172 Vx vs Short Field Takeoff speed

dr650

dr650

Well-Known Member
I'm reading through the POH for the 172RG I'm flying and I'm confused about Vx and the short-field takeoff speed. To me it seems they should be the same, however, Vx is listed as 67 KIAS and the short-field climb out speed is listed as 63 KIAS. Just looking for some clarity.
 
Any added parasite drag lowers the performance airspeeds of an airplane. Im guessing the difference is because of the 10º of flaps.
 
Read the expanded section of the Normal Procedures section.

I don't have my POH handy, but as I recall, you want to lift off as soon as the airplane will fly, and accelerate to reach Vx by the time you reach the theoretical 50' obstacle height
 
d_mckerrow06 said:
I'm reading through the POH for the 172RG I'm flying and I'm confused about Vx and the short-field takeoff speed. To me it seems they should be the same, however, Vx is listed as 67 KIAS and the short-field climb out speed is listed as 63 KIAS. Just looking for some clarity.
Click to expand...

Vx is a steady state best angle of climb speed, meaning that if you could instantly be at that speed, you would have the best altitude gain per horizontal distance.

That isn't the scenario in a short-field take-off. You're starting from zero KIAS and accelerating to optimal climb speed. This acceleration is essentially wasted as far as gaining altitude is concerned and the longer you accelerate, the more you eat up the horizontal distance that is available in the takeoff flight path.

If the obstacle is far away, no big deal, since the distance you use to accelerate is only a small percentage of the total distance to the obstacle, and you more than make up for that by having an optimal climb angle. But if the obstacle is close in, the longer ground run has a larger role in determining the horizontal distance to clear the obstacle, and you'd be better off to leave the ground at a non-optimal speed to begin your climb earlier.

Here is a diagram showing your takeoff technique choices:



climb.png





(One additional factor is that Vx is a clean airspeed; if you depart with some non-zero flap setting, Vx will be slightly slower, even ignoring the issue I just described.)
 
Mike H said:
you want to lift off as soon as the airplane will fly, and accelerate to reach Vx by the time you reach the theoretical 50' obstacle height
Click to expand...
:yeahthat:
Get the plane in ground effect; accelerate the aircraft to vx and commence your climbout.
 
The Vx of 67 is gear up while your 63 KIAS is gear down. It initially seems like you should pull the gear up right away, but with most RG (172 RG, 182 RG, V-35, etc), airplanes I have flown the process of gear retracting increases drag momentarily. If obstacles are close this extra drag could be enough to prevent you from clearing them. Once clear, bring the gear up while maintaining the same pitch angle and you will find the airplane accelerating to... 67 (if at max GW). If there are no other obstacles (such as rising terrain), adjust your airspeed.
 
Mike H said:
Read the expanded section of the Normal Procedures section.

I don't have my POH handy, but as I recall, you want to lift off as soon as the airplane will fly, and accelerate to reach Vx by the time you reach the theoretical 50' obstacle height
Click to expand...
When I was taught shortfield takeoffs I was always told to accelerate to normal rotation speed (55 for a 172) and then commence an immediate climbout at vx (I believe it was 56 knots for the 172 I flew). Keeping it in ground effect until Vy+ does you good in high density altitude conditions without immediate obstacles to clear , but that really has nothing to do with the way the PPL PTS (Not sure about Comm, almost there, but not yet) lays out short field takeoff procedures IMO.
 
A Short Field takeoff is designed to clear a 50ft obstacle near the end of the runway. That said, 50ft is your initial goal. That can safely be obtained at a slightly lower KIAS than Vx (published in the POH.) Of course, once you reach 50ft AGL, you should have accelerated to Vx. You can then continue your climb at Vx as if you need more climb performance than Vy would provide.

Keep in mind that any distant obstacle can always be cleared by climbing while in a rectangular pattern overtop of the airfield. There is no need to climb toward an obstacle at Vx from an airfield if you can arrive at that altitude safer while climbing at Vy in a rectangular pattern over the airfield, then departing the area.

Fly safe! Always use every option to make your flight safe and competent.:beer:
 
Thanks, yeah I think the biggest thing is that I forgot Vx is a clean airspeed, when the gear goes up I'm then accelerating to Vx from 63 KIAS.
 
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