Autofocus is likely failing because your camera uses contrast-detection AF, which needs a brief moment to evaluate contrast and move the lens until it finds maximum sharpness. In heavy prop wash, the camera is being buffeted constantly, so the AF point is shifting all the time and the camera may never settle long enough to lock focus.

Strong wind can also make sharp photos difficult in general, not just focus acquisition. The main issue is motion and vibration from the airflow around the camera.

Practical mitigations:

• Keep the camera out of direct prop wash if possible, using the windscreen or cockpit structure as shelter.
• Use manual focus if your camera allows it.
• If available, use an infinity or landscape/pan focus mode.
• A medium focal length with more depth of field can reduce the need for constant refocusing.
• Check that the lens is clean, since dust or debris can worsen performance.

So yes: manual focus or a preset long-distance focus mode is the best workaround, but reducing exposure to the airflow is just as important.