The shutter speed used here is way too fast. Aperture and ISO are fine and I agree it must have been BRIGHT. If I were taking the shot I would be aiming for around 15-30 degrees of prop blur which give an indication of motion. To achieve the right* amount of blur a certain shutter speed is required and I would have used shutter priority (1/125 to 1/250) should give good results. Remember the general rule for shutter speeds, the reciprocal of the focal length should be a minimum, may force photographer closer to the subject. At slower shutter speeds you need to track the body of the aeroplane to keep it still in the viewfinder and only allowing the prop to blur. The second shot would have looked great with a slow shutter although it would not have done anything for the prop, its the bushes that would have created a feel of speed. As this subject may take several passes to spray the entire crop field you may have the opportunity to try several different setups using various shutter speeds. Keep the aperture at around f8-f11 for sharpness and vary ISO to maintain aperture.
Sample photograph from the gallery at 1/125, (helicopters have large rotors that rotate slower than smaller props)
http://www.worldphotographyforum.com...php?photo=3095
*every photographers taste is different