# Can a wind gust below the rotor affect the lift force in a helicopter?

If a helicopter is hovering and a very strong, narrow and focused wind gust comes from the front and passes few feet below the rotor blade without touching the fuselage or any part of the aircraft for that matter, would the helicopter lift or altitude change in any way?

If it helps, another way to put it, imagine if you had a very powerful leaf blower with a rectangular nozzle of about 6 feet width X 1 foot height, and you are aiming to the helicopter downwash at a height of about 3 feet down from rotor blades. The red lines in the image represent the wind from the leaf blower.

PS: The helicopter is in-ground-effect hover.

• Is the gust acting upon the rotor? Commented May 1 at 0:50
• no, is a very narrow and focused gust few feet bellow the rotor. Another way to put it. Image if you had a very powerful leaf blower with a rectangle nozzle of about 8 feet width X 1 foot height, and you are aiming right trough the helicopter downwash about 3 feet down from rotor blades.
– Gabe
Commented May 1 at 0:57
• In-ground-effect or out-of-ground-effect hover?
– Ralph J
Commented May 1 at 1:19
• @RalphJ good question. In-ground effect. It would be interesting to see your answer in both cases though, if it is not too much to ask.
– Gabe
Commented May 1 at 1:41
• OGE I'd expect that it wouldn't affect things. IGE, I'd suspect that it could, but I'll let a rotorhead explain the real answer.
– Ralph J
Commented May 1 at 3:17

Yes the horizontal gust influences lift - initially it reduces lift, since the gust lowers static pressure $$p_s$$ underneath the rotor.

The helicopter hovers on top of a column of air accelerated downwards, with the rotor experiencimg total pressure $$p_t$$ equals $$p_s + \frac{1}{2} \rho V^2$$. All in a nice equilibrium, until an external differential wind gust is exerted. Which adds local velocity, thereby reducing static pressure. Velocity direction is [arallel to the rotor and therefore the increase in dynamic pressure does not contribute to lift. Easy to verify: just blow over a tree leaf resting on the ground, and it will float up.

If the gust continues for a while, the airflow direction through the rotor will change plus the lower static pressure underneath the rotor will suck in more flow through the rotor. The situation will now be more like that in forward flight, which provide more lift for a given power setting, as stated in this answer.

• Perfect, we have two opposing theories. I summon @PeterKämpf 😏 Commented May 7 at 11:36
• Yep, I thought this was going to be a easy question, someone must have done this experiment before like in a wind tunnel or something. I can't believe no one has tested this before, like a aircraft manufacturer or something.
– Gabe
Commented May 7 at 14:33
• @Gabe I think this is not much of an interest, other than purely academic and wind tunnel time is costly. IRL one will not meet the conditions you've described. It was fun to think about though 👍 Commented May 7 at 20:57
• "...just blow over a tree leaf resting on the ground, and it will float up." but if you slide a plate with zero AoA through the air it will not produce lift, right? Commented May 8 at 21:04
• @Jpe61 Correct, because in that case total pressure is equal on both sides of the plate. In the case questioned, an external factor causes increased air velocity on one side, parallel to the lifting surface., which reduces static pressure on that side while not increasing upwards dynamic pressure. And of course what happens over time after the first initial burst is more complicated. Commented May 9 at 23:53

If a helicopter is hovering and a very strong, narrow and focused wind gust comes from the front and passes few feet below the rotor blade without touching the fuselage or any part of the aircraft for that matter, would the helicopter lift or altitude change in any way.

Basically this confined gust blows only on the wake beneath the rotor without touching the rotor itself. If so, then the answer shouldn't be complicated.

Just like in a fixed wing, also behind a rotating blade a wake gets shed from the trailing edge. The wake mainly possesses a downward movement which is termed downwash. The following picture (from this paper) shows this downward-moving wake in a good way:

Note that the wake shed by the tip rolls up behind the blade creating a tip-vortex, just like behind a fixed wing. But unlike a fixed wing, as soon as the blade makes a complete rotation around the hub, it bumps into its own wake; and if there are more blades on the hub, then it bumps into their wakes as well. Note for example how, in the following picture, the blade at 270° bumbs into the downwash shed by the previous blade at 0° (figure from this paper):

This "swirling" environment that a blade encounters in its rotation is called inflow and has a quite complicated nature but, in general, its average movement is downwards. That implies that the AoA seen by the blade gets decreased due to the inflow.

Now, if we perform your experiment and blow this downward inflow away, what we could expect is, on average, a smaller decrease in the AoA seen by the blade so that the thrust should increase.

Lift would momentarily increase. I have reasoned this in the following manner:

The rotor produces lift by accelerating air downwards. The air mass then proceeds down, and in ground effect is diverted sideways, when out of ground effect, it continues downwards untill it eventually loses momentum due to turbulence.

Now, if we suddenly push a new slice of air with no vertical speed right under the rotor, the air pushed down by the rotor will run into this air and start pushing it down. While the new air layet is being accelerated downwards, it locally increases the pressure of the air mass above it. This would essentially be the same as if the ground was getting closer to the rotor.

I also envision that the increased lift is most prominent in the portion of the rotor disk that is facing the gust. The portion "downwind" of the gust will encounter less pressure rise since the air was already pushed into downwards motion by the upwind part of the rotor.

So if the gust came form the front and passed under the right side of the rotor disk, the nose of the helicopter would tilt up and the helicopter would roll to the left.

• Caveat! The answer above is pure reasoning, no science behind it. I'm quite good at seeing stuff like this with my mind's eye, but I'm not always right 😏 Commented May 5 at 20:39