rchamble
Pre-Flight
I have zero flight hours, have just been studying some book stuff. I cant get a complete grasp on reverse command or falling behind the power curve, can someone explain this to me???
falling behind the power curve
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poadeleted20
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COFlyBoy
Line Up and Wait
Concept is pretty basic, to go slower you point the nose higher. To maintain your altitude with that high nose and slower airspeed you need to add power. (that is the reverse command, that you added power to go slower)
I hate the term "reverse command". The term itself causes the confusion, not the concept.
It sounds like if you add power you'll go slower.
I hate the term "reverse command". The term itself causes the confusion, not the concept.
It sounds like if you add power you'll go slower.
COFlyBoy
Line Up and Wait
I've seen big long explanations of reverse command like the one in the link, but it really seems like overkill.
Ron, when you teach reverse command how long does it really take? (I can't remember my flight lesson during primary training that covered it)
Isn't it pretty much pull back on the attitude knob and push forward on the power knob. Voila! Reverse command. Didn't even need the power/speed graph.
bbchien
Touchdown! Greaser!
Think of it this way. Every wing has a most efficient angle of attack (lift made per drag created). Say you need to fly really really slow and maintain altitude. You have to pitch the aircraft to a higher angle of attack. The wing is less efficient, so you have to ADD power to maintain 1G lift.
That efficient AOA is approximated by Vy. You can go slower and slower (farther and farther below Vy) and so long as you don't run out of power, you can maintain 1G lift.
WHen you get vertical you'd better have Thrust=Mass of the airplane or you are going to descend.
Now say you are in cruise mode and you want to climb. You CAN just pitch back. That's because the AOA is now closer (from near flat, to up a few degrees) to the angle of attack represented by Vy.
But if you do that when really slow, you'll just get slower still.
That efficient AOA is approximated by Vy. You can go slower and slower (farther and farther below Vy) and so long as you don't run out of power, you can maintain 1G lift.
WHen you get vertical you'd better have Thrust=Mass of the airplane or you are going to descend.
Now say you are in cruise mode and you want to climb. You CAN just pitch back. That's because the AOA is now closer (from near flat, to up a few degrees) to the angle of attack represented by Vy.
But if you do that when really slow, you'll just get slower still.
PilotAlan
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Here's a key concept that helped me.
As you slow down, you get to the point where the wing isn't making enough lift to keep the plane level. By adding power (with the nose high), the prop is holding part of the weight of the aircraft.
So, by adding power, you can go slower.
"Reverse command" because normally adding power means going faster. Here, adding power means going slower.
As you slow down, you get to the point where the wing isn't making enough lift to keep the plane level. By adding power (with the nose high), the prop is holding part of the weight of the aircraft.
So, by adding power, you can go slower.
"Reverse command" because normally adding power means going faster. Here, adding power means going slower.
COFlyBoy
Line Up and Wait
No that is not the concept behind reverse command. Reverse command is caused by the induced drag of the wing going way up as the angle of attack is increased.
Only a tiny amount of thrust is going to holding the plane up.
Example: My plane probably produces roughly 500lbs of thrust. At 10deg nose up (which would be pretty slow) the thrust component that is fighting gravity is only 1.5lbs.
bob_albertson
Pre-takeoff checklist
PilotAlan said:
Not really Alan....
If the speed is allowed to become too slow, an increase in pitch and application of full power may only result in a further rate of descent. This occurs when the angle of attack is so great and creating so much drag that the maximum available power is insufficient to overcome it.
To understand that the thrust is not part of what holds your altitude in this instance you need to look at what happens at slow speeds. In keeping with the theme of simple as you can think of making it.... The amount of lift the wing produces increases and decreases with airspeed for a set angle of attack (L = (1/2) d v2 s CL). The slower you go the less lift the wing is capable of producing at a lower angle of attack. This puts you into a vicious loop of requiring a greater angle of attack to maintain the same amount of lift produced by the wing. What creates the vicious loop is that the slower you go the more induced drag is produced. In order to overcome that greater drag you need more thrust. So in order to allow the wing to increase the AOA enough to maintain the same amount of lift produced then you must also add more power to counter the increase drag. If you don't increase the thrust the aircraft will continue to slow down since the drag is increasing to be a force greater than that being countered by the thrust. The thrust is what counters the induced drag and prevents the airspeed from decaying further --- thus allowing the airplane to hold altitude by not allowing induced drag to continue to increase on it's own (establishes the equalibrium between thrust/drag).
That might be off by a bit, and I'm sure the nice guys will correct what I may be wrong about.... So, I'll digress to their expertise 
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I'll give it a shot. Here's an L/D Max curve, sorry I couldn't find a bigger image, but you should have something similar in your books:
Induced drag is that drag being caused by lifting the weight of the plane in the air. That is the force it takes to deflect the air at that angle and pick the plane up. The Form drag is the resistance of the airframe and is a result of going faster.You see where the two lines cross, that is your L/D max speed or Lift/Drag which ends up being your minimum sink speed at any power setting (which BTW means it's also your most efficient speed to loiter if you have to wait for a storm to clear and fuel is a concern). so you see, if we slow down from this point, in order to maintain altitude, we have to add power. Adding power to go slower is what they mean by "Reverse Command" and it's not refered to as "falling behind" but rather "on the back side" of the power curve, the backside being anything slower than LD Max.
The reason that this is important to a pilot is because when he is on final about to land, if he needs to slow his rate of descent, he needs to add power.
Did that work for ya?
BellyUpFish
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Backtothesand
In pretty simplistic terms:
Normally, you use pitch for airspeed and power for altitude.
In the region of reverse command, you use pitch for altitude and power for airspeed.
That is, if you're slow and at high angles of attack and trying to maintain this, to climb, you use the back pressure on the yoke and to maintain airspeed you add or decrease throttle depending on what speed you're at versus what you're looking for.
If you fly straight and level at 75% power in a 152/172, then push the throttle in, the first thing you're going to see is a change in pitch - a climb.
... but the concept is actually "reverse command." Sorta like if you get there and what normally makes you go left now makes you go right and right is left. Or sorta like inverted flight, pushing = pulling and pulling = pushing. You don't wanna pull when you shoulda pushed, when you're inverted and low.
Commands normally given for a regime of flight are now reversed, which is why it's called the region of reverse command.
Normally - throttle in = climb but now throttle in = airspeed change.
Normally - yoke back = airspeed change but now yoke back = climb.
cessna182h
Filing Flight Plan
The way my old instructor, Les Cothron, used to teach it, you started out at cruise and slowly pulled the throttle back and as the airplane slowed and started a descent, you countered the descent with nose up pitch. As the pitch increased and the aircraft slowed, you could no longer maintain flying speed unless you fed in more and more power. Especially with an underpowered craft like a 150, the pitch was so high that even with full power, you could barely keep from stalling...just mushing around 50 mph!
Jim_R
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Jim
G's are accelerations. Lift is a force. There is no such thing as "1G lift". You mean "lift = weight".
TMetzinger
Final Approach
The whole "reverse command" thing needlessly confuses people, in my opinion. I teach it as speeds above Best Glide (L/D max) and Below Best Glide. Assuming you don't reach the stalling AOA:
Above Best Glide, if you raise the nose, you'll get a climb and a reduction in airspeed until a new equilibrium is reached, as long as the new speed remains above best glide, because total drag DECREASES with the decrease in airspeed.
When the speed is below best glide, then raising the nose will actually result in an increased rate of descent unless power is added, because total drag is INCREASING with the decrease in airspeed.
Above Best Glide, if you raise the nose, you'll get a climb and a reduction in airspeed until a new equilibrium is reached, as long as the new speed remains above best glide, because total drag DECREASES with the decrease in airspeed.
When the speed is below best glide, then raising the nose will actually result in an increased rate of descent unless power is added, because total drag is INCREASING with the decrease in airspeed.
L/D max is Best Glide?
TMetzinger
Final Approach
Well, the FAA doesn't use the words "best glide" in the PHAK anywhere, but in the Airplane Flying Handbook, they say...
"
The best speed for the glide is one at which the airplane
will travel the greatest forward distance for a
given loss of altitude in still air. This
will travel the greatest forward distance for a
given loss of altitude in still air. This
best glide speed
corresponds to an angle of attack resulting in the least
drag on the airplane and giving the best lift-to-drag
ratio (L/DMAX)."drag on the airplane and giving the best lift-to-drag
I thought Best Glide was variable with wind.
TMetzinger
Final Approach
Nope... Best glide gives you the most forward motion for the least amount of drop. Wind matters in terms of if you can make a certain distance, since it's either a net gain or loss in distance covered, but it doesn't affect the best glide speed, to my knowledge.
There's also "minimum sink", which is least amount of drop per unit of TIME.
There's also "minimum sink", which is least amount of drop per unit of TIME.
Doggtyred
En-Route
How would the wing know what direction the prevailing wind is, as long as the airspeed vector over the wing is constant?
Dan Thomas
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It is, if you want to cover the most ground in the glide. If the wind is a headwind you need to increase airspeed. If we were gliding at 60 knots into a 60 knot wind we'd go nowhere, but if we stuck the nose down and glided at 80 knots we'd make 20 knots over the ground. Beats coming up short.
If we have a tailwind we're better off going closer to minimum sink speed, near the stall.
Dan
Minimum sink is L/D max.
Sure it does, to maximize distance into a headwind, you'll have to speed up.
COFlyBoy
Line Up and Wait
Tim, I think this is the best explanation I've seen. It should be a very simple demonstration. Bombarding people with a lift/drag curve or a power/airspeed curve requires that they be able to interpret the implications of a graph, which isn't all that easy. (Since someone is going to pick on the subjective statement I just made I should back it up. I am an aero engineer, I interpret vague graphs of data for a living. I know it is difficult for 95% of the population to simply look at a graph and in any way intuit how it is going to impact reality (even amongst engineers))
Everskyward
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Everskyward
Interesting, because graphs usually make things much clearer to me. Equations on the other hand....
I think it depends on your learning style.
bbchien
Touchdown! Greaser!
EdFred
Taxi to Parking
Wow, you guys are making this waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay too complex. He already said he was having trouble grasping it. Throwing out LDmax, graphs, and everything probably isn't helping since he's read this already.
Simply put:
There is a certain airspeed which is the "balance" point. An airspeed slower than that puts you on the "back side" of the power curve. Any airspeed higher than that puts you on the "front side" of the power curve.
If you are faster than that speed (on the front side) and you want to go higher, you increase pitch or raise the nose. You have extra power available, and raising the nose uses that extra power to gain altitude. If you add power, and keep the nose pointing straight ahead you go faster.
Now, if you are slower than that speed (on the back side) and you want to go higher, you can't just raise the nose, because you don't currently have that extra power to do so. You have to increase the power to go higher - add more throttle. By the same token if you want to go faster, you have to decrease the pitch or lower the nose to go increase your speed.
So let's say that speed is 80. You are currently going 60, maintaining altitude, and are operating at 50% of your current available power. If you want to go higher, you can't just raise the nose. You currently don't have enough extra power to do so. To go higher you have to add power. This is what being on the back side of the power curve is.
Simply put:
There is a certain airspeed which is the "balance" point. An airspeed slower than that puts you on the "back side" of the power curve. Any airspeed higher than that puts you on the "front side" of the power curve.
If you are faster than that speed (on the front side) and you want to go higher, you increase pitch or raise the nose. You have extra power available, and raising the nose uses that extra power to gain altitude. If you add power, and keep the nose pointing straight ahead you go faster.
Now, if you are slower than that speed (on the back side) and you want to go higher, you can't just raise the nose, because you don't currently have that extra power to do so. You have to increase the power to go higher - add more throttle. By the same token if you want to go faster, you have to decrease the pitch or lower the nose to go increase your speed.
So let's say that speed is 80. You are currently going 60, maintaining altitude, and are operating at 50% of your current available power. If you want to go higher, you can't just raise the nose. You currently don't have enough extra power to do so. To go higher you have to add power. This is what being on the back side of the power curve is.
bob_albertson
Pre-takeoff checklist
Not really, either
Your describing aircraft control with respect to the use of pitch and power. If that was really the region of reverse command then you would read about the Navy's instruction method of teaching pilots how to fly in the region of reverse command. That isn't true... The region of reverse command occurs at a specific point on the lift/drag curve (drag bucket). Using pitch and power for aircraft control occurs for all flight regimes.... The Navy happens to teach pilots to use power for airspeed and pitch for altitude --- which with your explanation would indicate they always fly in the region of reverse command
Aircraft control changes in the region of reverse command are from a cause/effect relationship more so then just how the controls are manipulated - unlike the Navy, the control manipulations in the region of reverse command are different from what most of us are taught. The region of reverse command is actually an aerodynamic point on a graph and not how the controls are manipulated. It shows us that an increase in AOA further increases induced drag which requires more thrust to overcome the increased drag. This reduces the amount of excess power available to climb. So the slower you go the more power it takes to overcome the drag, which means most of your thrust is gone and what is left is much less excess power than what you have in the region of normal command Also.... regardless of how you manipulate the controls the only thing that allows a plane to climb is excess horsepower (or thrust depending). Pushing the power in during the normal region of command flight the plane climbs not because of power but an increase in airflow over the wings that increases the lift - without adding a corresponding pitch change the airplane will climb at a given speed --- If, however you counter the increased lift with sufficient down force the extra horsepower is converted into speed... Run out of excess horsepower and you reach your absolute ceiling. Where the plane will maintain altitude and airspeed, but not be able to climb. If you vary the pitch - and change the AOA - you will either accelerate but trade altitude to do so or you will pitch up and find out what happens in the region of reverse command when you do not have any excess power available
.poadeleted20
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Perhaps so, but the OP asked for it.
Depends on whether the trainee is satisfied with just learning the effect, or wants to learn the theory, too. I can teach the effect in about 5-10 minutes in flight by starting at about 1.3 Vs0 and slowing in 5-knot increments, noting the power setting required to maintain altitude as we slow. Teaching the theory can take a few minutes or an hour depending on the trainee's prior level of aerodynamic knowledge and inherent grasp of math/engineering concepts.
The concept and its application are not quite that simple.
poadeleted20
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Not quite so. The difference between the front and back sides of the power curve is this:
On the front side of the power curve, as you go slower, it takes less power to maintain altitude. On the back side, as you go slower it takes more power to maintain altitude.
That's it.
As for your statement about the relationship between AoA/power and speed/climb rate, that relationship remains the same on both sides of the power curve -- if you increase AoA, you go slower with the same power, and if you increase power, you climb faster (or descend more slowly) at the same speed (and vice versa for reductions in AoA and power).
poadeleted20
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No -- minimum sink speed is below L/Dmax. While minimum sink gives you the lowest sink rate, the forward speed is lowered even more, so glide angle is reduced.
COFlyBoy
Line Up and Wait
Yep its a learning style thing.
The formula for the L/D curve is proportional to y= 1/x+x^2 which actually looks the same to me as the graph (as in I process them both the same way).
I was really referring to the fact that understanding the reverse command implication of the graph is more of a secondary impact. Its a question of what that curve means in relation to your power and pitch knobs. As the OP pointed out, its hard to make that leap if you're new.
OTOH, as Tim and Ron have pointed out, it only takes about 10 minutes in the plane to create your own power/airspeed graph and have a complete understanding of how it impacts your flight. Its almost like this concept shouldn't even be attempted in writing.
bob_albertson
Pre-takeoff checklist
Yep exactly - normal side = power has one of two effects - for the same AOA lift if increased and altitude increases or - lower the pitch and thrust become greater and greater than drag so airspeed increases.
Backside = drag increases inversely as the square of airspeed and requires more and more thrust to keep the thrust/drag ratio in equilibrium otherwise airflow over the wing degrades and as the AOA required to produced the same amount of lift increases (hold altitude) the drag continues to increase since induced drag is a by-product of lift (essentially it is caused by the wingtip vorticies). That's the cycle I was referring too earlier when I mentioned without power on the backside of the power curve you will stall the plane due to increased drag slowing the plane requiring a higher AOA to produce the same lift but that higher AOA and slower speed causes the vorticies to be closer together which increases the total induced drag -- so the plane slows further and needs a higher AOA --- only power - in that regime - can stop the drag from increasing.
I hope I didn't go and get retarded on everyone there
BellyUpFish
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Backtothesand
Were you taught to fly in the Navy or did you read Aerodynamics for the Naval Aviator and come up with how the Navy teaches their pilots?
I think the OP needs to google "pitch for airspeed and power for altitude."
This is one of those things is aviation that will continue to be and has been argued for decades..
The FAA can't even get it right. In the Airplane Flying Manual by the FAA they still emphasize controlling airspeed with pitch and altitude with power. However, at CFI courses all over the country they teach the direct opposite. The face of the matter is you can't change one (pitch or airspeed) without changing the other.
One in which both sides are right, in part. And both sides are wrong, in part. It depends on the phase of flight.
Power for altitude, pitch for airspeed -
You know that if you are in straight and level trimmed flight and you increase power, you will climb. If you decrease power, you will descend. So, in this case, power ~is~ for altitude.
Similarly, if you pitch the nose down, your airspeed will increase. If you pitch the nose up, your airspeed will decrease. So, again, pitch ~is~ for airspeed. This is often the technique used to maintain airspeed on final approach.
Power for airspeed, pitch for altitude -
However, sometimes, power is used to control airspeed. Heck, have you ever tried to take off with the engine at idle and pitching up the nose? Of course not. But if you want to fly faster, sometimes you need to increase power.
Finally, pitching can and should be used for some altitude changes or to initiate altitude changes.
All that being said, we need to teach our students that both pitch and power control both airspeed and altitude. They are not mutually exclusive. They work together. Even Langewiesche says pitch for airspeed, power for altitude - but it depends on the situation and aircraft. You can't fly an F-18 like you can fly a J-3 or a Spitfire.
Just remember not to think about it too much or overanalyze it. As you know, the key is to just fly the plane knowing some of these basic principles.
Here's a really good read on the whole thing..
dell30rb
Final Approach
You can compare it to a boat.
When you're near stall speed in an airplane, nose way high and lots of power, its like a boat coming onto a plane. Takes alot of throttle to get the boat on the plane, and in the transition the nose comes up and the boat starts pushing alot of water. Once you are on top of the water the nose comes down and the boat skims on the top of the water, and you pick up speed. This effect happens in an airplane around your best glide speed
This funky spot between actually being on a plane, and not being on a plane is your "backside". For the airplane this is somewhere between stall and best glide speed. The wings interacting with the air is similar to the hull of a a boat interacting with water, its "pushing" alot of air and so there is more drag.
When you're near stall speed in an airplane, nose way high and lots of power, its like a boat coming onto a plane. Takes alot of throttle to get the boat on the plane, and in the transition the nose comes up and the boat starts pushing alot of water. Once you are on top of the water the nose comes down and the boat skims on the top of the water, and you pick up speed. This effect happens in an airplane around your best glide speed
This funky spot between actually being on a plane, and not being on a plane is your "backside". For the airplane this is somewhere between stall and best glide speed. The wings interacting with the air is similar to the hull of a a boat interacting with water, its "pushing" alot of air and so there is more drag.
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BellyUpFish
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Backtothesand
I love how this thread is full of acronyms and pilot speak. Language that a person who doesn't fly planes will never understand.
Even better is that it's all in reply to a post that starts - "I have zero flight hours.."
Even better is that it's all in reply to a post that starts - "I have zero flight hours.."
zaitcev
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Pete Zaitcev
I suggest reading Denker. His (online) book explains it all in a very sensible way.
URL:
http://www.av8n.com/how/
EdFred
Taxi to Parking
Yeah, which is why I tried to avoid all that in post #25.
dell30rb
Final Approach
I agree with you, but the guy says he's been reading up for awhile.
I knew most of the terms and etc when I started taking lessons. I read a ton
bob_albertson
Pre-takeoff checklist
Even one better -- I was taught by a Navy IP.
Still, talking about pitch and power is nothing more then control methods. At the crux of the issue is simply nothing more then the 4 forces of flight. Thrust and pitch are both capable of changing the total amount of lift produced. Thrust is able to overcome drag and speed the plane up, and drag is able to overcome thrust and cause the plane to slow down. In order to really talk about the region of reverse command you need to leave out a description of aircraft control methods. All you are doing with the control inputs is varing the lift/thrust/weight/drag vectors.
My contention with describing the region of reverse command with control inputs is that the method of control is nothing more then playing/varing the aerodynamic forces on the plane. Kinda like describing pitch as causing a change in altitude -- the pitch change has nothing to do with the change in altitude -- the pitch change causes the AOA on the wing to change (in most but not all cases) which varies the lift produced and for the time the plane's VSI increases lift is greater than weight. As soon as the rate of climb equalizes all 4 forces are equal again (steady state climb). If you change the pitch and the airspeed decreases but the rate of climb remains unchanged you still have lift and weight in equalibrium.
So - control inputs are the cause side of the equation and the aerodynamic forces that vary are the actual effect side of the region of reverse command. No matter how you vary pitch and power you are only affecting lift/thrust/weight/drag in each flight regime. How you affect them can be complicated in the descriptions of what happens. But... to say the region of reverse command is where power controls airspeed and pitch controls altitude isn't correct. It is the region in which more thrust must be added to overcome the induced drag as the airspeed slows.... again, it's about the 4 forces -- how you manipulate those will allow you to safely operate in the region of reverse command... But, the description of pitch and power = region of reverse command isn't correct. Pitch and power describe the operational (control) charateristics when you are IN the region of reverse command. The region of reverse command is defined (and causually explained) by the definition I quoted earlier.
pitch and power = operational control
thrust and drag = the cause for the region of reverse command.
Bob
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bob_albertson
Pre-takeoff checklist
BellyUpFish
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Backtothesand
I dunno if that's better than having been in the Navy.
I would have love to flown in the Navy, but just never tried. Dunno why. I was taught by a Naval IP as well, well actually two. My father and uncle both flew in the Navy.