Why accelerate to Vy in ground effec w/ soft-field?

Mattio

Mattio

Well-Known Member
So I was doing some thinking about the FAA and overall concept that you should accelerate all the way to Vy (if applicable) in ground effect and then climb out. Why?

I can understand accelerating to a safer speed that is higher than normal rotation speed to make sure you don't slow down with the loss of ground effect (increase in drag) or settle back on the rough-surfaced runway but if we can safely climb out of ground effect at Vx, then why can't we just begin the climb out of ground effect upon reaching Vx and establish a Vy attitude if there are no obstacles instead of waiting until reaching Vy to begin the climb out. I don't understand the thinking of needing to keep the aircraft in ground effect for that extra 20 knots or whatever.

Anybody know if I'm missing something?

Matt
 
Mattio said:
then why can't we just begin the climb out of ground effect upon reaching Vx and establish a Vy attitude if there are no obstacles instead of waiting until reaching Vy to begin the climb out
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Why do you think you can't do that? Both the Airplane Flying Handbook and the PTS allow for that technique when appropriate.
 
tgrayson said:
Why do you think you can't do that? Both the Airplane Flying Handbook and the PTS allow for that technique when appropriate.
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In the AFH, it says to accelerate to Vy unless obstacles exist. Then it sort of contradicts itself and says that it should be held in ground effect until at least Vx. In the PTS it says accelerate to Vx or Vy "as appropriate".

I don't know why you would teach someone to stay in ground effect all the way until Vy. Besides getting them good at holding the a/c in ground effect, it seems pointless.
 
Mattio said:
I don't know why you would teach someone to stay in ground effect all the way until Vy. Besides getting them good at holding the a/c in ground effect, it seems pointless.
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The aircraft accelerates more rapidly in ground effect, so it will reach Vy more quickly. If your goal is to truly have the most altitude in a given amount of time, this technique is superior.

Your references to the AFM don't sound contradictory to me and seem compatible with the Airplane Flying Handbook and the PTS. Regardless, the AFM should be interpreted in light of what we know about maximizing aircraft performance.
 
tgrayson said:
The aircraft accelerates more rapidly in ground effect, so it will reach Vy more quickly. If your goal is to truly have the most altitude in a given amount of time, this technique is superior.

Your references to the AFM don't sound contradictory to me and seem compatible with the Airplane Flying Handbook and the PTS. Regardless, the AFM should be interpreted in light of what we know about maximizing aircraft performance.
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I was referencing the AFH (Airplane Flying Handbook) not the AFM.
 
minitour said:
Pointless indeed. Vy is an attitude, not an airspeed.

-mini
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tgrayson said:
The aircraft accelerates more rapidly in ground effect, so it will reach Vy more quickly. If your goal is to truly have the most altitude in a given amount of time, this technique is superior.

Your references to the AFM don't sound contradictory to me and seem compatible with the Airplane Flying Handbook and the PTS. Regardless, the AFM should be interpreted in light of what we know about maximizing aircraft performance.
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I realize that it'll reach Vy airspeed quicker if we accelerate in ground effect or even just in level flight, but if it's the attitude, not the airspeed that gets you the best rate (like Mini, this is what I've always taught), then what good does this do us? I'd be interested to try both methods and see which one got the best results.
 
Mattio said:
but if it's the attitude, not the airspeed that gets you the best rate (like Mini, this is what I've always taught), then what good does this do us?
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IMHO, it doesn't.

Get off the ground, then put the nose up into the Vy attitude and let it climb out of ground effect when it's ready. Why hold the plane down close to the ground when it's (clearly) ready to fly. That airspeed will jump in a hurry once you get it out of ground effect in the Vy pitch attitude.

-mini
 
Mattio said:
I was referencing the AFH (Airplane Flying Handbook) not the AFM.
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The parts you cited aren't contradictory, whatever volume they came from.
accelerate to Vy unless obstacles exist. it should be held in ground effect until at least Vx. In the PTS it says accelerate to Vx or Vy "as appropriate".
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They all say the same thing - stay in ground effect until Vy unless Vx is more appropriate (such as when there are obstacles to clear). But don't climb out of ground effect until at least at Vx.
 
Mattio said:
but if it's the attitude, not the airspeed that gets you the best rate (like Mini, this is what I've always taught)
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It isn't. Achieving and maintaining Vy will result in a specific attitude at a particular weight and density altitude, but trying to force the attitude prior to achieving Vy will result in poorer performance, because you will using some of the airplane's excess power accelerate, rather than climb.

Vy is a speed where your excess power is maximized and that depends on your powerplant output and the airplane's AoA (airspeed). It has nothing to do with attitude. You will see the concept come into play as you accelerate in ground effect. As the aircraft gets faster, it has more excess power, which increases the acceleration even further. Even though the airplane will gain no altitude during this acceleration, when it finally begins its climb at Vy, it will overtake the airplane that started climbing at Vx and accelerated to Vy.

Here's a chart showing the concept:

Best%20Climb%20Chart.png
 
That's a good chart and explanation TG. One problem I seem to have is accelerating to Vy with the flaps set to the recommended setting. What affect would flaps have on the situation? It is pointless to accelerate to Vy with flaps down. But no way am I going to teach anyone to retract the flaps in ground effect.

Another question is... If this is the real reason the FAA wants us to accelerate to Vy first, why wouldn't they recommend we put the nose over on every takeoff until reaching Vy?

Personally, I have known it to be a good skill builder to maintain ground effect while accelerating. Continually increasing forward control inputs are required to do it correctly. And after all, the PTS is about displaying piloting ability, not necessary always the proper way to fly the plane.
 
lhornaday said:
That's a good chart and explanation TG. One problem I seem to have is accelerating to Vy with the flaps set to the recommended setting. What affect would flaps have on the situation? It is pointless to accelerate to Vy with flaps down. But no way am I going to teach anyone to retract the flaps in ground effect.
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Your Vy with flaps down is going to be lower than your clean Vy. If the soft-field procedure calls for use of flaps, I teach to accelerate to the appropriate Vy for the flap setting, climb away from the ground, and then once at a safe altitude, retract the flaps and pitch for the clean Vy. If there are obstacles, then I teach to accelerate to the appropriate Vx, climb out of ground effect, and stay at the appropriate Vx until clear of obstacles, then accelerate to the clean Vy while retracting the flaps.

If there is no published Vx/Vy for the flap setting, then the best you can do is pitch for whatever Vx/Vy is published, and then retract the flaps when safely away from the ground.

I agree that the flaps should definitely not be retracted while in ground effect.
 
lhornaday said:
What affect would flaps have on the situation? It is pointless to accelerate to Vy with flaps down. But no way am I going to teach anyone to retract the flaps in ground effect.
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As Xiph pointed out, Vy with flaps is slower than with; otherwise, the process should work identically. Personally, I have no problem retracting flaps in ground effect, and in fact this technique probably maximizes performance.


If this is the real reason the FAA wants us to accelerate to Vy first, why wouldn't they recommend we put the nose over on every takeoff until reaching Vy?
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I don't know if this is the real reason for the FAA recommendation. I was merely defending the practice. I wouldn't make the assumption that the FAA technique is *always* optimal for a given airplane or situation.

Note, too, that the profile I depicted is an idealization. Realistically, you can't make instantaneous changes to the flight path. You can't wait until exactly Vy before starting your climb or you're likely to overshoot the speed. Since power required varies as the cube of your airspeed, only a slight overshoot of your airspeed may undo your use of an otherwise optimal technique.
 
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