MERGED power curve / floats posts

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aeroncasuperchief
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MERGED power curve / floats posts

Post by aeroncasuperchief » Fri Nov 16, 2018 1:59 am

In Fort Frances, I once debated with a fellow pilot concerning the backside of the power curve, In that , as you increase power, you reduce your speed and land at a slower groundspeed ! He could not believe it !!! Well lets open this discussion to newbies and seasoned bush pilots who know better about these beyond training school tools in the industry .
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Re: The backside of the power curve

Post by aeroncasuperchief » Fri Nov 16, 2018 2:04 am

As all professional pilots have NEVER operated an airplane beyond gross weight and survived, I suspect that there are NO pilots who have operated in the backside of the power curve and survived to tell about it and therefore I must be speaking about something that defies physics
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Re: The backside of the power curve

Post by youhavecontrol » Fri Nov 16, 2018 3:17 am

aeroncasuperchief wrote:
Fri Nov 16, 2018 1:59 am
In that , as you increase power, you reduce your speed and land at a slower groundspeed ! He could not believe it !!!
If you described it like that, I would not believe it either. Under normal conditions, power is reduced to allow you to slow the aircraft down, until you enter the back-side of the power curve, where it is added again to maintain altitude. Adding power does not simply allow you to slow down. The power added compensates for the loss of lift resulting in flight at a lower airspeed. Adding power allows you to maintain your altitude while flying at a higher angle of attack, by providing a vertical component of lift.

Operating behind the power curve is often used for Vref short field approaches in most small single engine aircraft I've flown. Maybe your fellow pilot flies something larger with more engines or swept-back wings and wouldn't dream of flying it slower to land shorter for risk of losing control in the event of an emergency.
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Re: The backside of the power curve

Post by photofly » Fri Nov 16, 2018 4:15 am

Most small airplanes’ usual approach speed is at the bottom of the power curve, (hence “pitch for airspeed, power for altitude”) . but what I read in Handling the Big Jets (Davies) is that all transport jets approach well into the backside of the power curve. Any airlines pilots want to confirm or deny?
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Re: The backside of the power curve

Post by PilotDAR » Fri Nov 16, 2018 5:30 am

Yes, it is possible to approach and land behind the power curve. Doing so is an advanced technique, fraught with risks if not properly understood and executed. If you're needing to do it to a runway to get in, you're landing that plane on the wrong runway, go around!

A few examples of where one might get into that odd phase of flight would include landing a floatplane on very rough water, where you want to slow it right down before contacting the water, and, when "testing" the surface with one ski while landing on snow.

Some of the risks: Your nose will be unusually high, and block your vision forward, you are more vulnerable to drift in a crosswind (and with blocked vision, less able to perceive it), If the engine quits while you're doing this, the plane is going to instantly stall and drop to the surface, and, it is possible to bang the tail tiedown ring of your STOL kitted C 150 on the surface before a mainwheel touches - ask me how I know!

If attempting this in a Robertson STOL equipped high wing Cessna, get qualified training first! The way that wing "holds on", you can get the plane settling with near full power, and you're stuck now, going down, with no way to arrest your descent. You've got all of your power committed, and zero stored speed or inertia with which to flare - ask me how I know :shock: !

If I need to do this, I'll plan a long landing run area, fly a normal approach to a few feet off the surface, and then start feeding in some power to allow the plane to be flown more slowly. It would only be in an emergency that I would attempt this in a taildragger, as it will certainly contact the tailwheel first, and then slam down the mains. Remember that as soon as the tailwheel is on the surface, you don't have elevator control of the plane any more, you can't hold the nose off anymore. Again, if you need to do this, you're already into specialized types of operation, which should be properly trained.
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Re: The backside of the power curve

Post by wrightflyer » Fri Nov 16, 2018 5:53 am

The normal approach in the C-17 is a backside approach. Power for Glidepath and Pitch for Airspeed. Of course Power and Pitch are a couple and you need to change both in varying degrees for a consistent performance.

I will say this, the plane was designed to work this way. That is partly why the flaps are in the exhaust stream of all four engines. (powered lift)
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Re: The backside of the power curve

Post by AuxBatOn » Fri Nov 16, 2018 6:36 am

The backside of the power curve is a regime where airspeed is unstable: it has a tendency to run away from you. You have to be constantly making small corrections in order to maintain a constant airspeed.

On the front side, if your speed deviate from trimmed airapeed and you release the controls, it will tend to return to the trimmed airspeed. On the backside, if you deviate from trimmed airspeed, it has a tendency to worsen (ie: ie you get slow and release the controls, it will get slower)

Some airplanes approach on the backside (Hornet for example) in order to minimize their kinetic energy.

And yes, drag increases as you get slower on the backside. The airplane may not stall before your available power is not enough to maintain level flight. If you get there, there is no other way than increasing airspeed by trading altitude to reduce drag enough to have some excess power.
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Re: The backside of the power curve

Post by aeroncasuperchief » Fri Nov 16, 2018 11:07 am

I see there are still pilots out there that do not believe that increasing power while on the backside allows you to fly SLOWER !
There are 2 ways to learn how to ride a bike 1 with training and training wheels and 2 with trial and error ( best to have soft grass and padding little one when you do this ) you can practice landings on the backside 4000 feet above the runway until you know you can do it for real and of course you should not use full or near full power when doing it for the obvious reasons stated before. Some bush strips require backside approaches to INCREASE the level of safety ! otherwise, 1 error and you prang the plane or loose it over the cliff on the far side of the landing strip . This same strip would require you to take off towards the cliff so yes, a strip where it is short enough to make the landing not possible without the backside approach but possible to roll off the end cliff on take off without good rotation speed
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Re: The backside of the power curve

Post by aeroncasuperchief » Fri Nov 16, 2018 11:12 am

With a backside taildragger approach, you will likely hit tail first UNLESS you go from 3/4 power to full or near power AND push forward to land 3 point ! do try this at altitude or demonstrated by a seasoned pilot
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Re: The backside of the power curve

Post by Zaibatsu » Fri Nov 16, 2018 11:59 am

Useless for most light aircraft.

You’ll be limited by takeoff performance, so there’s no point. You’re increasing risk just so you can have a bunch of extra runway ahead of you.
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Re: The backside of the power curve

Post by youhavecontrol » Fri Nov 16, 2018 12:04 pm

aeroncasuperchief wrote:
Fri Nov 16, 2018 11:12 am
With a backside taildragger approach, you will likely hit tail first UNLESS you go from 3/4 power to full or near power AND push forward to land 3 point ! do try this at altitude or demonstrated by a seasoned pilot
You're now talking about the power use in the final phase of the landing - the transition to a touch-down attitude, and not specifically the approach in general... which proves that we don't know what you are talking about?
I see there are still pilots out there that do not believe that increasing power while on the backside allows you to fly SLOWER !
Somewhat true, but somewhat not... and please don't think we're ignorant about this. On the back-side of the power curve, increasing the angle of attack increases the drag on the wing while increasing the lift, but inevitably as the speed decreases from the induced drag, the lift will also decrease with the slower airflow over the wing, which means that excess thrust is required to provide a vertical component of lift as well as preventing the aircraft from slowing down farther while maintaining altitude. So in saying that, yes, adding power allows you to fly slower in the sense that you're able to maintain level flight.
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Last edited by youhavecontrol on Fri Nov 16, 2018 4:42 pm, edited 1 time in total.
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Re: The backside of the power curve

Post by Chris M » Fri Nov 16, 2018 12:22 pm

aeroncasuperchief wrote:
Fri Nov 16, 2018 11:07 am
I see there are still pilots out there that do not believe that increasing power while on the backside allows you to fly SLOWER !
I think you need to qualify this statement a little... Last I checked the airspeed that you stall at is more or less unchanged whether you have power on or off (discounting aircraft with blown flaps and the like). To fly at the edge of stall in level flight is going to take a whole bunch of power, but you can fly just as slowly without any power - you'll just be coming down like a brick.

The correct statement would be "As the aircraft slows below best L/D, power must be added to maintain either altitude or descent angle".

Your initial statement is also oddly worded.
...as you increase power, you reduce your speed and land at a slower groundspeed...
Makes it sound like more power = less speed, which is not the case at all.
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Re: The backside of the power curve

Post by jakeandelwood » Fri Nov 16, 2018 1:58 pm

Other than "showboating" I don't really understand the point. Any airplane I've flown needs more runway to takeoff than to land when operated normally anyway. So what, you are pulling the nose up and increasing power, the propeller is now being used more for lift to stop the plane from stalling. Sounds dangerous to me. Model airplane guys play around with that, they can nearly make the over powered model nearly hover!
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Re: The backside of the power curve

Post by Gannet167 » Fri Nov 16, 2018 4:50 pm

The semantics are confusing the issue. The OP almost sounds like some trolling. You can fly an approach at almost any angle of attack. If you fly out at double the normal approach speed, AoA week be very low. Most planes fly at somewhere around 1.3 Vs. The 30% extra speed gives a margin above stall for safety and manoeuvrability but keeps speeds low enough to have minimal float as the plane decelerates in the flare and keeps landing distances low. But that speed is merely a means of achieving a target AoA which is truly what matters. If you take the airspeed indicator away and only fly approach AoA (assuming you have an AoA indicator) you'll be accomplishing the same thing, arguably more directly.

At any speed above Vs, you can fly at any AoA up to the max, critical AoA. Of course as speed increases, so does g force for a given AoA. If you want to fly straight and level, 1g, at a high AoA, you'll simply need more power to counter the increased drag. At any AoA (or corresponding airspeed) lower than L/D max, the wing is below it's sweet spot and drag increases more than lift as AoA increases.This is what every student does when they learn slow flight. If you want to fly at a high AoA on approach, you simply need to change the power to get the desired vertical rate. Slow flight in a descent. Yes, it will be a higher power setting than at lower AoAs because drag is higher.

It is not the higher power that reduces ground speed. It's higher AoA, putting the plane at a lower airspeed and therefore lower ground speed, all other things (wind) being equal. You need more power because in that flight regime drag is higher. Higher power is required as a consequence of the slower approach, it is not the thing that makes your approach slower. Cause vs effect.
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Re: The backside of the power curve

Post by photofly » Fri Nov 16, 2018 5:16 pm

Chris M wrote:
Fri Nov 16, 2018 12:22 pm
I think you need to qualify this statement a little... Last I checked the airspeed that you stall at is more or less unchanged whether you have power on or off (discounting aircraft with blown flaps and the like).
In theory you are right; but the 1xx Cessnas display a lot of the "blown wing" effect because the strong propwash keeps the airflow attached to the wing at higher angles of attack. So the unaccelerated stalling speed is reduced while power is applied.
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Re: The backside of the power curve

Post by aeroncasuperchief » Fri Nov 16, 2018 6:26 pm

Here is an article from another conspiracy theorist like me lol and he mentions not once but twice that on the backside as you increase power, you slow down !

BEHIND THE POWER CURVE
THE PLACE YOU DON'T WANT TO BE
January 5, 2002 By Larry Randlett
Jet fighters are easier to fly than most general aviation aircraft. It's true! I came to this conclusion after 30 years of flying during which I logged more than 2,000 hours in the F-4 Phantom II, 500 hours in the F-16 Fighting Falcon, 500 hours in the F-15E Strike Eagle, and more than 1,000 hours in a wide variety of single-engine GA aircraft. And I say this for a number of reasons.
First, being considerably heavier than light airplanes, fighters are much more stable in the wind. You fly them to the runway and, hence, they are a lot easier to land in a crosswind. Second, redundant systems that are integral to jets make most emergencies a lot less critical in fighters. Besides, there's always an ejection system that allows you to walk home if circumstances so dictate. But, perhaps the greatest difference between the two that makes fighters so much easier to fly is excess power.
Each generation of fighters has had a progressive increase in the power-to-weight ratio. To give you an idea, consider that the latest model of the F-15 has 30 percent more power than the model that immediately preceded it. The latest fighter, the F-22, can even cruise supersonic. It is the excess power of jet fighters that can correct for a multitude of sins. But there is one sin it can't correct for - getting too far behind the power curve.

Every airplane has a power curve, and every power curve has a back side. Understanding the power curve is essential if you want to be in control of your airplane at all times. And, to understand the power curve you must first understand drag.

You may recall that during your ground-school days you learned drag in an airplane comes in two forms: parasite and induced. And after taking your FAA knowledge test, that is probably the last time you thought about drag at all. Parasite drag is primarily caused by the shape of the airplane and the friction of the air over the skin of the plane. The faster the airplane flies, the more parasite drag it creates. In fact, if you double your speed, you increase parasite drag by four times! Parasite drag is very noticeable. It is why those last few knots of increased airspeed increase fuel consumption so much. It is also why we normally fly at around 75-percent power.

Less noticeable, but even more important, is induced drag. Induced drag is the result of lift. Without going into great detail of the physics of the phenomenon, suffice it to say that induced drag is greatest at slow speeds and decreases at a geometric rate as speed increases. Of course, when you stall, you have no drag; you only have gravity.

The reason we need to know about these two types of drag is that they both act upon our airplanes any time we are flying, and together they dictate such factors as maximum endurance airspeed, minimum range airspeed, and power required.

In addition to displaying the total drag acting on your airplane, this "total drag" curve also dictates the required thrust to overcome it. But don't confuse the two. Thrust is the force generated by the propeller. Power is the actual work done by the engine turning the propeller.

Despite the similarity in the shape of their curves and the fact that they are both the result of drag, the power curve and the thrust curve do not have corresponding points. For example, the lowest point on the power curve represents the speed for maximum endurance and minimum sink rate, while the lowest point on the thrust curve represents optimum glide speed providing maximum range.

So let's take a closer look at that power curve. Remember that it shows the power required to maintain a given indicated airspeed while flying at a constant airspeed. Now let's put some numbers on the curve.

We are flying our theoretical airplane at a constant indicated airspeed and at a constant altitude. It is amazing to most pilots that it takes the same power to fly at 30 kt as it does to fly 100 kt. This is the effect of induced drag.

You will also notice that our power curve has a "front side" and a "back side." We spend most of our time flying around on the front side. To go faster while flying on the front side (maintaining constant altitude), you must add power. That makes sense. However, on the back side, to fly slower you must actually add power. This takes a little thought. So let's take a closer look at the back side of this power curve. If the numbers I've picked for our theoretical aircraft look a lot like those you see when on final approach, it's because that is when we spend most of our time flying on the back side.

The VASIs tell us that we are too low as we approach the airfield. So we maintain our airspeed of 65 kt and trim the aircraft to maintain our attitude. If you drop the nose momentarily, your airspeed will increase and move off the power curve. The laws of physics require you to reduce power if you are going to maintain this higher airspeed and not change your altitude. The opposite is also true.

If on your "dragged-in" final, you perceive that you are a bit low on altitude and you raise the nose, your airspeed will slip a little bit. And if you don't add power, you will actually increase your sink rate because induced drag has caused you to have a deficiency of power on the back side of the power curve.

Adding more back pressure to stop the sinking only exacerbates the situation. To get out of trouble, you must lower the nose and add power. If you wait too long or don't have enough power to recover, you are going to hit the ground short of the runway. This is why your instructor probably told you that, on final, you control altitude with power and airspeed with pitch.

So how does your airplane fly "behind the power curve"? I suspect that your pilot operating handbook doesn't have a set of power curves. The next time you have some free time, you might decide to collect data for drawing your own power curves. It's really very simple. Trim the aircraft for level flight at a given power setting. Write down both the airspeed and power setting. Then reduce your airspeed in 10-kt increments without changing your altitude and continue to record airspeed and power settings.

Eventually, you will reach a point at which more power is required to maintain a slower airspeed. You are now officially "behind the power curve." Once you know you're on the back side, push the nose over and observe what happens. Then raise the nose and make some similar observations. Continue to do this while maintaining altitude. When you have finished, you will know exactly how your airplane will act while flying under these circumstances.

Once you have mastered how to physically fly your airplane behind the power curve, you can better keep from entering this flight regime unintentionally
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Re: The backside of the power curve

Post by aeroncasuperchief » Fri Nov 16, 2018 6:39 pm

Here is another foolish pilot :
Chris Habig
Chris Habig, Experimental test pilot, fighter pilot, airline pilot, flown 70 types
Answered Nov 4 2016
This question is unclear and open to interpretation. Here are two answers in one…hopefully one of them addresses the real question:

Approach speed is always less than minimum drag speed in the clean configuration. Why slower? Because flaps are extended for approach, which permits flying slower, typically 1.3 times Vstall.
Approach speed is nearly always greater than minimum drag speed in the landing configuration. That means that the airplane is speed stable. More thrust is required to fly faster, and less thrust is needed to fly slower. This is also known as flying on the front side of the power curve. The airplane is typically also flight path stable, meaning that if you pull back on the flight controls while keeping thrust constant, the flight path shallows and if you push forward with constant thrust the flight path steepens. If the airplane is slower than the minimum drag speed, we call that flying on the back side of the power curve and the relationships are reversed—more thrust is needed to fly slower and less to fly fast. The airplane is also not flight path stable.
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Re: The backside of the power curve

Post by aeroncasuperchief » Fri Nov 16, 2018 6:44 pm

Another 50 hr wonder pilot :

Back Side of the Power Curve
Imagine you are on final approach. On long final you are maintaining a speed near VY (a normal approach speed in many aircraft), using 1700 RPM of engine power. Then, suddenly, the tower controller asks that you land and hold short of a crossing runway. You decide to convert the normal approach to a short-field approach. This requires slowing down from VY to a somewhat slower speed. The procedure is shown in figure 7.3.

You need to shed some kinetic energy, as shown by the shaded area in the figure. Since that always takes time, you should immediately retard the throttle. You are now getting rid of mechanical energy (via drag) faster than it is being replaced (via the engine). You want to pay for this energy deficit by cashing in airspeed, not altitude, so you must pull back on the yoke and then roll in some nose-up trim to get rid of the force on the yoke. When the airspeed reaches short-field approach speed, you re-open the throttle. Returning to 1700 RPM will not suffice; you will need more power to complete the approach at this low speed than it would have at the higher speed.


Figure 7.3: Slowing Down on the Back Side of the Power Curve
This is an interesting contrast with the previous situation (e.g. figure 7.2). The required power increases as the airspeed decreases. Therefore you do not even have the option of making the speed-change with only one power-change. It requires two (opposite and unequal) power changes.
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Re: The backside of the power curve

Post by aeroncasuperchief » Fri Nov 16, 2018 6:55 pm

Dumb NASA they got it all wrong!!!

NASA TECHNICAL NOTE NASA TN D-7791
C ir
h
?
(NASA-TN-3-7791) FLIGHT-PATH AND AIRSPEED N74-34481
CONTROL DURING LkN3ING APPROACH FOE
POWERED-LIFT AIRCRAFT (NASA) 63 p HC
83.75 CSCL C1B Unclas
H

Considering the response to an attitude change at constant thrust (fig,
2(a)), the nose-up attitude change produces a typical flight-path response for
operation on the backside of the drag curve. The glide path initially becomes
more shallow but eventually steepens. Speed response is conventional since
the aircraft decelerates following 3 nose-up change in attitude. For
increase in thrust with attitude held constant (fig. 2(b)), flight 'iliii:r
responds quickly and is substantially sustained in the long term. .iie spee
response in this instance is decidedly adverse in that
the aircraft dece1era;-s
after an increase in thrust.
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Re: The backside of the power curve

Post by digits_ » Fri Nov 16, 2018 7:02 pm

Not sure who you are arguing with. Nobody is disagreeing with you. Yes you need more power to fly slower at some point, but you will also need to manipulate your yoke/stick to do so.
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Re: The backside of the power curve

Post by AuxBatOn » Fri Nov 16, 2018 7:04 pm

Perhaps your wording is inadequate. You don’t add power to slow down. You need more power to maintain a given flight path angle when you slow down would be more appropriate.
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Re: The backside of the power curve

Post by Gannet167 » Fri Nov 16, 2018 7:09 pm

There's no conspiracy here. Behind L/D max, more power is required. You're quoting sources and stating the obvious, even if the way you word your posts shows you don't fully grasp the aerodynamic concepts at work. No one is disputing that it takes a lot of power to fly slow. Congratulations. In other news...
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Re: The backside of the power curve

Post by aeroncasuperchief » Fri Nov 16, 2018 7:26 pm

Digits says,,,,,,,,,,,,Not sure who you are arguing with. Nobody is disagreeing with you

Gannet says: It is not the higher power that reduces ground speed.
Youhavecontrol says: Adding power does not simply allow you to slow down
ChrisM says: .Makes it sound like more power = less speed, which is not the case at all.
Auxbaton says: You don’t add power to slow down.

Digits, you might want to change your glasses prescription !
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Re: The backside of the power curve

Post by aeroncasuperchief » Fri Nov 16, 2018 7:38 pm

The Be-18 driver that I spoke with over a beer in the OP was likely never on the backside to learn or experience what I am saying . I suspect many pilots are in the same boat.
One of my job descriptions is a test pilot and yes, I do take the A/C to the near limits where I am told, one should never go to !
power_curves-300x244.jpg
power_curves-300x244.jpg (18.22 KiB) Viewed 1165 times
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Re: The backside of the power curve

Post by aeroncasuperchief » Fri Nov 16, 2018 7:50 pm

In my previous post, in the chart, one can clearly see that as speed decreases to the left of the best endurance speed ( near best L/D ) power MUST increase to maintain altitude! Without additional power, ( while maintaining altitude in steady state, unaccelerated flight ) you will NOT be able to go slower than Best endurance speed. So, it does not matter how smart you think you are, or how you coach your words to make yourself right and others wrong, The fact of the matter is this: POWER is INCREASED to fly SLOWER in the backside regime !


Maybe we can also look into the bernoulli verses newton debate with regards to how "lift" is generated! Maybe it is a combination of both????
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