User talk:בנצי

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Lift coefficient and drag coefficient
Hi! On 30 August 2017 you wrote: ''I'd like to know the values of these coefficients, say, for cessna 172, when it's on the runway towards liftoff ? Of course, these values depend on the angle of attack, namely, it's required 1st to know this angle while running. I hope there's a pilot or an aeronautical engineer in the audience. Thanks,'' See the diff. Unfortunately no-one answered your question before it disappeared from the Science Reference Desk. Here is my answer:
 * At Cessna 172 it gives the following information that can be used to calculate the maximum lift coefficient achievable by the 172R:


 * wing area 16.2 square metres
 * gross weight 1,111 kg (or 10888 newtons)
 * stall speed (at gross weight) with flaps down 87 km/hour


 * An aircraft’s maximum lift coefficient is the lift coefficient at the stall.


 * At sea level in the International Standard Atmosphere, the density of air is 1.225 kg/m3. A stall speed of 87 km/hr is 24.17 m/s so the dynamic pressure :q, half rho times speed squared, is 357.8 Pa (pascal, or newtons per square metre.)


 * Inserting q = 357.8 Pa into the definition of lift coefficient (lift L divided by dynamic pressure q and divided by wing planform area S) gives a lift coefficient of 1.878 (with flaps down.)


 * So the maximum lift coefficient with flaps down, regardless of aircraft weight, is about 1.88.


 * In the take-off configuration with flaps retracted the maximum lift coefficient won’t be has high as it is with flaps down. Let’s assume the maximum lift coefficient with flaps retracted is only 1.6, and let’s assume lift-off occurs at 110% of stall speed. Therefore, the lift coefficient at lift-off will be about 17% less than 1.6 so let’s say 1.32.


 * To calculate drag coefficient requires information about the drag on the aircraft at a known speed. We don’t have that information. Dolphin
 * ( t ) 13:40, 4 September 2017 (UTC)
 * Thank you for your answer & analysis, after all. How did you notice the question after it disappeared ?
 * Do you fly a plane yourself ?
 * What kind of information is required regarding drag coeff ? בנצי (talk) 14:17, 4 September 2017 (UTC)


 * I first saw your question a couple of days after you posted it on the Reference Desk (Science) but I didn't have time to answer it. I finally had time to answer it but by then it had disappeared from the Reference Desk.


 * I have done a lot of flying of gliders and single-engine propeller-driven airplanes. I am an aeronautical engineer.


 * To calculate a drag coefficient for an aircraft it is necessary to have some data relating weight, drag and airspeed. For example, if you knew that at a weight of 12 kilonewton a particular aircraft requires thrust of 1500 newton to maintain a constant indicated airspeed of 200 km/hour, you could calculate a drag coefficient for the aircraft because in level flight at constant speed, thrust is equal to drag. You would need to select a representative area S for the aircraft. The relevant formulae are given at Drag equation.


 * At Drag coefficient it states that the drag coefficient for the Cessna 172 and 182 is 0.027. Dolphin  ( t ) 03:56, 5 September 2017 (UTC)

Your help desk question
You have a response.— Vchimpanzee  •  talk  •  contributions  •  20:32, 20 September 2019 (UTC)