THE INFLUENCE OF RUNNING PROPELLERS

The forces on a single propeller are illustrated in Fig. 7.5, where olp is the angle of attack of the local flow at the propeller. It is most convenient to resolve the resultant into the two components T along the axis, and NP in

the plane of the propeller. The moment asociated with T has already been treated above, and does not affect G„ . That due to N„ is

’ m p

(7.3,7)

where CN = Npl^pV2Sv and Sp is the propeller disk area. To get the total

ДGm for several propellers, increments such as (7.3,7) must be calculated for

each and summed. Theory shows (ref. 7.4) that for small angles CN is

proportional to ap. Hence Np contributes to both Gm and dCmJda. The

latter is _

ЭС™ sp xp dCN dctP

If the propeller were situated far from the flow field of the wing, then dajda would be unity. However, for the common case of wing-mounted tractor propellers with the propeller plane close to the wing, there is a strong upwash e„ at the propeller. Thus

aP = a + eP + const (a)

where the constant in (7.3,9a) is the angle of attack of the propeller axis relative to the airplane zero-lift line. Finally,

dCmp = XpL depdCNv da S с да/ dap

INCREASE OF WING LIFT

When a propeller is located ahead of a wing, the high-velocity slipstream causes a distortion of the lift distribution, and an increase in the total lift. This is a principal mechanism in obtaining high lift on so-called deflected slipstream STOL airplanes. For accurate results that allow for the details of wing and flap geometry powered-model testing is needed. However, for some cases there are available theoretical results (refs. 7.5 to 7.7) suitable for estimates. Both theory and experiment show that the lift increment tends to be linear in a for constant GT, and hence has the effect of increasing awb, the lift-curve slope for the wing-body combination. From (6.3,36) this is seen to reduce the effect of the tail on the N. P. location, and can result in a decrease of pitch stiffness.