14.4.3. Mutual influence of prop and airplane

It is convenient to divide study of mutual influence of prop and airplane into two parts: influence of airplane parts onto prop and prop influence onto plane.

Axial speed component decreases under engine nacelle influence in the place of prop installation. The flow becomes decelerated and radial speed component appears.

The wing influence onto prop located before the wing is similarly to engine nacelle influence or fuselage effect, but flow is decelerated before a wing much less, as a rule.

The wing influence onto prop located above the wing can be substantial and is exhibited in increasing or reduction of flow velocity incoming on the prop, in comparison with speed of undisturbed flow.

The prop influence onto the plane is shown, first of all, through a pressure rising in jet directly behind the plane of rotation, where engine nacelle, wing and other parts of the airplane are located. Besides, the jet behind the prop has speed exceeding speed of the incoming undisturbed flow and distinguished from it by direction due to twist and lack of coincidence of the prop axis with direction of the undisturbed flow or deflection of jet from the prop by other parts of the airplane.

The pressure rising in jet behind the prop causes an additional pressure increasing at nose sections of the airplane elements located in jet, that is an additional drag.

The increasing of speed and change of flow direction in jet behind the prop causes changes of forces of pressure and friction and their distributions. It results in occurrence of additional drag and additional lift on parts of the airplane blown by jet from the prop.

Influence of the prop on horizontal tail located behind the props is the same as on the wing. However, the increasing of positive pitching moment occurs on the normal configuration airplanes at negative lift of horizontal tail which is necessary to counteract by additional deflection of elevators for diving.

As the greatest effect from the airplane parts blown by props occurs at small flight speeds , i. e. at takeoff modes, then it is necessary to take into account the influence of ground proximity onto the airplane aerodynamic characteristics while props activity.

The influence of ground proximity is shown first of all in decreasing of flow downwash caused by the prop jet. It results in increasing of wing lift, reduction of its induced drag, decreasing of horizontal tail negative lift and reduction its pitching moment.

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V.V.Chmovzh 5/27/2025