RV12 Drag curve
Goal: Develop an understanding of RV12 performance by estimating the RV12’s Drag curves
Disclaimer: I know enough about aerodynamics to know that I do not understand it.
Intent of this post: To start a discussion about these fundamental characteristics of the RV12.
Assumptions: Sea Level, standard day, aircraft at maximum gross weight, flaps up
Start: RV6a Drag information from the Café Foundation APR (aircraft performance report) of the RV6a. This test can be found at: http://cafefoundation.org/v2/pdf_cafe_apr/RV-6A Final APR.pdf
From the RV6a drag graph in the report, the coordinates of the induced and parasitic drag data points were obtained. This data was then entered into Excel and curves fit. Induced drag is supposed to equal a constant divided by the square of the airspeed. Parasitic drag is supposed to equal a constant times the square of the airspeed (from Aerodynamics for Naval Aviators). My goal was to find the constants that resulted in the best fit. These are the formula of the best fit lines.
Induced Drag (pounds) = 764669.98 / CAS(mph)^2
Parasitic Drag (pounds) = 0.00594791 * CAS(mph)^2
Total Drag is the sum of the Induced and Parasitic drag values at each of the airspeeds.
Creating the RV12 Drag Curve Estimates:
Theses estimates were created following these (possibly misguided) ideas:
Began with the induced curve
1) Shifted the stall speed by 3 mph (RV6a reported stall 55 mph cas, RV12 is 52). Since both share essentially the same airfoil (RV6a 23013.5, RV12 23014) they should therefore stall at essentially the same angle of attack.
2) Adjust for different gross weights (RV6a 1650, RV12 1320). Induced drag is proportionate to weight; after all it is the backward vector component to the wing’s lift.
3) Induced Drag (pounds) = (1320/1650) * 764669.98 / [CAS(mph) + 3]^2
Then proceeded to the Parasitic curve.
1) The two aircraft have essentially the same fuselage cross-section and length. The RV12 has a longer wingspan and area. The RV6’s gear was faired while the RV12’s are not. The RV6a is flush riveted while the RV12 uses low profile rivets.
2) The RV12’s best glide speed and therefore the speed where parasitic drag and induced drag should be equal (by definition) is, according to the POH, 98 mph.
3) The coefficient for the parasitic drag curve was adjusted until the parasitic and induced drag at 98 mph were equal.
4) The resulting coefficient for the RV12 (0.00624409) is larger than the RV6a (0.00594791) and steepens the curve, indicating that the RV12 has more parasitic drag than the RV6a. The ratio of the coefficients (RV12/RV6a) is 1.04979. In other words the RV12’s parasitic drag is about 5% greater than the RV6a.
5) RV 12 Parasitic Drag (pounds) = 0.00624409 * CAS(mph)^2
From these curves the following can be determined (maybe):
1) The Total Drag curve, also known as Polar Drag, can be drawn by adding the induced drag and parasitic drag values at each of the airspeeds.
2) Carson’s speed, (optimal cruise) can be determined by finding where a line drawn from the origin will be tangent to the Polar Drag curve. By visual inspection this appears to about 129 mph
3) According to the Café papers:
Thrust Horsepower Required = AS (mph) * Drag (lbs) / 550:
Which at best glide speed (98 mph) is 21.37 THr, at minimum decent speed (min power required) (72.6 mph) is 18.47 THr, and at 135 mph is 35.82 THr.
4) Minimum decent speed is the low point on the Thrust Horsepower required curve. According to the Café papers: Sink rate can be calculated from:
Sink rate (fpm) = drag (lbs) * speed (fpm) / weight.
At the Minimum decent speed of 72.6 mph the calculated sink rate is 677 fpm producing a glide angle of 6.1 degrees.
5) Maximum range glide (best glide speed) is 98 mph (from POH). At 98 mph the sink rate calculates out to a 784 fpm glide, which results in a minimum glide angle of 5.2 degrees.
6) From the RV 12 and RV6a parasite drag curves coefficients, and knowing that the RV6a flat plate area is 2.32 sq ft (Café Report) the calculated equivalent flat plate drag area of the RV12 calculates to 2.436 sq ft. Again about a 5% difference.
The Graph:
[Tried to post without any luck.]
Assistance/corrections from someone who actually understands this stuff would be appreciated.
Can someone add the plot of the Thrust Available from the Rotax and prop used?
Regards, Dave
Goal: Develop an understanding of RV12 performance by estimating the RV12’s Drag curves
Disclaimer: I know enough about aerodynamics to know that I do not understand it.
Intent of this post: To start a discussion about these fundamental characteristics of the RV12.
Assumptions: Sea Level, standard day, aircraft at maximum gross weight, flaps up
Start: RV6a Drag information from the Café Foundation APR (aircraft performance report) of the RV6a. This test can be found at: http://cafefoundation.org/v2/pdf_cafe_apr/RV-6A Final APR.pdf
From the RV6a drag graph in the report, the coordinates of the induced and parasitic drag data points were obtained. This data was then entered into Excel and curves fit. Induced drag is supposed to equal a constant divided by the square of the airspeed. Parasitic drag is supposed to equal a constant times the square of the airspeed (from Aerodynamics for Naval Aviators). My goal was to find the constants that resulted in the best fit. These are the formula of the best fit lines.
Induced Drag (pounds) = 764669.98 / CAS(mph)^2
Parasitic Drag (pounds) = 0.00594791 * CAS(mph)^2
Total Drag is the sum of the Induced and Parasitic drag values at each of the airspeeds.
Creating the RV12 Drag Curve Estimates:
Theses estimates were created following these (possibly misguided) ideas:
Began with the induced curve
1) Shifted the stall speed by 3 mph (RV6a reported stall 55 mph cas, RV12 is 52). Since both share essentially the same airfoil (RV6a 23013.5, RV12 23014) they should therefore stall at essentially the same angle of attack.
2) Adjust for different gross weights (RV6a 1650, RV12 1320). Induced drag is proportionate to weight; after all it is the backward vector component to the wing’s lift.
3) Induced Drag (pounds) = (1320/1650) * 764669.98 / [CAS(mph) + 3]^2
Then proceeded to the Parasitic curve.
1) The two aircraft have essentially the same fuselage cross-section and length. The RV12 has a longer wingspan and area. The RV6’s gear was faired while the RV12’s are not. The RV6a is flush riveted while the RV12 uses low profile rivets.
2) The RV12’s best glide speed and therefore the speed where parasitic drag and induced drag should be equal (by definition) is, according to the POH, 98 mph.
3) The coefficient for the parasitic drag curve was adjusted until the parasitic and induced drag at 98 mph were equal.
4) The resulting coefficient for the RV12 (0.00624409) is larger than the RV6a (0.00594791) and steepens the curve, indicating that the RV12 has more parasitic drag than the RV6a. The ratio of the coefficients (RV12/RV6a) is 1.04979. In other words the RV12’s parasitic drag is about 5% greater than the RV6a.
5) RV 12 Parasitic Drag (pounds) = 0.00624409 * CAS(mph)^2
From these curves the following can be determined (maybe):
1) The Total Drag curve, also known as Polar Drag, can be drawn by adding the induced drag and parasitic drag values at each of the airspeeds.
2) Carson’s speed, (optimal cruise) can be determined by finding where a line drawn from the origin will be tangent to the Polar Drag curve. By visual inspection this appears to about 129 mph
3) According to the Café papers:
Thrust Horsepower Required = AS (mph) * Drag (lbs) / 550:
Which at best glide speed (98 mph) is 21.37 THr, at minimum decent speed (min power required) (72.6 mph) is 18.47 THr, and at 135 mph is 35.82 THr.
4) Minimum decent speed is the low point on the Thrust Horsepower required curve. According to the Café papers: Sink rate can be calculated from:
Sink rate (fpm) = drag (lbs) * speed (fpm) / weight.
At the Minimum decent speed of 72.6 mph the calculated sink rate is 677 fpm producing a glide angle of 6.1 degrees.
5) Maximum range glide (best glide speed) is 98 mph (from POH). At 98 mph the sink rate calculates out to a 784 fpm glide, which results in a minimum glide angle of 5.2 degrees.
6) From the RV 12 and RV6a parasite drag curves coefficients, and knowing that the RV6a flat plate area is 2.32 sq ft (Café Report) the calculated equivalent flat plate drag area of the RV12 calculates to 2.436 sq ft. Again about a 5% difference.
The Graph:
[Tried to post without any luck.]
Assistance/corrections from someone who actually understands this stuff would be appreciated.
Can someone add the plot of the Thrust Available from the Rotax and prop used?
Regards, Dave
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