... continued
Reprinted from  Flight
Magazine and the Aircraft Engineer, December 1935, London,
England
Experimental Verification
Since all of the above hinges greatly upon the assumptions
made, the writer made extensive studies of available experimental
data in Mr. Burnelli's files and in every instance found confirmation
of the possibility of simple superimposition of body and wing
effects since the relative position of the zero lift line, slope
of the lift curve and Lmax always check very well. It was decided
to attempt to break down the experimental lift and drag curves
from the latest test of a Burnelli monoplane as given in the New
York University Report No. 715.
The general procedure outlined above was followed to obtain
the combined effect of aerofoil body and wing, and it was noted
that the zero lift point of the body was shifted considerably
in the range of positive angles of the wing. Upon completion of
the analysis, which showed remarkably close agreement with experimental
data, as can be seen from Fig. 3, it was decided that a better
coordination could be obtained by approaching closer the two
zero lift point. This was confirmed by the study, the results
of which are given in Fig. 4. It was further observed by an approximate
stability calculation that this arrangement would further lead
to an improvement in the condition of downwash with a consequently
better stability. To prove definitely all these findings it was
decided to design a new body form which would fulfill all the
desired conditions and locate the wing in such a way as to approach
the two zero lift points as close as it was feasible without extensive
changes in the wind tunnel model.
The body was made 33 per cent thicker, since the investigation
indicated that this increase could be well afforded because of
better coordination. Fig. 4 shows the remarkably close agreement
of the wind tunnel test (New York University Wind Tunnel Report
No. 715B) and the predicted characteristics. The zero lift point
checks extremely well while being different from the former zero
lift point by 2.5 degrees. The slopes of the lift curves are in
very close agreement. From all these investigations, confirmed
by experimental verification, it is evident that the simplified
analysis gives sufficient practical accuracy. It further proves
that interference effect for the case of Burnelli design is negligible,
that with properly coordinated aerofoil body and wing location
the fuselage carries a considerable percentage of lift constant
through the whole flying range and easily determinable. Structural
significance of this fact is apparent since the weight of the
body now becomes useful weight contributory to the lift somewhat
in the same sense in which we regard the drag of wings as compared
to the parasite drag.
The weight of the wings is reduced partly because of the smaller
area required and partly because of smaller wing root bending
moments.
The points of safety, accessibility
and volume were not touched upon but they cannot be disputed.
In conclusion it can be definitely
stated that for the same aerodynamic efficiency the Burnelli type
of design results in considerable saving of weight and for equal
weight gives higher aerodynamic efficiency.
