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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