Discussion of a method for eliminating turbulence caused by the formation of insect roughness upon the leading edges and fuselage, particularly in aircraft using BLC. The proposed technique foresees the use of elastic surfaces on which insect roughness cannot form. The operational characteristics of highly elastic rubber surface fastened to the wing leading edges and fuselage edges are examined. Some preliminary test results are presented. The technique is seen to be advantageous primarily for short-haul operations

Wortmann, F. X.

English

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NASA TECHNICAL HEMORANDm1 NASA Tr'-1 -77419 
A POSSIBILITY OF AVOIDING SURFACE ROUGHNESS DUE 
TO INSECTS 
F.X. Hortmann 
Translation of "Uber eine Moglichkeit zur Vermeidung der 
Insektenrouhigkeit",(jSTIV Congress, 9th, Junin, Argentina, 
Feb. 1963). Aero-Revue, Vo138, Nov. 1963, pp 644-646. 
JAN 1 5 i985 
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WASHINGTON, D.C. 20546 HARCH1984 
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APOSSIBfLITY OF AVOIDING SURFACE 
ROUGHNESS DUE TO INSECTS 
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Uber elne M5ilichkett zur Vermeidung der Insekten-
rauhigkeit,(OSTIV Congress, 9th, 'Junin, Argentina, 
Feb. 1963).Aero-Revue, Vo138, Nov. 1963, pp 644-646 
. (A64-11564) 
Discussion of a method fo~ eliminating turbu~ence 
caused by the formation of "insect roughness" 
u~on the lead~ng edges and fuselag~, partic~larly 
irraircraft:using ~LC. The proposed technique 
foresees' the.use of elastic surfaces'on which 
·Lnsect roughness cannot form .. Th~ operational· 
characteristics of highly elastic rubber surface 
'fa~t~ned to the wing l~ading ~dges and. fuselage 
edges are examined. Sqme prelimin~ry test.results 
are presented. The techDique is seen to be advan-
tageo~s p~imarily'for short-haul ·operatton~. 
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A POSSIBILITY OF AVOIDING SURFACE ROUGHNESS DUE 
TO INSECTS 
F.X. Wortmann 
1. Overview 
Upon collision of insects with fuselage and leading wing 
edges a roughness is produced which is normally great enough to 
generate an immediate turbulence. Thus, the insect-roughness 
considerably deteriorates the aerodynamic performance of an air-
craft. A premature turbulence is undesirable especially in those 
cases where a significant reduction in resistance could be 
achieved on aerodynamically smooth surfaces by keeping the boun-
dary layer laminar. This insect problem will only be solved by 
taking into account that the boundary layer is to be kept com-
pletely laminar through suction. The methods proposed heretofore 
to eliminate insect roughness are relatively complicated. They 
are all aimed more or less at protecting the leading wing edges 
wi th protective coatings before take-off; these coatings would 
then be jettisoned or washed off at greater altitude, i.e. above 
the insect zone. Such methods and pertinent questions about 
them are reported in detail in [lJ. 
It is instructive that cumbersometake-off~preparations can 
however, also include a suction step for long-range aircraft. 
For short-range aircraft which do not even leave the insect zone, 
or for aircraft which only partly desire a laminar flow, e.g. 
through laminar profiles, the solution to the insect problem 
should rely on simpler methods. In the author's opinion, elastic 
surfaces offer one such possibility, since they do not allow the 
generation of insect roughness in the first place. 
* Numbers in the margin refer to pagination in the original text. 
----- ---.- _. - - -- --- -
- ---~- ------- ~------ -----
/644* 
1 
Fig. 1: Impact of a Drop of Water on a Foam Rubber 
Surface on the Right Side of the Photo. On 
the left, a similar drop at the end of the 
Impact. Impact velocity about 5 m/s. Picture 
sequence: 6000 exposures per second. The 
numbers indicate the picture numbers of the 
film. 
2. Action of Elastic Surfaces 
At higher impact velocities even small insects have a 
kinetic energy sufficient to cause disintegration of the 
insect shell and distribution of the viscous body fluid upon 
impact on solid surfaces. 1 If the second process can be prevented, 
then the first process can be viewed as insignificant. Thus 
below, the insect will be viewed as a viscous drop of liquid. 
Now it is suggested to store the impact energy for a brief time 
in an elastic "spring" and to use it to decelerate the drop of 
liquid. Whether this will succeed will depend primarily on the 
following parameters--as one can easily see. 
1 
2 
If one's hand is held in an air stream moving at about 150 km/h 
and filled with fruit flies, then each impact is felt as a 
small pain. 
18 
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Fig. 2: Impact of a Drop of Oil on a Foam Rubber Surface. 
Left: A drop at the end of the impact Process. 
Impact Velocity about 7 m/s. Image frequency: 
6000 exposures per second. 
1. The mass of the spring must be sufficiently small, otherwise 
the spring will not be compressed. 
2. The vibration period of the system must be so small that 
the viscous drop is not distorted too much during this time. 
3. The spring damping should also remain sufficiently small, 
even at high frequencies, in order for sufficient energy 
to be available to separate the drop from the wetted surface. 
4. The separation process should be promoted by poorly wettable 
surfaces. 
3 
Fig. 3: Contour of the Oil Drop 
upon Impact on a Foam 
Rubber Surface. The 
numbers denote the picture 
numbers of the film. 
Image frequency: 6000 
exposures per second. 
Figure 1 shows the impact of a droplet of water at low 
velocity. The droplet has such low viscosity that it splits 
apart before any reflection occurs. 
Figure 2 shows the same process with a drop of oil. Now 
we clearly see the vibration shape of the droplet, which is 
shown enlarged in figure 3. The impact energy is not sufficient 
however, to separate the droplet from the surface. 
Figure 4 shows several photos of a water droplet impacting a 
silicon rubber surface at a speed of 150 m/s.2 Now we clearly see 
the reflection and separation of the drop from the elastic surface. 
A small part of the drop evidently adheres to the surface. 
3. Tests With Insects 
Naturally these few tests simulating insects by a drop of 
fluid and using only perpendicular impact, will permit only a few 
basic findings on the effectiveness of elastic surfaces. Thus, 
additional tests with real insects were conducted in summer 1961 and 
1962 using various elastic surfaces under different types of condi-
tions. 
2These photos were taken by Mr. E. Wieland of Dornier-System Co. 
-us-ing-a-high-f-requenc-y--camera--devel-oped-by G.- Hahn [2]. . ------
4 
Figure 4: 
8 10 
Reflection of a Water Drop upon Impact on a 
Silicon Rubber Layer. Impact speed about 150 m/s. 
Image frequency: 85,000 per second. The numbers 
denote the picture numbers of the film. 
Figure 5: Fruit Flies in Perpendicular Impact on a 3 mm-
thick Silicon Rubber Plate. Impact speed about 
50 m/s. The dark spots on the rubber are points 
impact. The white adhesive strip is used to 
attach the rubber. 
The tests were concentrated on solid rubber and foam rubber 
surfaces of 1 - 3 mm thickness with Shore-hardnesses of 10 - 35. 
Figure 5 shows a typical result of a wind tunnel test with per-
pendicular impact of fruit flies on a 3 mm-thick solid rubber 
surface. 3 Figure 6 shows the result of another wind tunnel test 
with various rubber samples and variable angles of impact. 
3My thanks go to Mr. Hamma for his untiring assistance in these 
tests. 
5 
-----.-----~-.--.--------------- - -.-.. --~.~--.--- - .. - -
Fig. 6a: Testing of Various Rubber Samples with Fruit 
Flies. Impact speed about 50 m/s. Right: A 
3 mm-thick Silicon Foam Rubber 
Fig. 6b: Sectional Enlargement of Fig. 6a. The 3 mm-thick 
Silicon Foam Rubber Allows the Generation of NO 
Insect Roughness. The same Rubber can also be 
Produced with a Smooth Surface. 
Besides these wind tunnel tests which were limited to 
practically one type of fly--the fruit fly--similar rubber samples 
were also attached to vehicles and training aircraft and observed 
daily in suitable weather. In all these tests practically no 
. 4 
insect roughness was ,found on several rubber surfaces. However, 
tiny traces of liquid remain which change the optical appearance 
of the surface somewhat, so that one can indeed determine whether 
or not insects have indeed impacted. Thin rubber . membranes about 
1 mm-thick are not fully effective above a speed of about 100 km/h. 
The 3 mm-thick rubber plates were satisfactory in the entire 
investigated velocity range, i.e. from 40 km/h to about 200 km/h, 
4 
In aircraft with a pneumatic rubber dei~ing _~~~_~~~_ th!s effect 
-------is pract1cally not observeabecause the rubber is hardly elas-tTc:--- ----
6 

operating mainly at lo\v and medium altitudes and on aerodynamically 
smooth surfaces having a partial or complete laminar friction 
layer. 
Summary 
It is proposed to solve the problem of insect roughness 
by highly elastic rubber surfaces attached to wing and rubber 
leading edges. Such elastic surfaces are suitable--as high-
speed photography shows--to reflect impacting insects or viscous 
liquid drops elastically. This alone will prevent the generation 
of insect roughness and the endangered fuselage and leading wing 
edges remain aerodynamically smooth. Perhaps the simplicity of 
this method will contribute to the possibility of reducing 
friction through retaining laminar flow at the boundary layer. 
[1] H.S. Coleman: "Roughness due to Insects," Lachmann, "Boundary 
Layer and Flow Control," Pergamon, London 1961. 
[2] G. Hahn: NA Simple, High-Frequency Cinematographic Tool 
for Interferbmetric Photog~aphy of Instationary Fl~ws at 
a Frequ~ncy up to 3.3 x 10 frames/second." Kurzze~tphoto­
graphie pp. 257-266, Hellwig, Darms tadt 1960. 
8 
End of Document 


A possibility of avoiding surface roughness due to insects

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