Short field climb speeds in a light single

RipCurl

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The POH of the Arrow I've been flying lists the obstacle clearance speed on the takeoff performance chart. It increases depending on weight, from 54 up to 62 knots. Vx with the gear down is 72. Is this lower initial climb speed basically a best angle of climb adjusted for the 25* of flaps that I have cranked in there (as per the POH)?
 
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MauleSkinner

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Without having an Arrow manual handy...what configuration does the manual say the 72 knots Vx is for?

My guess is that Vx of 72 is for flap 25, but it gets up quick enough at the 54-62 numbers that it will clear 50 feet in a shorter distance than it will if you accelerate to 72.

In other words, you clear the trees at the departure end with the 54-62 number, and accelerate to 72 to clear the mountains beyond.

Fly safe!

David
 

Singlecoil

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25 degrees seems like a lot of flaps to takeoff with in an Arrow, but if that is what the POH is saying, then that's what you do. You're right, the 54-62 is essentially the Vx in that listed configuration. The Vx of 72 with the gear down is probably a gross weight number. It too will change with weight, getting lower at lighter weights. I don't think I can type the formula here, but I'll try.

This works for any speed except Vne.

b = Weight b

c = Weight c


Vc = Vb * Sq.Root of (c/b)

So if the 72 was for gross weight. Take your current weight, divided by gross weight. Take the square root of that, multiply that new number by 72 to get your new climb speed at your current weight.
 

Lead Sled

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Singlecoil said:
25 degrees seems like a lot of flaps to takeoff with in an Arrow, but if that is what the POH is saying, then that's what you do. You're right, the 54-62 is essentially the Vx in that listed configuration. The Vx of 72 with the gear down is probably a gross weight number. It too will change with weight, getting lower at lighter weights. I don't think I can type the formula here, but I'll try.

This works for any speed except Vne.

b = Weight b

c = Weight c


Vc = Vb * Sq.Root of (c/b)

So if the 72 was for gross weight. Take your current weight, divided by gross weight. Take the square root of that, multiply that new number by 72 to get your new climb speed at your current weight.
And I would suggest that if you have to go through all of those mental gyrations to be able to determine if you're going to be able to safely operate out of a particular airport you choose a different airport.

'Sled
 

NYCPilot

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Most figures for the Vx speed will be determined for gross weight operations. Therefore, in order to obtain maximum climb performance under lighter conditions, you will need to reduce the value by some margin. It is usally best to always use the published value for standardization and checkride purposes. Usually the difference is marginal. If you were even in a situation that required a really narrow obstacle clearance, you probably should think twice about departing.

I believe that the POH for the Arrow will state that the flaps should be set to 25 degrees for short and soft field takeoffs requiring an obstacle clearance.

The inital Vx speed is usually lower with the gear down. Once the gear comes up, you'll need to increase the speed to the published value for gear up. Afte obstacle clearance you should accelerate to Vy until at a safe altitude. Once you have enough altitude you should transition into a cruise climb speed. This will give you better forward speed, visibility and cooling.
 

Garritto

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My 2 cents

Just finished up my CFI in the arrow, and was wondering the same thing. Let me guess......Vx (gear down) is 72, Vy (gear down) is 78. Vx (gear up) is 78, and Vy (gear up) is 90. Since these numbers are absolutes, they are calculated at gross weight. The 54-62 knots, like you said, are adjusted for the 25 degrees of flaps you have put in, because with the flaps in the wing has changed shape. My arrow POH calls for accelerating to 55-65 knots immediately after takeoff. After the gear is retracted, the climb would be accelerated to 78 (Vx with gear up) if obstacle clearance was an issue.

In my arrow POH, Vx is 72 knots for gear down, flaps up. The key is that at gross weight, with flaps up, and gear down, 72 knots will give you the best angle of climb. This number is adjusted for differing weights, but there is little sense in adjusting it for different flap settings too.
 

RipCurl

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Thanks for the answers. Leadsled & NYC Pilot, I couldn't agree more. What I'm essentially looking for is a way (more properly, the correct way) to explain the theory behind the numbers to my future students.

For clarification, the 72 figure is Vx gear down, flaps up, max gross. Proper short field technique (as per POH) is: 25* flaps, rotate just prior to liftoff speed (59 @ max gross), pitch for obstacle speed (62 @ max gross), then Vx or Vy once clear of obstacle, depending on what's ahead of you. Gear and flaps come up after primary obstacle is clear.

My instructor's opinion is that it would take a greater distance to accelerate up to 72 before climbing to 50' than it would to simply climb at the lower speed, despite the slightly lower climb gradient. I have a feeling that it is a combination of that and the fact that the flaps (and weight) will decrease Vx.
 
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midlifeflyer

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RipCurl said:
My instructor's opinion is that it would take a greater distance to accelerate up to 72 before climbing to 50' than it would to simply climb at the lower speed, despite the slightly lower climb gradient. I have a feeling that it is a combination of that and the fact that the flaps (and weight) will decrease Vx.
I have a feeling your instincts are right. There are two things at work. One is that Vx with flaps is different than Vx without. The other is that if you "pitch" for Vx, the airplane doesn't get from rotation to Vx instantaneously. It takes time to accelerate to Vx. If you pitch for Vx, you'll hit that 50' speed on the way up - at about 50'.
 

dmspilot00

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Every aircraft handbook I have seen has used a much lower speed for a short-field takeoff with obstacle clearance than Vx.

For example, the older 172s specify that short-field takeoffs are made with flaps retracted, but the airspeed given for obstacle clearance is still lower than Vx. Like RipCurl and midlifeflyer said: in order to accelerate Vx in the first place, you would have to temporarily decrease your climb angle. By the time you accelerate to and reach Vx, you're at 50 feet already.

If the obstacle was three times as tall and three times farther from the runway, I think using Vx would make more sense.
 

pilotmiketx

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Whatever you do, make sure you use flaps 25 for short field takeoff. Beat it into your students. I'm pretty sure it's the same for everthing from Cherokee 140s thru Saratoga. You will not make it off with flaps up.
 
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