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The Osprey is a tilt-rotor which takes off as a helicopter but flies like a plane. It is known for it's nasty downwash which can knock people off their feet and even wreck helipads.

Its rotors are a compromise: larger than a plane's propeller but smaller than a helicopter's rotor. Its weight must be supported on a smaller swept area (higher disk loading). This means a more concentrated downwash. Is my reasoning correct?

Edit: it seems that my physics reasoning is correct. Sophit's link compares, quantitatively, disk area and disk loading between an Osprey vs helicopter of similar mass.

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  • $\begingroup$ This might help. $\endgroup$
    – sophit
    Aug 31, 2023 at 9:57
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    $\begingroup$ The reference you linked to is highly misleading. The Osprey did not "demolish" a concrete or asphalt helipad. It just blew away some thin mats that were sitting in a field. It was not on the mats and the mats did not have any helipad markings. $\endgroup$ Aug 31, 2023 at 16:36
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    $\begingroup$ You wreak havoc. You wreck helipads. $\endgroup$ Sep 1, 2023 at 1:22

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higher disk loading... means a more concentrated downwash

Correct and this can be demonstrated even using a very simplified approach like the momentum theory by which the airspeed $w$ under a rotor of area $A$ generating a thrust $T$ is:

$w=2\sqrt{\frac{T}{2 \rho A}}$

Being the thrust equals the weight, a smaller rotor implies a higher speed of the air pushed under it. It also implies a higher power which has to be supplied to the rotor to give that thrust. This is why the disc area for an helicopter is normally chosen to be as big as possible.

As you've also highlighted, the disk area of the MV-22 is a compromise among contrasting requirements and the resulting induced velocity of 80kts under it can be classified as a hurricane. That implies, among other things, that the MV-22 cannot for sure land on some unprepared field; the risk of brownout/whiteout is increased; and the fuselage is better accessed from behind instead from the side where the high induced velocities can make it difficult.

To all these it should also be added that at landing, when the engines are rotated vertically, the hot exhaust impinges on the airfield requiring not only a prepared airfield but also a sturdy surface.

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Yes, your reasoning is correct.

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  • $\begingroup$ Do you have any information about the speed of the downwash, and the speed of the downwash of a helicopter of similar size? That information would be pretty interesting. $\endgroup$ Aug 31, 2023 at 16:37
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    $\begingroup$ Simple, correct, concise and to the point... bravo 😅 $\endgroup$
    – sophit
    Aug 31, 2023 at 18:23
  • $\begingroup$ @TannerSwett Does the question ask for downwash figures? If you want to know more, please ask a new question. All I did was to answer the question asked by Kevin. My understanding of this site is that people ask questions and others answer them. If that understanding is wrong, please educate me! $\endgroup$ Sep 1, 2023 at 13:38

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