gross Weight = 3,000 lbs Disk Area = 547 square feet Air Density = .002378 slugs/feet cubed (assuming sea level) Plugging in the values into the above formula and hitting the square root key on the calculator results in the downwash velocity at the rotor being approximately 34 ft/sec, or about 23 mph. But we haven’t completely answered the question yet! Keep in mind that this speed is at the rotor disk. As the column of air is forced down below the rotor, it constricts, much like molasses being poured out of a pitcher does. In doing so, it reaches its maximum velocity at 1.5 — 2 rotor diameters below the disc. Consequently, the final fully developed downwash velocity can be shown to be 2x the above calculated amount, or 46 mph in our Hughes 500 example. Points to remember: Higher weights, smaller rotor disk areas, and higher altitudes will all produce higher downwash velocities.

Author

Frank Lombardi is a Police Helicopter Pilot,Testing & Evaluation
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Old 5th Feb 2015, 07:54
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Old 5th Feb 2015, 12:27
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Having spent a LOT of time underneath hovering Chinooks, Pumas, and to a lesser degree Wessexes and SKs, the theoretical figures posted by TM seem about right to me. The Hughes 500 is a far smaller beast than any of those that I got under, But the downwash seemed to be like trying to walk into a 50-70mph wind. Much easier to wait with the load than try and approach it with the helo in the hover. But riskier with the strop, especially at night. The Wokka's 3 hooks made that a little entertaining too.

Unsurprisingly, the CH47 had the most, by far, but the the SK wasn't far behind it. Puma was a walk in the park compared to those two. Puma 2 with new engines and rotors - I can't see it being much different, because as the calculations show, mass of the object(s) being held aloft and area of the rotor disc(s) are the variables.

Which brings me to another point - a Bell 212 or 214, never went under either of those (do they even have hooks?), but I'd expect the downwash to be about the same even though the 214 has twice as many blades.
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Old 5th Feb 2015, 14:24
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The Bell 412 EP (Griffin) at DHFS had a hook for basic load handling on the RAF AFT course.

The SARTU Griffins used winches for decks and wets.

As a thread diversion, I have seen numerous Chinooks flying low over the City of London all week...anyone from Odiham here know what's occuring? (I assume standard helilanes training or posibly the Hon Arty Coy HLS?)
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Old 5th Feb 2015, 15:20
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A quick call to the RW GS tutor at ETPS will give you a mathematical answer. Ask for DL. A flying tutor there could dig out a preview report where practical measurements of down wash at various hover heights and distances have been measured. S92 from 2009 iirc. Or if you fancy a day out of the office, just take a trip down the 303...
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