Slow Flight

[Shiny Side Up]

Is this really all the attention this thread is going to get?

I think topics about handling the airplane on the backside of the power curve, and training to that regard have been well hashed. This is merely the latest revision about it, and I assume that it will keep coming up whenever there's new blood on the forums. If you want to continue the discussion I'd encourage you to PM 767 about it.

[niss]

Maybe it's my constant need for validation, but could someone at least please confirm or correct my post?

[Old Dog Flying]

I used to use the old line...Attitude+Power=Performance...but what the heck did I know? Changing one thing will change the resultant.

[Shiny Side Up]

Maybe it's my constant need for validation, but could someone at least please confirm or correct my post?

Get used to disappointment.

I used to use the old line...Attitude+Power=Performance...but what the heck did I know? Changing one thing will change the resultant.

It still holds true, just doesn't quite give the full description. I remember my instructor had a saying comparing it to messing with a group of women of ill repute, the idea is the same. The big thing to get across to the student is they have to think ahead of how the airplane is going to react depending on which part of that equation they "mess with". The one thing I always have hated with Slow flight is a lot of the misleading terminology that often gets associated with it, "The area of reverse command" is the one that comes to mind. Even the term "slow flight" is bad I feel. One after all is still in the process of "flight".

[niss]

Maybe it's my constant need for validation, but could someone at least please confirm or correct my post?

Get used to disappointment.

The biggest disappointment is that your picture didn't work!

[hairdo]

The back side of the power curve is any speed below Vy...

Umm... correct me if I'm wrong, but the back side of the power curve would start at the point where you need more power to go slower. So what would immediately preceed it would be minimum power to maintain level flight. Which would also be known as maximum endurance (least amount of fuel flow/power to stay in the air). Vy would be where you have the most excess power (thus allowing for the maximum rate of climb). This doesn't necessarily occur at the point where minimum power to maintain level flight does unless you are able to turn the engine at maximum RPM while at max endurance airspeed (which you likely can't do with a fixed pitch propeller).

In response to the OP, to maintain a descent rate, while increasing a/s:
1: If you simply drop the nose, descent rate momentarily increases, airspeed increases, and then a descent rate less than the original descent rate is achieved.
2: If you drop the nose and reduce power, descent rate increases momentarily (more than Ex 1), airspeed gradually increases, and then descent rate returns to original ROD (assuming you got the power right).
3: If you drop the nose and increase power, descent rate stays the same (assuming you got the power right), airspeed gradually increases, and then descent rate becomes less than original descent rate (a lower ROD than Ex 1)
4: If you raise the nose and increase power, you will get a momentary reduction in ROD (perhaps even a climb), a loss of airspeed, and then when the speed is stable again, and then if you get the power right, the original ROD.
5: If you raise the nose and decrease power, you should get a momentary reduction in ROD (perhaps a brief climb), a loss of airspeed, and then a higher than original ROD.

Now, all that depends on how smoothly you can make the transition. For example. From S&L, you should be able to transition out of slow flight to the opposite side of the power curve simply by very gradually reducing the AOA on the wings. So if you can do it very smoothly, you could very gradually reduce the nose attitude, allowing the aircraft to increase its speed, and as the speed increases, gradually reduce the power.

[iflyforpie]

Umm... correct me if I'm wrong, but the back side of the power curve would start at the point where you need more power to go slower. So what would immediately preceed it would be minimum power to maintain level flight. Which would also be known as maximum endurance (least amount of fuel flow/power to stay in the air). Vy would be where you have the most excess power (thus allowing for the maximum rate of climb). This doesn't necessarily occur at the point where minimum power to maintain level flight does unless you are able to turn the engine at maximum RPM while at max endurance airspeed (which you likely can't do with a fixed pitch propeller).

Actually, Vy and endurance speed are aerodynamically the same speed--the speed where you have the most excess power available. Of course at endurance you don't have excess power, but any other speed will result in a power deficit. Because power settings and density change the shape of the power curve, these speeds are not always the same (indeed, Vy will change as you climb). Vy also corresponds with minimum sink when gliding (slightly slower than best glide).

I could try to explain it further, but this guy explains it best...

http://av8n.com/how/htm/energy.html#sec ... urve-intro

[Av_Av]

Actually, Vy and endurance speed are aerodynamically the same speed--the speed where you have the most excess power available.

No. Vy is the speed at which you have the most excess thrust power available. Endurance is the speed at which the least power is required. Excess power not only depends on how much power is required, but how much power is available -- which varies with airspeed due to changes in prop efficiency as the advance ratio (prop AOA) changes. For a fixed pitch prop, prop efficiency starts at zero (at zero airspeed), increases to a maximum at some airspeed selected by the designer, and then drops off again at higher airspeeds. Vy can change depending on whether you have a climb, cruise, or compromise prop installed. Endurance does not change -- it's a fixed AOA speed and is independent of prop efficiency.

It's possible for both speeds to be the same, but in general, they are not. The most likely scenario for the two speeds to be equal is in an aircraft with a variable pitch prop -- which can therefore maintain maximum prop efficiency over a wider range of airspeeds. It's also possible -- maybe even likely -- with a fixed pitch climb prop. It's far less likely with a cruise prop or a compromise prop (which is the most common type on primary training aircraft).

Because power settings and density change the shape of the power curve, these speeds are not always the same (indeed, Vy will change as you climb).

Yes, but endurance speed doesn't (well, it changes a little because of fuel burn and the corresponding weight change).

Vy also corresponds with minimum sink when gliding (slightly slower than best glide).

No again. Endurance speed corresponds with minimum sink when gliding.


With regard to the OP:

Others have already made these points in their own words. Here they are in mine. Take 'em for what they're worth (what you paid for 'em).

I would say lower nose, decrease power. Lowering nose (increasing airspeed) would bring you to the right, down the backside of the power curve, reuiring less power to maintain level flight (or FPM). Thoughts?

You're on the right track, but your initial pitch change will also result in an increase in descent rate. If you don't want this, you'll need some corresponding power changes.

When considering the effect of the power curve, remember that it is developed based on the assumption that the aircraft is in equilibrium. During the maneuver you're asking about, the airspeed is changing, so the aircraft is not in equilibrium. You will need to make a power change (increase) to accelerate the aircraft initially while maintaining a fixed descent rate, and then another power change (reduction) once your target airspeed is reached in order to stop the acceleration and maintain your fixed airspeed and descent rate. The final power setting will be lower than your original one (assuming your airspeed change remains within the slow flight range). While these power changes are being made, you'll need to pitch down to reduce your AOA as the airspeed increases.

[hairdo]

Thank you Av_Av for saying it better than I could. It makes sense in my head, but somehow it doesn't always make sense when I post it.

[Big Pistons Forever]

In the first post on this thread TKdowel asks for the recommended actions if while in slow flight you want to increase your airspeed while maintaining your descent rate.
Well at the risk of being rude my first thought was "well what did you do the last time you got a bit slow on a short field approach ?" I am guessing you just added some power without a whole lot of analysis of the situation.

I think the flight training industry sometimes has a tendancy to over think situations in an attempt to come up with a one size fits all solution to a problem. In other words if "X" occurs in flight than you always do "Y". There are some things that obviously require this certainty, like if the airplane stalls you have to immediately lower the nose, but the slow flight regime is a good example of taking the necessary action to make the aircraft do what you want it to, or simply just fly the aircraft. If you are barely into the slow flight regime than small adjustments of attitude and power will cause the desired change, but if you are deep into the slow flight regime than lots of power and carefull management of the attitude will be required to get the aircraft starting to accelerate without having the bottom fall out. So IMO the only answer to the poll question is you adjust the attitude and power as required to get the desired performance. The magnitude and the order of the adjustment will be different depending on the situation. The best way to get students to understand this is by practicing the various scenarios in the air, and the most important part of the slow flight lesson is not flying in slow flight it is the ability to recognize when the aircraft is entering the slow flight regime.

[niss]

Well at the risk of being rude my first thought was "well what did you do the last time you got a bit slow on a short field approach ?" I am guessing you just added some power without a whole lot of analysis of the situation.

I always lower the nose to get my speed back up and add power as required so I don't get low. That's how I was taught.

[Av_Av]

...my first thought was "well what did you do the last time you got a bit slow on a short field approach ?" I am guessing you just added some power without a whole lot of analysis of the situation.

This is a perfect example of rule-based decision making. It's very effective for experienced pilots, who have a good experience base to work with. But it doesn't work so well for students and low timers who haven't had the time to develop intuitive "rules" to work with. Low timers need more interpretive knowledge to reason through problems and to make knowledge-based decisions. The problem is that these knowledge-based decisions take time. So in situations where decisions must be made quickly (eg. - low and slow on approach), it's important for us to spend some time on the ground helping students reason through the process in advance. This will help them respond in a more timely manner once in flight.

I think the flight training industry sometimes has a tendancy to over think situations in an attempt to come up with a one size fits all solution to a problem. In other words if "X" occurs in flight than you always do "Y".

Failure to recognize that a solution will no longer work if some parameter is changed is more a symptom of underthinking than overthinking. The resulting "one size fits all" solution is indeed common in flight training. But the only way to fix this is to put more thought into how and why we do things.

...but the slow flight regime is a good example of taking the necessary action to make the aircraft do what you want it to, or simply just fly the aircraft.

Again, the just fly the aircraft approach is very effect for experienced pilots - it calls for rule-based decisions. But a student or low-timer needs more knowledge-based emphasis on how to fly the airplane.

So IMO the only answer to the poll question is you adjust the attitude and power as required to get the desired performance.

Well, strictly speaking, that's true. But the next time you're flying a little airplane with a non-pilot passenger, give them control, make that statement, and ask them to bring you home and land. Let me know how it goes .

The magnitude and the order of the adjustment will be different depending on the situation.

True. But the situation -- including a starting point and an objective -- was provided in the original post. The order of the necessary adjustments can be anticipated from this. The magnitude of the adjustments require more information (A/C type, starting and target speeds, etc.), and even with this information, we'd probably only be able to provide rule-of-thumb estimates.

The best way to get students to understand this is by practicing the various scenarios in the air,

Absolutely. But this practice can be complimented and made more effective (not to mention cost effective) by reasoning through the maneuvers on the ground.

and the most important part of the slow flight lesson is not flying in slow flight it is the ability to recognize when the aircraft is entering the slow flight regime.

I'm not sure I'd agree that it's the most important part, but it's certainly important and often underemphasized in flight training.

I always lower the nose to get my speed back up and add power as required so I don't get low. That's how I was taught.

This is correct for fixing an approach. But on an approach, we are usually more concerned with descent angle than descent rate. The two are related, of course, but for a fixed descent angle, our descent rate needs to change as airspeed changes. The original question for this thread specified a fixed descent rate. In any case, if your approach is conducted in the slow flight range, you will still likely find yourself making additional power changes (reduction) after you airspeed has stabilized where you want it (assuming you want to maintain a constant glidepath).

[Big Pistons Forever]

Av Av

As a current class 1 Instructor I think I have a pretty good idea of how to teach people how to fly. I absolutely agree that students must understand the theory that underlies and explains each manoever, where we part company is the idea that ab inito students can't make what you call rules based decision making, (but is actually better called conditions based decison making). It doesn't matter what the aircraft should be doing, the only thing that matters is what the aircraft is actually doing. That is determined by always knowing what the attitude of the aircraft is what power has been set and the resulting performance. Early in training students need help, so verbal coaching is required to help them look at the right thing at the right time but those habits are essential to good flying skills. It can be quite tedious for the instructor at the early stages which is why IMO it often gets replaced by a if "this" happens you always do "that" style of instruction.

[tkdowell]

Ok kids the verdict is in. I wrote the AIRAT this morning and this question, sure enough, was on there. I answered option 2 in the poll or "Lower nose, decrease power" and I did not recieve any feedback relating to slow flight, so I'm fairly confident that was what they were looking for. Cheers.

[B-rad]

the answer is lower the nose and reduce power. same time and not readding power or increasing before.

[niss]

the answer is lower the nose and reduce power. same time and not readding power or increasing before.

I'm still a little confuses. Maybe its just too hard to visualize on the ground. But lowering your nose and reducing your power will increase your descent rate no? To stabilize my descent rate I need to either raise my nose again or add power as necessary?

[hairdo]

the answer is lower the nose and reduce power. same time and not readding power or increasing before.

I'm still a little confuses. Maybe its just too hard to visualize on the ground. But lowering your nose and reducing your power will increase your descent rate no? To stabilize my descent rate I need to either raise my nose again or add power as necessary?

Initially, yes, you will get an increase in your rate of descent. However, when you lower the nose (reduce the angle of attack), you will reduce the amount of induced drag being created. When you achieve your new (higher) speed, the increased airflow over the wings will allow for a lower AOA to create the same amount of lift. So, with a lower AOA to generate same amount of lift, you won't have as much drag, hence the reason you won't need as much power to maintain the desired rate of descent.

[B-rad]

just remember, attitude equals your airspeed and power equals your altitude

[niss]

just remember, attitude equals your airspeed and power equals your altitude

Exactly, that is why power would equal your descent rate.

[B-rad]

ok i can't tell if your just fucking around. are you really this confused about it or just being a wise ass?

[tkdowell]

Maybe it's my constant need for validation, but could someone at least please confirm or correct my post?

Well I'm afraid you're not my girlfriend, so no validation from me. Even if you put out.

On the attached picture (I know it's fairly crude, but it's also late) I drew up a power curve. Basically slow flight is the range between V1 and V2. V1 represents, for all intensive purposes, the slowest speed you can possibly fly in level flight and V2 represents, for all itnensive purposes, endurance speed. Now the poll question asked us to go from V5 to V6, or increase our speed while in slow flight. Now how do we maintain the constant descent rate?

For the sake of simplicity, the question could have just as easily asked how do we maintain a constant altitude? Or a constant climb? If you are not satisfied on this point I can re-draw the picture and show you that the same power change is required to maintain the same defecit.

So now P5 corresponds to the power required to maintain altitude at V5 and P6 corresponds to the power required to maintain altitude at V6. Simply put, as we go from V5 to V6 the power required reduces (notice that P5 is greater than P6). Therefore we need to reduce power. If we maintained power then we would have more power than that required and we would climb and if we increase power than we would have even more power than required and we could climb.

Or, put into the context of the poll question, the answer is to lower the nose and reduce power. If we were to lower the nose and maintain power (not that that was an option), we would slow our descent rate and if we were to lower the nose and increase power we would slow our descent rate (possibly to the point of midigating it entirely or causing a climb, depending on the airspeed change and power change).

Hairdo said it well here:

In response to the OP, to maintain a descent rate, while increasing a/s:
1: If you simply drop the nose, descent rate momentarily increases, airspeed increases, and then a descent rate less than the original descent rate is achieved.
2: If you drop the nose and reduce power, descent rate increases momentarily (more than Ex 1), airspeed gradually increases, and then descent rate returns to original ROD (assuming you got the power right).
3: If you drop the nose and increase power, descent rate stays the same (assuming you got the power right), airspeed gradually increases, and then descent rate becomes less than original descent rate (a lower ROD than Ex 1)
4: If you raise the nose and increase power, you will get a momentary reduction in ROD (perhaps even a climb), a loss of airspeed, and then when the speed is stable again, and then if you get the power right, the original ROD.
5: If you raise the nose and decrease power, you should get a momentary reduction in ROD (perhaps a brief climb), a loss of airspeed, and then a higher than original ROD.

Point 1 would be if you maintain power. Point 2 is if you reduce power. Point 3 is if you increase power.

Point 4 would be if we went from V6 to V5 and from P6 to P5 and Point 5 would be if you go from V6 to V5 and took the power down below P6 (this is me using developemental - good luck!). In any case these options are incorrect as the intention was to increase speed not decrease speed.

TD

[niss]

ok i can't tell if your just fucking around. are you really this confused about it or just being a wise ass?

Certainly not fucking around, or being a wise ass, I just can't visualize it on the ground. I didn't get a good chance to read the next post as I'm on my iPod but I will definatley be reviewing slow flight next time I'm up, I just have a hard time relating to the lower power and lower nose as a controlled fpm. Again, once I get a chance to dick around I'm sure it will clear up.

[hairdo]

I just have a hard time relating to the lower power and lower nose as a controlled fpm.

Two words: less drag. If you lower the nose (angle of attack), you lower the total drag. Less drag means you need less power to maintain your flight path.