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

[tgrayson]

TGrayson, where did you get this chart from?

I made the chart, but the underlying theory is based on the "Energy Height" concept that you will find in any decent aircraft performance textbook. The one in particular that I used was "Aircraft Performance and Design", by John D. Anderson.

 

[Mattio]

Ah, ok. As far as stall recovery goes, I've always advised establishing Vy attitude and get the aircraft climbing away from the "ground" ASAP, rather than accelerating to Vy in level flight. How would you tie this in to stall recovery?

Looking at the chart, it looks like there's a small period of time where establishing Vy attitude is going to keep you further away from the ground....

 

[tgrayson]

Ah, ok. As far as stall recovery goes, I've always advised establishing Vy attitude and get the aircraft climbing away from the "ground" ASAP, rather than accelerating to Vy in level flight. How would you tie this in to stall recovery? Looking at the chart, it looks like there's a small period of time where establishing Vy attitude is going to keep you further away from the ground....

You're right, if ground contact is imminent, you won't want to accelerate to Vy. But then, there's nothing special about the Vy pitch *attitude*, either, except that it will eventually result in Vy (maybe). For terrain clearance, you're better off at Vx or slower; the Vy pitch attitude only makes sense to me if you've already assured yourself of obstacle clearance and wish to resume a best rate climb.

I'm not sure I like the "pitch for Vy" concept; that will probably work fine in airplanes and environments you're familiar with, but the pitch attitude for Vy will vary with the quantity of excess power the aircraft has, and that depends not only on the airplane's rated power, but also the weight and density altitude. If you try to achieve the same pitch taking off from Leadville, CO, as you do in Orlando, you may stall the airplane. Your optimal takeoff attitude will be much shallower when excess power is low because your flight path is shallower. I tend to slowly increase the backpressure as the airspeed increases, with the rate of backpressure increase proportional to how fast I'm accelerating. That let's the airplane essentially determine its own pitch angle.

Also, note that in my chart the horizontal axis is time, not distance, so the profile is not exactly what you would see if you were standing on the ramp watching an airplane takeoff.

 

[Mattio]

Thanks for the feedback TG. I've always been barking at students that practice (and have been taught to practice) stall recovery by leveling the airplane until reaching Vy airspeed and then climbing out. I've always barked at them to get a positive rate of climb going and simulate that it is happening close to the ground, the most critical situation. I suppose a real situation, it would depend on the circumstances.

 

[minitour]

If you try to achieve the same pitch taking off from Leadville, CO, as you do in Orlando, you may stall the airplane.

Doubtful. Same pitch. At higher altitude, your Vy reference airspeed will be less than at a lower altitude.

The pitch attitude is the same.

Yes, it varies between aircraft types. Know your airplane.

-mini

 

[tgrayson]

Doubtful. Same pitch. At higher altitude, your Vy reference airspeed will be less than at a lower altitude. The pitch attitude is the same.

Performance decays faster than Vy does and this is probably true on any normally aspirated airplane. The C172N shows a 5 knot reduction in Vy from 0 to 10,000 feet, which translates into an AoA increase of about 1 degree. According to the POH, the flight path is reduced by 2 to 3 degrees, so your pitch attitude will constantly decrease with density altitude. This may not matter that much when the aircraft has plenty of excess power, but it can be a big deal for a normally aspirated piston.

 

[MidlifeFlyer]

Doubtful. Same pitch. At higher altitude, your Vy reference airspeed will be less than at a lower altitude.

The pitch attitude is the same.

Maybe in commercial aircraft but not the piston singles I fly. I did my private and instrument on the east coast, my commercial and CFI in Colorado.

When I go back to the lowlands I have to remind myself the Vy will be more nose-high and recognizing the Vy will be more nose low is #1 on the hit parade when a lowlander flies out here for the first time.

 

[lhornaday]

Maybe in commercial aircraft but not the piston singles I fly. I did my private and instrument on the east coast, my commercial and CFI in Colorado.

When I go back to the lowlands I have to remind myself the Vy will be more nose-high and recognizing the Vy will be more nose low is #1 on the hit parade when a lowlander flies out here for the first time.

Sounds like an experiment to try on my next day of hauling death divers up to 10k. I've never really noticed much of a pitch attitude change as a climb.

 

[Blip16]

I teach to Vx because none of the new planes I fly have published speeds for either climb speed w/ flaps. You can interp. The short field takeoff to be Vx w/ flaps but I always just use published Vx because it is close to what Vy w/ flaps would be. There is no point in staying in ground effect w/ a negative aoa, let the aiplane fly itself out which is always around published Vx

 

[shdw]

When I go back to the lowlands I have to remind myself the Vy will be more nose-high and recognizing the Vy will be more nose low is #1 on the hit parade when a lowlander flies out here for the first time.

I agree with both techniques. I also trust midlife and understand tgrays post, knowing the level will be different, visually. However, won't the yoke pressures be the same?

If I am not mistaken the pressures won't change, nor should the kinesthetic feel.

This is why I teach a visual pilot to use their senses like we teach instrument pilots to use their instruments. First we explain what the instruments (our senses) are:

• Vision: To see aircraft attitude. Most effective when focus is on or near the horizon.
• Hearing: Our ears recognize volume change well, but do a horrible job judging level. Optimally used to recognize speed changes or when making throttle changes. Note: Each notable increase, moving swiftly, equates to approximately 100-200 rpm in a cessna. Know your aircraft!)
• Kinesthetic: G-forces. Is what you feel comparable to what you think you should feel in the current flight attitude? Also can be used to maintain level flight in turbulence.
• Control Pressures: Compare these pressures to the maneuver.

To complete this little lesson I cover instrument (our senses) cross checking. Talking through a few maneuvers: What pressures would you expect on the controls? What would you see? Will you show me what this looks like with this model aircraft?

If they can learn to compare their senses for accuracy, much like an instrument pilot does with their instruments, I believe they can be a far safer pilot. That is why I agree with both of your techniques, adopted to be all inclusive.

 

[tgrayson]

However, won't the yoke pressures be the same?...If I am not mistaken the pressures won't change, nor should the kinesthetic feel.

Assuming you didn't trim, why would the yoke pressures be the same? Since Vy is a lower IAS at a higher density altitude, the yoke pressure would be higher.

 

[shdw]

Assuming you didn't trim, why would the yoke pressures be the same? Since Vy is a lower IAS at a higher density altitude, the yoke pressure would be higher.

Are you sure you mean indicated?

I am assuming trim set to takeoff for both, just to keep it stationary.

It seems very contradictory. Why would a lower pitch attitude result in a higher yoke pressure anyway? (Assuming higher DA)

 

[tgrayson]

Why would a lower pitch attitude result in a higher yoke pressure anyway? (Assuming higher DA)

Your pitch attitude has nothing directly to do with your yoke. Your yoke controls the elevator which controls your AoA which controls your lift coefficient which controls your airspeed. The pressure you feel in your hand is merely the resistance against the elevator deflection produced by the changing pressure distribution around the horizontal stabilizer.

The pitch attitude that results will depend on the quantity of excess thrust you have as you change your airspeed, but the pressure you feel is solely due to the elevator deflection angle, which isn't affected by your angle of climb.

 

[minitour]

Your pitch attitude has nothing directly to do with your yoke.

Just wow.

-mini

 

[FDX8891]

Tgrayson, you probably are the smartest guy alive, but I don't understand how 25% of what you say gets across to your students in the manner you intend it to. I'm a CFI, and I don't understand half of the graphs or explanations you give based on how you present the information. I am by no means a genius, but I can hold my own when it comes to understanding things that require a little bit of knowledge and mind power.

I understand that everyone's learning styles are different, but when it comes to understanding these things that are a little more complex, what enables me to understand them consists of two things: dummying it down, and using simple examples (for instance, teaching wind correction in terms of a boat trying to follow a straight course when going across a river).

I felt like there was a simple answer to the question that was posed. My answer is that when you accelerate to Vy, you're eating up unnecessary distance over the ground that should be used for climbing out at Vx. If you're clearing an obstacle on takeoff, climb out at Vx, because at Vy, you're climbing at a shallower angle that might not allow you to clear the obstacle.

When these math formulas and complex graphs are presented, just like in high school, I automatically lose interest.

When people start to split nose hairs over 1/2 a knot indicated, or 1/16 of a degree of pitch, I say "why"? To me it seems like a game of "whose ***** are bigger," know what I'm sayin'?

This is starting to turn into an opinionated rant, but I guess I'm just tired of seeing nose-hairs split over things on this forum.

I know, I know, if I don't like seeing things happen on this forum, then don't visit it, right?

I guess I just want this place to be more open to those who might have difficulty understanding something and is afraid to speak up.

When these forums are continuously clogged by the same people over and over, I feel like those who don't post much don't do so because of the fear of instant retaliation over whatever they would say.

I'm just sayin...

 

[tgrayson]

When these forums are continuously clogged by the same people over and over, I feel like those who don't post much don't do so because of the fear of instant retaliation over whatever they would say.

I see. I hear you saying that I'm "clogging the forum" and you'd prefer to read responses by people who don't have any better understanding of the subject matter than you do, so that you don't feel intimidated. Do I understand you right?

 

[shdw]

Tgrayson, you probably are the smartest guy alive, but I don't understand how 25% of what you say gets across to your students in the manner you intend it to.

Pay attention to the person he posts to. When he speaks to someone newer on here, he dumbs it down to be less technical. When he talks with me he will give a formula and explain it, because we understand it that way.

If he talks with fish, I go get a bag of chips and pretend I know what is going on.


Here is a translation, see if I can get this right:

Your pitch attitude has nothing directly to do with your yoke.

I wouldn't argue this, but I'd be skeptical taking it for face value or presenting it to a student. That said, I have no translation until tgray gives more information into his thinking here.

Your yoke controls the elevator which controls your AoA which controls your lift coefficient which controls your airspeed.

This might be over simplified for you, but here:

Using a basic formula to demonstrate relationship of lift to speed and AOA we can write:

Lift = speed + AOA

Since lift always equals weight, whenever pitch is constant, we can assume lift to be a constant number. Let us keep them small, don't want to scare anyone with big math numbers, and say lift is 10. We have 5 speed and 5 AOA.

10 lift = 5 speed + 5 AOA

We can change AOA to but now we still have to keep lift at 10, so:

10 lift = some speed + 4

Some speed must be 6 to keep lift at 10. This is a clear demonstration of lifts relationship to speed and AOA. Speed and AOA vary inversely to keep lift constant when pitch is constant.

Knowing for sure now that speed varies inversely with AOA, and changes because AOA changes, we can refer back to this quote. Reread it and now understand the relationships, ignoring lift coefficient.

...did you reread!...? ok...

Lift coefficient = pi * AOA

So all the last part says is that changing AOA will make lift coefficient change. Here:

Remember pi from math class? It equals 3.14, so..

6.28 lift coefficient = 3.14 * 2 AOA

Let's change AOA from 2 to 3 and see what happens:

3 AOA * 3.14 = 9.42 lift coefficient

So we can see that is true.



Finally...

The pressure you feel in your hand is merely the resistance against the elevator deflection, produced by the changing pressure distribution around the horizontal stabilizer.

Reread this a few times over, paying attention to the comma that was added and substituting the bold word with: lift. Since lift is merely a difference in pressure, why not use a simpler word to reduce brain strain.

The pitch attitude that results will depend on the quantity of excess thrust you have as you change your airspeed, but the pressure you feel is solely due to the elevator deflection angle, which isn't affected by your angle of climb.

Did you word this funny or did I have too much to drink (my bday in 15 min:beer?



Ok T, a quick thought experiment:

Takeoff at sea level, trim for a perfect 75 knot climb. As you progress through 5,000, have you changed trim? How bout 10,000?

It has been a long time since I have done a progressive climb to 10,000+. However, the last time I did, I don't recall changing trim, even to 11.5 to get over Philadelphia.

Am I wrong? If I am right about this I am right about feeling the pressures on the yoke too. I'll explain after we agree on this though.




Mini - Are we on the same page with use of senses like instruments and such?

 

[lhornaday]

Ok T, a quick thought experiment:

Takeoff at sea level, trim for a perfect 75 knot climb. As you progress through 5,000, have you changed trim? How bout 10,000?

I don't think the trim would need change to hold 75 knots. I can't ever remember having to trim in a climb to 10,000 except due to people moving around.

Of course the pitch attitude (deck angle) to hold 75 KIAS would decrease as you climb.

 

[tgrayson]

Takeoff at sea level, trim for a perfect 75 knot climb. As you progress through 5,000, have you changed trim? How bout 10,000?

Generally, no, although reduced thrust on the horizontal stabilizer *might* result in a slight increase in airspeed. However, note that even if you keep 75 knots, you're no longer at Vy, since Vy reduces with altitude. This is where the increase in back pressure comes from. As thrust decays with altitude, you flight path will start to become flatter.

BTW, your use of 2*pi * AoA is not really appropriate, for three reasons: 1) That only applies to two dimensional airfoils, not three dimensional ones. A 2D airfoil is one with an infinite wingspan, i.e., no wingtip vortices. In reality, the lift slope would be less than that. 2) The AoA would also have to be measured in radians, not degrees, and 3) A cambered airfoil generates lift even at zero AoA, so you'd have to add a constant in after multiplying the AoA by some slope value.

Reality is that you'd have to look up the lift coefficient on a lift diagram.

 

[shdw]

As thrust decays with altitude, you flight path will start to become flatter.

Does the flight path or flight attitude or both get flatter with altitude increase? I suspect we have been discussing both, interchangeably, throughout this discussion.

Reality is that you'd have to look up the lift coefficient on a lift diagram.

To explain the relationships it should suffice. IMO, to a pilot of any level, understanding the relationships is the goal. Relationships lead to correlations.

I didn't claim radians or degrees, though I know it is radians, because it isn't relevant. They aren't reading this to learn to calculate lift coefficient.

The purpose was to explain your technical talk in layman terms, as best and accurately as I could. Consider that early training with basic flight dynamics would likely include discussions/calculations on an infinite wing, for simplicity of course. Why can't we give the same benefit to pilots?