Barrier Speed

jsstevens

jsstevens

Final Approach
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jsstevens
I’m in process of knocking off some rust and getting an IPC. I ran across something I’ve never heard of and it doesn’t make sense to me. CFI said he couldn’t explain but Cessna has it in their POH (which he showed me).

Barrier speed. It’s a column in the takeoff performance tables for a P model 172. It varies with weight getting lower as weight drops. For a takeoff of 2400 lbs, it’s 56 KIAS. He says to fly that speed until above a 50 ft obstacle, then Vx until clear of trees/hills etc. then Vy.

If Vx is best angle of climb (most altitude gain for distance traveled) what is the point of “Barrier Speed”?

John
 
Vx is the best angle climb speed once in a steady-state condition. But for a takeoff, where you are accelerating, it often results in a shorter overall distance over an obstacle if you climb at a slightly slower speed - because to get to Vx, you'd have to accelerate longer in more-level flight before climbing.

So, to clear a 50-ft tree or other close-in obstacle, fly the POH procedure with the slightly slower speed. If you need to clear a distant mountain, climb at Vx.
 
You don’t rotate at Vx…you rotate and accelerate to barrier speed by the time you cross the 50-ft “barrier” On the way to Vx.

whats Vx at 2400 lbs?
 
MauleSkinner said:
You don’t rotate at Vx…you rotate and accelerate to barrier speed by the time you cross the 50-ft “barrier” On the way to Vx.

whats Vx at 2400 lbs?
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65 KIAS.
 
MauleSkinner said:
You don’t rotate at Vx…you rotate and accelerate to barrier speed by the time you cross the 50-ft “barrier” On the way to Vx.

whats Vx at 2400 lbs?
Click to expand...
So, does it matter how far away the 50 ft obstacle is? Seems you'd want to get to Vx as soon as you can then climb until clear of any obstacles?
 
jsstevens said:
So, does it matter how far away the 50 ft obstacle is? Seems you'd want to get to Vx as soon as you can then climb until clear of any obstacles?
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Basically you’ll establish the pitch attitude for 65, and let the acceleration happen.
 
MauleSkinner said:
Basically you’ll establish the pitch attitude for 65, and let the acceleration happen.
Click to expand...
That's what I'm trained to do. So clearly you pass through this barrier speed on your way to Vx, but I'm still kind of curious what the value of publishing this specific number is?
 
jsstevens said:
That's what I'm trained to do. So clearly you pass through this barrier speed on your way to Vx, but I'm still kind of curious what the value of publishing this specific number is?
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That’s the manufacturer telling you that you won’t actually get to Vx by 50 feet.
 
MauleSkinner said:
You don’t rotate at Vx…you rotate and accelerate to barrier speed by the time you cross the 50-ft “barrier” On the way to Vx.

whats Vx at 2400 lbs?
Click to expand...

And as I recall the P Model 172 recommends 10 degrees flaps for Obstacle Clearance so what is Vx at 2400lbs and 10 degrees Flaps.

Brian
 
MauleSkinner said:
If you fly the manufacturers procedure you won’t.
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The manufacturer's procedure is to climb at 56. If you climb at 64, you won't be at 56. Seems like your argument is a tautology.
 
dmspilot said:
The manufacturer's procedure is to climb at 56. If you climb at 64, you won't be at 56. Seems like your argument is a tautology.
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You’re saying that if you establish a pitch and power for 64 the airplane wont accelerate?
 
MauleSkinner said:
Because I said you establish the pitch and power for Vx and let the airplane accelerate, and it seemed like you disagreed.
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So when you said, "that’s the manufacturer telling you that you won’t actually get to Vx by 50 feet", you are saying the manufacturer follows your procedure? I thought pilots are supposed to follow the manufacturer's procedure, not the other way around.
 
dmspilot said:
So when you said, "that’s the manufacturer telling you that you won’t actually get to Vx by 50 feet", you are saying the manufacturer follows your procedure? I thought pilots are supposed to follow the manufacturer's procedure, not the other way around.
Click to expand...

Not worth the discussion if you’re going to carry on both sides yourself. Have a nice day.
 
What you are reading in the AFM is a procedure to reproduce the desired value in a performance chart. It is the procedure Cessna specifies for the best takeoff performance.

What is altering the procedure from Vx is ground effect. In ground effect, the wing will produce more lift for a given AOA and TAS while experiencing less drag. Cessna’s procedure utilizes this aerodynamic principle.

If you are at the specified conditions and rotate to the perfect angle at the given rotation speed, the aircraft will accelerate to the barrier speed as soon as tire friction is zero and the plane is in ground effect. A few seconds later you are over the obstacle.

If you rotate at the given value and accelerate to Vx, the distances increase.
 
I’m guessing that if you’re cutting it so close that this actually matters - you’re cutting it way to close.
 
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