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BrianW

Vx and Best angle of climb

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Hello...I am well aware of the two definitions and the difference between theBest angle of climb, (Vx) and Best Rate of Climb (Vy)...etc.However, the Cessna 172 POH seems to be vague on a particular question I have. Vx (Best angle of climb speed) is documented to be 64 KIAS. Yet in the procedures for a clean climb to an obstacle, itlists the obstacle climb speed as 59 KIAS. Both senarios relate to a clean configuration at sea level.My question is, "why would there be an obstacle climb speed LOWER than the speed given for BEST ANGLE?"Darryl

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Hi Darryl; Well in the event you need to clear an obstacle basically you tradealtitude for air speed. If you're looking for best flight efficiency,then the 64 KIAS would be be appropriate. However if ALTITUDE in a HURRYis what you're after then 59 KIAS would be your target. You're gainingaltitude over a short span by CLIMBING faster and reducing IAS. Don't know if that makes sense or not?:-) DennyProfessional Tourist

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I agree, tt doesn't make sense. Vx by definition is the most height for the least distance. However, I seem to recall that Vy on 172 was 65 knots and that Vx is 55! I don't have a PoH to hand so this is from memory.

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Just to add that a PoH that quotes a single number for a speed will be for for max weight. Other versions of may quote lower numbers for expected min weights (single pilot).

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Because if you rotated at the recommended speed, the obstacle speed is the speed you would be at when passing over the 50 foot obstacle. You are not going to achieve the published Vx unless you held off your rotation or you held the nose down a little for acceleration, either of which will defeat the purpose of clearing the theoretical 50 foot obstacle and not the technique they use to determine your distance numbers.

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Hi Darryl,Which year and model of C172 are you referring to?The reason I ask is that starting with the C172N, the procedure for short field takeoff changed and 10 degrees of flaps was recommended. Prior to that (Model M and earlier), the recommended short field procedure was with no flaps (unless a minimum ground run was desired).I believe the lower speed you are citing is for a flaps 10 takeoff.John

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John is correct. The later models use 10 degrees of flaps for short field takeoffs. 56 KIAS and flaps 10 should be mantained until obstacles are cleared then you would accelerate to Vx (60 KIAS for a 172P) and then retract flaps. If the max climb gradient isn't required then you would continue to accelerate to Vy or to a cruise climb. Brian

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The C172RG (from around 1980) that I fly has a Vx of 67 KIAS, but the climb speed for a short field takeoff is 63 KIAS (which is the same as the approach speed). We still use 67 KIAS when training short field takeoffs (but on a long runway). Flaps up is recommended for short field takeoff.Martin767 fetishistIt's a lot like life and that's what's appealing

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Kevin's and Denny's answers in my opinion are closest to the truth.To put it in other words - Vx relates to 'enroute' climb, clearing obstacle at the end of the runway (assuming the runway is relatively short) brings into play more factors - like place of rotation, distance to the obstacle, etc. Contrary to a 'static' Vx climb this situation is highly dynamic - you rotate and then you keep bringing the nose up with speed bleeding off in the 'desperate' attempt to clear this obstacle. You are trading speed for altitude - this tradeoff would not work very well for a longer period of time - it just works for seconds necessary to get this job done.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2http://www.precisionmanuals.com/images/forum/747400.jpg

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Micheal,I can't speak for the 172. But:Vy (not Vx) is best rate of climb. Enroute or cruise climb is usually more than Vy. The only possible reason to use Vx enroute is for collision avoidance or training.As stated before, Vx is the best angle - where distance/height is least. If you fly slower or faster than this speed then you will be lower over the same distance in every case, it doesn't matter how you look at it and over what time frame you measure it. If you keep bringing the nose up you are increasing the wing loading and my find yourself with a power on stall which is very , very , very nasty.Short field take-off is simply a matter of rotating at the right speed, any sooner, or any later and your take-off run to 50' will be longer.

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>Pilots do not fly on assumptions, they operate the aircraft>only according to the POH for the aircraft.Correct - read your POH how to perform takeoff over the obstacle (and re-read Kevin's post - he is an ATP).Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2http://www.precisionmanuals.com/images/forum/747400.jpg

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