Comments, please

[spiderweb]

This is from a procedures guide published by a flight school:

"SOFT FIELD TAKEOFF

1. Perform all pre-takeoff checks before taxiing onto soft surface.
2. Taxi onto the runway will full up elevator.
3. Without stopping on the runway, apply full power, holding full up elevator.
4. As nose rises, reduce back pressure only enough to keep tail from striking the runway.
5. Lift off at as slow a speed as possible.
6. As soon as you lift off, lower the nose and accelerate in ground effect to 81 knots (Vx).
7. At 81 knots, pitch to climb attitude and climb out normally.
NOTE: It is very difficult, if not impossible at light weights, to accelerate in ground effect to Vy."

Please comment on that last note.

 

[Dave Krall CFII]

One can accelerate in ground effect to any speed in my experience.

This is from a procedures guide published by a flight school:

"SOFT FIELD TAKEOFF

1. Perform all pre-takeoff checks before taxiing onto soft surface.
2. Taxi onto the runway will full up elevator.
3. Without stopping on the runway, apply full power, holding full up elevator.
4. As nose rises, reduce back pressure only enough to keep tail from striking the runway.
5. Lift off at as slow a speed as possible.
6. As soon as you lift off, lower the nose and accelerate in ground effect to 81 knots (Vx).
7. At 81 knots, pitch to climb attitude and climb out normally.
NOTE: It is very difficult, if not impossible at light weights, to accelerate in ground effect to Vy."

Please comment on that last note.

 

[fgcason]

NOTE: It is very difficult, if not impossible at light weights, to accelerate in ground effect to Vy."

Please comment on that last note.

Doesn't make sense to me at least for the stuff I fly or can think of.
I'm notorious at soft/short field liftoffs then Vx before climbing out of ground effect and Vy isn't that much faster. Why wait for Vy in ground effect anyway? Ground effect to Vx, clear the obstacles then worry about Vy when you get around to it.

 

[Richard]

Perhaps they mean that it is difficult to remain in ground effect because of increasing lift. And such difficulties are more pronounced at lighter weights.

 

[poadeleted3]

Perhaps they mean that it is difficult to remain in ground effect because of increasing lift. And such difficulties are more pronounced at lighter weights.

That's the only thing that makes sense to me. If that's what they meant, though, they really should re-write it so that's what it says.

 

[Michael]

Are they saying it is difficult because you would run out of runway? how long is the runway?

 

[Skip Miller]

Doesn't make sense to me at least for the stuff I fly or can think of.
I'm notorious at soft/short field liftoffs then Vx before climbing out of ground effect and Vy isn't that much faster. Why wait for Vy in ground effect anyway? Ground effect to Vx, clear the obstacles then worry about Vy when you get around to it.

Frank, it is a soft field procedure, not soft and short. The recommended procedure gets you to altitude sooner, and in most emergencies altitude is your friend.

Yes, I know the difference in time to altitude between your procedure and the recommended procedure is small. Therefore I am not jumping up and down screaming at you. Fly it as you will.

The procedure is common wisdom for the best way to do it. Yours isn't wrong but the high and mighty procedure writers don't think yours is the best.

-Skip

 

[Len Lanetti]

Ben,

It could be a typo or simply poorly worded so the intent of the message is not conveyed. You might be the first person to read all the way to the end.

What do the folks at the flight school say?

I'm also curious as to how long the document has been around.

Len

 

[poadeleted20]

Sounds like this school doesn't recognize the change in Vy due to weight. I can accelerate in ground effect to virtually any speed of which the aircraft is capable, but it requires a whole bunch of trim change or a lot of stick force. In any event, it ain't worth bothering about. There's no point accelerating to Vy in ground effect -- once you've got Vx made, you want altitude before accelerating further.

 

[MauleSkinner]

I'd agree with Len...ask the folks at the flight school.

Having said that, I would say that the "level off and accelerate" phase (step 6) of a soft field takeoff at very light weights is so short as to be nonexistent in a lot of airplanes...simply pitching to the attitude to attain Vy will get you there.

Fly safe!

David

 

[gismo]

This is from a procedures guide published by a flight school:

"SOFT FIELD TAKEOFF
.
NOTE: It is very difficult, if not impossible at light weights, to accelerate in ground effect to Vy."

Please comment on that last note.

I think the comment is an attempt to accomodate the fact that at light weights where there is significant climb performance available, it would require a rather rapid pitch down as soon as the wheels leave the ground in order to remain in ground effect, and this is indeed true of many airplanes. Conversely if you are loaded very heavy (perhaps too heavy) the airplane might not even climb above a few feet AGL with no pitch change.

What's missing from the note (and procedure) is that prior to breaking ground you should pitch to an attitude that is expected to produce an airspeed 5-10% above stall rather than what you'd get with full up elevator and once the plane is free of the ground you should pitch for an attitude that would give you Vy (or Vx if there are obstacles) at a rate that keeps you within a few feet of the ground until Vx is reached. If you do all that you will remain in ground effect exactly as long as necessary to generate the maximum performance without any danger of climbing out of ground effect at too low a speed. And at lighter weights, the time needed close to the ground will be much less than it would be at MGW and if you don't manage to pitch forward rapidly enough, you will begin climbing before reaching Vx or Vy. Also part of the issue here is that the procedure probably assumes the same Vy for light and heavy weights and we all know the reality is that Vy decreases with a decrease in weight (at the square root of the change in weight).

 

[SkykingC310]

I can remember learning soft-field takeoffs and I remember the day it finally clicked. I was struggling with the liftoff then leveloff in ground effect. I was always very abrupt and choppy with my control inputs. It's as if I were forcing the plane to do something it didn't want to do. Then, one day, my regular instructor was unable to make our appointment, so I went out with a different guy. He had a way of explaining the procedure that really made sense to me. After he saw the problems I was having, he suggested to me that I think of the airplane as pushing a big ball down the runway. As I reached liftoff speed, I would simply roll up the side of the ball and ride on top of it. I don't know why, but this worked for me. I'm not saying it's the solution for everyone, but for me, it was. From that point on, my soft-fields were awesome. All that is to say, well, a few things. One, go up with a different instructor once in a while, especially if your struggling with something. Maybe they'll have the perfect analogy that will get you past that plataeu. Secondly, I really appreciate flight instructors.

 

[TMetzinger]

Another way of putting it is that you should not allow the plane to continue climbing out of ground effect (it might, you know) until you have reached at least Vx, and that at light weights once you release the forward pressure you used to keep the airplane in ground effect to Vx you're going to climb well - no sense waiting for Vy, because you've ALREADY used more runway (because it's soft) than normal - get high fast in case there are obstacles.

It's probably the procedure that they feel works best in all situations, soft-only and soft-short.

 

[mgkdrgn]

Somehow don't sound quite right to me ... wonder what they meant, as opposed to what they wrote.

This is from a procedures guide published by a flight school:

"SOFT FIELD TAKEOFF

1. Perform all pre-takeoff checks before taxiing onto soft surface.
2. Taxi onto the runway will full up elevator.
3. Without stopping on the runway, apply full power, holding full up elevator.
4. As nose rises, reduce back pressure only enough to keep tail from striking the runway.
5. Lift off at as slow a speed as possible.
6. As soon as you lift off, lower the nose and accelerate in ground effect to 81 knots (Vx).
7. At 81 knots, pitch to climb attitude and climb out normally.
NOTE: It is very difficult, if not impossible at light weights, to accelerate in ground effect to Vy."

Please comment on that last note.

 

[spiderweb]

Thank you for your comments.

A few notes:

1) Even if you know the name of the flight school, I think for the sake of our discussion, it is best to keep the name out of it. Suffice it to say, it is from an excellent flight school.

2) The flight school has a complete set of procedures for every type of aircraft on the line. This segment comes from the procedures for a hi-performance retract.

3) The PIM for this aircraft publishes TWO (2) sets of Vy and Vx speeds--one each for gear down and one each for gear up. In this case, one could probably accelerate to the gear-down Vx in ground effect, with gear DOWN (of course), then continue with the procedure. However, because the gear-down Vx airspeed is slower than gear-up Vx (81 KIAS), I think it is probably safer to do it they way they proscribe it: get your 81 KIAS first. When you retract gear at that airspeed, you are at gear-up Vx.

4) For reference, gear-up Vy is 95 KIAS.

5) Although there is a published gear-up Vy, I don't know what the point of that would be beyond having more information.

6) Gear retraction was taught to me in this way (by a few CFIs): get the gear up once you have a positive rate, and no possibility of landing on remaining runway. I always wait at least until Vx to do this. (Maybe I should say, I am always at least at Vx before I have a positive rate.)

 

[Dave Krall CFII]

Thank you for your comments.

A few notes:


6) Gear retraction was taught to me in this way (by a few CFIs): get the gear up once you have a positive rate, and no possibility of landing on remaining runway. I always wait at least until Vx to do this. (Maybe I should say, I am always at least at Vx before I have a positive rate.)

It's safer in some models like Cessnas to wait until all obstacles are cleared before gear retraction, because during retraction more drag is created by the transitioning gear, which can be a significant issue in low clearance take offs.

 

[spiderweb]

It's safer in some models like Cessnas to wait until all obstacles are cleared before gear retraction, because during retraction more drag is created by the transitioning gear, which can be a significant issue in low clearance take offs.

Funny. As I was typing 6), I was thinking the same thing. It is probably safer not to fly retract Cessnas!

 

[Dave Krall CFII]

Funny. As I was typing 6), I was thinking the same thing. It is probably safer not to fly retract Cessnas!

Not fly Cessna retracts?
That might be going a bit too far although it probably would ultimately be safer...

 

[spiderweb]

Not fly Cessna retracts?
That might be going a bit too far although it probably would ultimately be safer...

Just kidding.

 

[tonycondon]

Ah the fun cessna retracts. Genius design, hydraulic pressure holds the gear down, so if you lose hydraulic pressure, just belly it in, no sense in maybe having gravity do that job, noooo.

I love em though. 182RG is a hell of a plane.

 

[spiderweb]

I still don't understand, though, why it would be impossible to accelerate to any speed in ground effect, unless it means stick force.

 

[Richard]

I still don't understand, though, why it would be impossible to accelerate to any speed in ground effect, unless it means stick force.

Ben, I think you got it. They worded it wrong. And they do mean stick force.

Try it sometime, you're pushing down on the elevator and still climbing as the speed increases. But put yourself in their shoes, do you really want the avg renter doing that in your plane? And if there is an accident and it is found they followed your printed instructions you just became very liable.

 

[Nav8tor]

I still don't understand, though, why it would be impossible to accelerate to any speed in ground effect, unless it means stick force.

I don't think they mean that its impossible to accelerate while in ground effect, just that it is nearly impossible to keep the airplane in ground effect as you accelerate.

 

[SkykingC310]

Airplanes are designed to fly. It's what they were born to do. Much like many of us. You can look at ground effect through the 'eyes' of Bernoulli. As you increase speed the pressure over top of the wing will be decreasing. Pressure under the wing is building and building in a 'space', if you would, that is not increasing, assuming you are straight and level. It will take more and more effort to hold the plane in ground effect as that pressure differential increases. It's kinda like that 'ball' I was talking about earlier in this thread. Once you're on top it begins to 'inflate' directly proportionate to your velocity.

 

[spiderweb]

Very cool. This post turned out spark a lot of conversation!

 

[SkyHog]

Airplanes are designed to fly. It's what they were born to do. Much like many of us. You can look at ground effect through the 'eyes' of Bernoulli. As you increase speed the pressure over top of the wing will be decreasing. Pressure under the wing is building and building in a 'space', if you would, that is not increasing, assuming you are straight and level. It will take more and more effort to hold the plane in ground effect as that pressure differential increases. It's kinda like that 'ball' I was talking about earlier in this thread. Once you're on top it begins to 'inflate' directly proportionate to your velocity.

Does that mean, then, that after a certain speed in ground effect, it could be impossible to continue to accellerate, since you'd run out of elevator authority to keep the nose down?

 

[tonycondon]

you wouldnt get that far nick. your speed would stabilize before you ran out of forward pressure.

 

[TMetzinger]

unless you're flying a lifting surface so close to the ground and so fast that you reach the limits of compressibilty in the available air volume.

 

[tonycondon]

well, trying to keep it realistic...

 

[Ed Guthrie]

What's missing from the note (and procedure) is that prior to breaking ground you should pitch to an attitude that is expected to produce an airspeed 5-10% above stall rather than what you'd get with full up elevator.

I would agree with the above advice on grass where misconceptions abound and "soft field technique" is in reality just "don't abuse the landing gear on mole tunnels any more than necessary" technique. However, if the surface really is soft field and a full effort, soft field technique really is required to get airborne, then the technique you suggest will leave lift unaccessed and may result in the landing gear never lifting free of the high drag surface.

 

[gismo]

I would agree with the above advice on grass where misconceptions abound and "soft field technique" is in reality just "don't abuse the landing gear on mole tunnels any more than necessary" technique. However, if the surface really is soft field and a full effort, soft field technique really is required to get airborne, then the technique you suggest will leave lift unaccessed and may result in the landing gear never lifting free of the high drag surface.

That's true, for the absolute maximum lift in ground effect you need an AOA slightly higher than the stalling AOA, but as you said, few pilots ever take off with the wheels mired in mud, and IME the difference between the best AOA and the one I suggested is only about 10%.

 

[MauleSkinner]

I would agree with the above advice on grass where misconceptions abound and "soft field technique" is in reality just "don't abuse the landing gear on mole tunnels any more than necessary" technique.

True...just because it's grass doesn't make it a "soft field". Mole tunnels make it a "rough field", which is essentially the same technique, but there's some really fine grass out there (Speaking of which, I have to get out and mow my really fine airstrip )

Just as an attempt to correct one of those misconceptions, the 10% increase in runway (or whatever your POH/AFM specifies) required for grass is NOT for soft-field technique...it is the increase required when using short-field technique on grass. Soft field technique will increase the distance substantially more.

Fly safe!

David

 

[Dave Siciliano]

Don't forget one can adjust the trim also! If the plane is trimmed for Vx (and you may want to keep it there to break gound) adjusting trim down can make the task a little easier. One can also begin with less trim, pull back a little to break ground, then, release slightly to bring the nose down.

In my planes, where I accellerate in ground effect anyway to keep CHTs low, I normally use less trim for takeoff than what the POH calls for. If I don't, as soon as I break ground, I wind up cranking in forward trim anyway.

Best,

Dave

 

[Dave Krall CFII]

That's true, for the absolute maximum lift in ground effect you need an AOA slightly higher than the stalling AOA, but as you said, few pilots ever take off with the wheels mired in mud, and IME the difference between the best AOA and the one I suggested is only about 10%.

So then which would be best, the best AoA or the one you suggested?

 

[gismo]

So then which would be best, the best AoA or the one you suggested?

You would do best with the AOA that provides the maximum lift in ground effect which would be slightly higher than the stalling free air AOA, maybe an extra 2 degrees. Of course that's the full flaps stalling AOA which is at least a few degrees shy of the flaps up stalling AOA. I'd guess at something like +15 to 16 degrees, but you could measure it yourself on a calm day by fully stalling the plane six inches above the runway.

 

[poadeleted20]

That's true, for the absolute maximum lift in ground effect you need an AOA slightly higher than the stalling AOA,

Unfortunately, for most light planes, if you lift off that way, it will leave you behind the power curve (i.e., with insufficient thrust to overcome drag), and you will be unable to accelerate without lowering the nose and thus losing some lift, in which case you will settle back to the runway. There's a very small range in which this is a problem, but it does exist. Practice on a long, hard runway to get the feel of just how high you want the nose to be before you try this on a short, soft one.

 

[gismo]

Unfortunately, for most light planes, if you lift off that way, it will leave you behind the power curve (i.e., with insufficient thrust to overcome drag), and you will be unable to accelerate without lowering the nose and thus losing some lift, in which case you will settle back to the runway. There's a very small range in which this is a problem, but it does exist. Practice on a long, hard runway to get the feel of just how high you want the nose to be before you try this on a short, soft one.

I agree completely and that's part of the reason I believe that it's better to aim for a little less pitch than max lift. At such a high AOA you have to dump the nose real quick if a gust happens to lift you up a few feet as soon as the wheels break loose. I think a better technique might be to use a bit less pitch and "horse" the wheels out of the muck by yawing the plane like you do to get floats unstuck from the water. That said, when you are truly mired in muck on the takeoff roll, the plane should accelerate quickly once the wheels clear the ground so as long as you pitch down some, you should achieve sufficient airspeed to stay in the air.

 

[Ed Guthrie]

Unfortunately, for most light planes, if you lift off that way, it will leave you behind the power curve (i.e., with insufficient thrust to overcome drag), and you will be unable to accelerate without lowering the nose and thus losing some lift, in which case you will settle back to the runway.

For most light aircraft the "behind the power curve" AOA is well beyond the tail strike AOA. IOW, you can't get where Ron suggests you might find yourself.

 

[Ed Guthrie]

Unfortunately, for most light planes, if you lift off that way, it will leave you behind the power curve (i.e., with insufficient thrust to overcome drag), and you will be unable to accelerate without lowering the nose and thus losing some lift, in which case you will settle back to the runway.

Let me see if I understand your premise correctly. An aircraft begins with the wheels mired in some high drag media (mud, slush, whatever). The aircraft has sufficient excess horsepower such that it can accelerate the wheels through that high drag media and thereby create enough lift to increase altitude a foot or so such that the wheels clear the high drag media. But at that point, with the wheels now clear of the high drag media and overall drag now very much reduced, you actually believe horsepower will somehow at this point be limited and the aircraft will be unable to climb another foot or accelerate? If that were even remotely the case the wheels would never leave the high drag media.

IOW, if the aircraft cleared the mud/slush/whatever, it definitely isn't at that point on the back side of the power curve such that a descent will be required in order to accelerate/climb.

I agree completely

You need to reconsider the drag function, too. Ron's premise is absolutely bogus.

and that's part of the reason I believe that it's better to aim for a little less pitch than max lift.

As stated before, if you do that that aircraft may not fly. Soft field is for the most part an experimental test event. We don't really know how much excess lift we'll have available. Furthermore, a nose over is a decided possibility. Anything less than a max lift effort is somewhat flirting with disaster.

At such a high AOA you have to dump the nose real quick if a gust happens to lift you up a few feet as soon as the wheels break loose.

Once the wheels lift loose there will be buckets of excess horsepower, plus, settling back to the soft field in that attitude is nothing worse than a soft field landing. Big deal.

I think a better technique might be to use a bit less pitch and "horse" the wheels out of the muck by yawing the plane like you do to get floats unstuck from the water.

This technique works for a floats because floats are hydrodynamic lifting devices; the excess speed lifts the float out of the water. Tires are not exactly highly efficient hydrodynamic lifting devices.

That said, when you are truly mired in muck on the takeoff roll, the plane should accelerate quickly once the wheels clear the ground so as long as you pitch down some, you should achieve sufficient airspeed to stay in the air.

Exactly.