Soft Field Takeoff - How high prior to Vx or Vy?

JimNtexas

JimNtexas

Well-Known Member
I've been working on CFI-A stuff, and have a question about how high to climb in ground effect while accelerating to Vx or Vy.

The AFH points out that the lower you are in ground effect, the less induced drag you have. At 1/10 wing span the reduction is 50%, at 1/4 span it's about 25% .

I've generally climbed up to what I perceive as about 1/4th wing span (about ten feet). This protects from sinking to close to the ground should I have some issue, pilot error, downdraft, or windshear for example and I still benefit from accelerating in ground effect.

A CFI told me I should be about 6" above the ground while accelerating in ground effect. While this does get you to Vx/Vy a few seconds sooner, it also increases the risk of retouching the runway, at least that's how it seems to me.

I might be missing it, but I can't find anything in the AFH or PTS that says how high you level off for the ground effect acceleration portion of the soft field takeoff, just that you be in ground effect.

What are your recommendations? What are the Examiners/DPEs expecting?

TIA
 
Stay as low as possible - if the field is all mangled up and you risk dragging a wheel - go higher, but I usually just went a foot or three up and accelerated from there, 10' is too high in my opinion, but it's not going to hurt you much unless there are things to run into at the end of the runway.

Break ground as close to stall as you can without risking settling back in. As you apply power, hold the yoke back in your gut, as soon as the nose wheel loosens and comes off the ground reduce backpressure to keep the nosewheel 1" or 2" above the ground until you have the speed to break free of the contaminated surface, if you have manual flaps, you may be able to use them to help pull you out of the mud by feathering in some flaps - do not do this with electric flaps, and only when you know exactly what flying speed feels like in your airplane - otherwise your takeoff roll will most likely be longer. Right at flying speed, slightly increase your back pressure and pull the mains out of the mud. As soon as you feel you break ground, you reduce the back pressure and accelerate to the appropriate speed as low to the ground as possible before climbing out. If you are in really icky mud, it may be helpful to pull a tire out of the mud with aileron after you've got the nosewheel off the ground, but only do that if you think the drag reduction from the tire is greater than the drag you'll get from control inputs and if you think you can maintain directional control - in deep snow this is harder than you would think, especially if there are ruts in it.
 
This to me is a commonly overthought procedure. If your CFI taught you stalls and slow flight properly, you'll know when the airplane is ready to fly.

You don't need to hold the plane a foot off the runway all the way to VX or VY, as ground effect occurs up to about a wingspan height off the ground, albeit less as you go higher.

When you first lift the mains off, relax backpressure just enough to keep the plane airborne. As the speed builds, the backpressure control force will relieve itself because you'll have the plane trimmed for climb. Once you reach that point of no backpressure, let the plane do its thang.
 
gotWXdagain said:
This to me is a commonly overthought procedure. If your CFI taught you stalls and slow flight properly, you'll know when the airplane is ready to fly.

You don't need to hold the plane a foot off the runway all the way to VX or VY, as ground effect occurs up to about a wingspan height off the ground, albeit less as you go higher.

When you first lift the mains off, relax backpressure just enough to keep the plane airborne. As the speed builds, the backpressure control force will relieve itself because you'll have the plane trimmed for climb. Once you reach that point of no backpressure, let the plane do its thang.
Click to expand...

This is why it's really important to practice a soft field takeoff from an actual soft field at least once. When you takeoff from wet grass you have to keep the nose off the ground if you want to go anywhere. The plane will start to lumber into the air before it's ready to fly, then you relax backpressure and let it accelerate a bit to climb out. The way it's taught on a paved runway doesn't really make sense - there's no way realistically that a 172 is going to sit 5 feet in the air and accelerate to Vy but somehow not have enough lift to climb out of ground effect.
 
thevideographer said:
This is why it's really important to practice a soft field takeoff from an actual soft field at least once. When you takeoff from wet grass you have to keep the nose off the ground if you want to go anywhere. The plane will start to lumber into the air before it's ready to fly, then you relax backpressure and let it accelerate a bit to climb out. The way it's taught on a paved runway doesn't really make sense - there's no way realistically that a 172 is going to sit 5 feet in the air and accelerate to Vy but somehow not have enough lift to climb out of ground effect.
Click to expand...

Unless the pilot trims to maintain low-height ground effect to Vy. Which I have seen.


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gotWXdagain said:
Unless the pilot trims to maintain low-height ground effect to Vy. Which I have seen.
Sent from my iPad using Tapatalk Pro
Click to expand...

I've learned to set some forward/down trim ahead of time on a soft-field takeoff in a 172, and it makes things a lot easier.
 
JimNtexas said:
I've been working on CFI-A stuff, and have a question about how high to climb in ground effect while accelerating to Vx or Vy.

A CFI told me I should be about 6" above the ground while accelerating in ground effect. While this does get you to Vx/Vy a few seconds sooner, it also increases the risk of retouching the runway, at least that's how it seems to me.

What are your recommendations? What are the Examiners/DPEs expecting?
Click to expand...
You've answered your own question! As low as you can be without touching the ground again.
Sounds like you've studied the topic, sussed out the important information and correlated it well, but that you are now looking for someone to give you an answer that is not published. Congratulations: You've just reached the APPLICATION stage of learning! In aviation you will often find yourself in this situation. Stop looking. Act on the informed answer you've already made. Take this as a lesson to apply as you yourself advance in your career, and take this as a lesson to apply when teaching others about how to study, how to learn, and how to make decisions.
BTW, by no means do I intend to imply you should stop learning. This is a wheel-like cycle. As you APPLY what you've learned first time round, you're starting the cycle over at EXPERIENCE. That new experience is going to give you more material to assimilate, analyze, and apply again, and again, and again.
 
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Crop Duster said:
You've answered your own question! As low as you can be without touching the ground again.
Sounds like you've studied the topic, sussed out the important information and correlated it well, but that you are now looking for someone to give you an answer that is not published. Congratulations: You've just reached the APPLICATION stage of learning! In aviation you will often find yourself in this situation. Stop looking. Act on the informed answer you've already made. Take this as a lesson to apply as you yourself advance in your career, and take this as a lesson to apply when teaching others about how to study, how to learn, and how to make decisions.
BTW, by no means do I intend to imply you should stop learning. This is a wheel-like cycle. As you APPLY what you've learned first time round, you're starting the cycle over at EXPERIENCE. That new experience is going to give you more material to assimilate, analyze, and apply again, and again, and again.
Click to expand...

Jeez, it's like you've taught people to fly or somethin'....
 
Crop Duster said:
You've answered your own question! As low as you can be without touching the ground again.
Sounds like you've studied the topic, sussed out the important information and correlated it well, but that you are now looking for someone to give you an answer that is not published. Congratulations: You've just reached the APPLICATION stage of learning! In aviation you will often find yourself in this situation. Stop looking. Act on the informed answer you've already made. Take this as a lesson to apply as you yourself advance in your career, and take this as a lesson to apply when teaching others about how to study, how to learn, and how to make decisions.
BTW, by no means do I intend to imply you should stop learning. This is a wheel-like cycle. As you APPLY what you've learned first time round, you're starting the cycle over at EXPERIENCE. That new experience is going to give you more material to assimilate, analyze, and apply again, and again, and again.
Click to expand...
Love this post :)

Minor nit: correlation is a higher level than application. Application is using the information to do that takeoff or landing at a soft field (simulated or real). Correlation is when you realize the same techniques are at play for a snowy, slushy field, a landing with a suspected bad nosewheel, a takeoff from a very high density altitude airport, getting "on the step" in a water takeoff...
 
z987k said:
No but you can overfly looking for logs and other objects you don't want to run over, then drag your wheels. Then if you're happy with how all that went come in for a landing.
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Doesn't just apply to soft fields. I try to do at least a midfield flyover at every rural airport - low passes at night. You never know what critters might be lurking or crazy activities folks might be up to. And I'm not just talking about the old dudes in their Cubs or cropdusters with no radios... Little airports are often a euphoric blender of delightful crazy.
 
In a tailwhee leave the tail low and let it fly off when ready. Also popping in flaps can work to pry you off, but it's best to get a good feel for when she'll fly and add the other techniques as you progress.
 
The master of wet grass field take offs.:stir:
Screen Shot 2017-03-11 at 8.52.04 AM.png
 
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