Wing Tip Drag
by David Freid
Date: Mon, 13 Nov 1995
From: smtpgwy.dehavilland.ca!david_fried@matronics.com
Encoding: 84 Text
To: RV-LIST@matronics.com
Subject: RV-List: Aircraft Drag, Nav Lights & Baggage pods
Sender: owner-rv-list@matronics.com
Precedence: junk
Reply-To: rv-list@matronics.com
A few of the recent posts have involved the drag penalties resulting
from
the addition of such things as baggage pods and navigation lights.
Calculating the drag is not too difficult and the resulting numbers
are
quite small. Here is some food for thought.
Dynamic pressure is symbolized by "q". This is the pressure resulting
from
motion through the air. It is felt at areas of a body that bring
the
relative speed between air and body to zero. This area is known
as
stagnation and is found along flying surface, air inlet and gear
leg
leading edges. There are also stagnation areas at the forward
edge of the
canopy, spinner, wheel pants and the boundary layer diverter
(just above
the carb inlet).
Air density on a standard day at sea level rho = 0.002377 lb/ft^3
Factor to convert miles/hour to feet/second Vfps = Vmph * 1.467
q (lb/ft^2) = 0.5 * rho * Vfps^2
The profile drag coefficient (CDp) is based on the frontal area
of
streamline bodies. For convenience, D divided by q, "D/q" is
used to
calculate the area of an equivalent flat plate with a Cd of 1.0.
Increments to aircraft drag are noted as delta D/q with units
of
square feet.
CDp = D /(q * S front)
2 baggage pylons S front = 2.0 ft^2 (total)
Cd = 0.05
q = 102.3 lb/ft^2 (200 mph)
D = 10.2 lb deltaD/q = 0.1 ft^2
2 wing tip lights S front = 0.1 ft^2 (total)
Cd = 0.1
q = 102.3 lb/ft^2 (200 mph)
D = 1.0 lb deltaD/q = 0.01 ft^2
An article in the April 1995 issue of Sport Aviation gives drag
data for the
RV-6a. The data was set up to compare with another type of aircraft
at a
reduced weight. The result when corrected to 1600 pounds gives
the following
drag coefficient based on wing area.
CD = .02026 + .08825 * CL^2
CL = Weight /(q * S wing) Weight = 1600 lb S wing = 110 ft^2
= 0.142
CD = 0.02204
D = CD * q * S wing q = 102.3 lb/ft^2 (200 mph)
= 248.1 lb
D/q = 2.42 ft^2
Looking at it from another direction, a 160/CS powered -6 will
max out at
sealevel at something like 200 mph.
Thrust horsepower = BHP * Prop efficiency
Prop efficiency nP is approximately 0.8 to .85
Power = Thrust(lb) * V(fps) / 550
Solving for Thrust (Drag) at 200 mph gives:
Drag = BHP * nP * 550 / (200 * 1.467) nP = 0.83
= 248.9 lb
For propeller driven aircraft where power output is independent
of speed
(constant speed props) there is a simple relationship between
deltaD/q and
the resulting deltaV. It works best if the drag increments are
not too large.
Fixed pitch props probably have similar results under the same
condition.
deltaV = (deltaD/q / D/q) * V / 3 D/q = 2.42 ft^2 V = 200 mph
= 2.8 mph For two baggage pods
= 0.3 mph For a tip light set
David Fried
DF-6 C____
dfried@dehavilland.ca
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