Engine power output and adjusting it.

[Ivan]

Hi Brett_Henderson, fliger747

I already saw your correction regarding 70 mph. I was just trying to post the actual data I had which BTW was from IIS 82 VIA the book "Zero" by Robert Mikesh. Seems like Mr. Mikesh converted the numbers from the original MPH to Knots. If I have the opportunity, I will have to ask why he did that.

The actual numbers from IIS 85 were the following:
Gear + Flaps Up, Power On ==> 74 mph
Gear + Flaps Up, Power Off ==> 78 mph
Gear + Flaps Down, Power On ==> 61 mph
Gear + Flaps Down, Power Off ==> 69 mph

Level Speeds:
Sea Level ==> 270 mph
5000 feet ==> 287 mph
10000 feet ==> 305 mph
16000 feet ==> 326 mph
20000 feet ==> 321.5 mph
25000 feet ==> 315 mph
30000 feet ==> 306 mph

I have also attached an extract from a report from Wright Field regarding A6M2's performance.
I'll be glad to send what I have from IIS 85 if you email me at Ivan1GFP@yahoo.com.

One of the things to keep in mind is that this aircraft was hardly in perfect condition. The operators also had no flight manuals. I am fairly convinced that this aircraft wasn't quite up to the mark of a A6M2 in good condition. I believe there is actually quite a lot of inaccuracies in recorded Japanese WW2 aircraft performance due to various combinations of poor condition, bad test protocols and bad records keeping.

The test reports state that the gross weight of the aircraft was 5555 pounds. The typical quoted figure for loaded weight of the A6M2 is 5313 pounds. Even the A6M2-N floatplane was only 5423 pounds. Perhaps this accounts for some of the reduction in climb performance.

Brett_Henderson: I may try to adjust Vy when I get a chance by adjusting the propeller efficiencies.

Fliger747: I AM working on finding a better propeller. That is why I am messing with a stock Hurricane propeller. I only steal from the stock aircraft so that no one can make the claim that I stole something from their plane.

BTW, the actual consideration of propeller selection is Cp, the Coefficient of Power which is a combination of power, reduction gear, prop diameter, engine RPM, etc. I worked out a spreadsheet after reading a document from Sparks' site and doing a bit of poking around on the Internet for some additional information. I needed the spreadsheet because the numbers are very non-intuitive.

- Ivan.

P.S. My F4F-3 Wildcat is actually using a P-51D propeller. ;-)

 

[Brett_Henderson]

I'd like to see that data.. This is a good discussion, and I think we're all learning something.. I'll PM my email address.. :salute:

Mean-time, you're going to discover the frailty and flaws of the flight-model... and see where trying to build the flight-model in a realistic manner (ala aiming for an accurate engine power curve), just doesn't work.

The key focal point for in-flight realism, is to get Vy / Vref set, and to get them to be aprox 130% of Vs... and then build from that foundation for an airplane that will start flying AT the proper airspeed, and AT the proper length of runway,,,, and then maintains an accurate max speed at altitude, AT a reasonable power setting. That's a very tall order in itself.. If you manage to also get a realistic climb performance profile through the service-ceiling, you've done very well.

Here's the method that you've either discovered is problematic, or are just unknowingly going to attempt:

Brett_Henderson: I may try to adjust Vy when I get a chance by adjusting the propeller efficiencies.

Vy is not changeable by propeller performance, nor power (either by adjusted curve or actual throttle setting). A V-speed (any V-speed), is essentially an AoA. It's a function of the wings and airframe. Vy at 50% power is the same as Vy for 100% power. Yes, those two power settings will yield two different RATES of climb, but at either of those power settings, the best-rate-of-climb happens at the same airspeed (Vy). Not to mention that if you try to adjust V-speeds by thrust (engine power or prop performance), you'll throw the other parts of the performance envelope out the window. You have to adjust for V-speeds by airframe numbers.

Now the evil-twist from MSFS. You actually can change Vy by thrust (albeit by less of a factor than airframe tweaking.. as it should be), which is utterly unrealistic. Which is why I mentioned in an earlier post, that when designing a flight-model, you have to assume that a sim-pilot will be pitching for Vy at high power settings (and pitching for Vref at low power settings). It's just something we live with.

There's always more than one way to skin a cat.. I'm not saying it's impossible to build an accurate model around an accurate engine, but it's pretty near impossible. I've done countless flight models (even a few big jets). I started out determined to do it just like intuition would lead you.. I.E.. setting up absolutely, dead-on geometry, weights, MOIs, and control-surface deflection for the airframe... and using ultra-realistic engine specs. The initial testing reveals that only for a light-single (C172-ish), will you be anywhere close to something that can be flown realistically. I did a Saab 340 a couple years back.. and I literally gave up (as bad as the MSFS model is.. it's out of this world bad for turbo-props), and just tried to make it something that was pleasant, and a bit challenging to fly, with accurate takeoff performance, and accurate cruise performance.

If I were to build a flight model for an A6M; I'd use the P-51 air-file (and not touch it).. and then build a cfg file with realistic airframe/engine numbers. The construction would then entail tweaking airframe numbers, and power-scalars to get as close as possible for the entire flight envelope. If there was some "bending" of the engine power curve needed at that point, or some prop-performance tweaking (you'll be hard-pressed to do a better job of tweaking the prop by something other than min/max pitch in the cfg.. especially while trying to keep manifold-pressure and RPM number relationships accurate).. I'd open the air-file. Doing so before getting everything else right, makes the whole process infinitley more difficult. And 99 times out of 100, the "bent" engine won't be all that realistic anyway.

When a sim-pilot pushes the throttle forward for takeoff.. it's of little significance that the modeler can claim that his engine could be put on a virtual dynamometer, in an pressure controlable wind tunnel, and produce a data-sheet that rivaled real-world numbers (or that the model demonstrates accurate, full-throttle airpseeds at a given altitude), if it leaps off the runway and climbs like a jet (or cant' get airborne at the proper airspeed, using up realistic ruway length, and takes forever to get to altitude), or a realistic/stable approach can't be set up at/near Vref.

ANYway.. one last reminder. When you work on Vy.. don't do it from the engine/prop, and do your testing at takeoff power first. Vy should be chased by wing-efficiency an drag .. normally inverting the drag numbers (increasing induced means decreasing parasitic, etc). If Vy is unknown, just use 130% Vs. If you get it to where the vertical-speed is highest at Vy, you're off to a good start (you'll tune for the actual vertical speed by engine/prop power/thrust scalars.. or if you're ambitious, this is where you'd mess with the engine power curve in the air-file .. hoping that you keep the altitude effect on the curve to stay reasonably predicatable, and not make chasing takeoff-perfromance vs cruise-performance a tail-chasing nightmare.. (why I leave the air-file alone until the very end.. and tweak for in-sim results, not realistic dyno readings)..

 

[Ivan]

Hi Brett_Henderson,

File sent.

I believe that perhaps the formulas you state may apply to modern General Aviation aircraft but don't necessarily apply all that well to WW2 fighters. The A6M2 Zero we have been discussing is an example where Vs * 1.30 << Vy. Vy according to IIS 85 is 105 knots or 120 mph. Vs (Clean) = 74 mph, so the Vy using this formula should be 96 mph. The Spitfire Mk.IX and F6F Hellcat are two others that don't come close to this ratio either.

I don't believe I agree with you that Vy is a constant AoA no matter what the power is. Vx I believe won't vary as much as Vy, but isn't constant either. The reasoning is thus: Drag (Parasitic + Induced) is a U shaped curve plotted against velocity. At the low speed end, it rises because Induced Drag rises even though Parasitic drag drops. At the high speed end, it rises because although Induced drag is dropping, Parasitic drag rises as to V^2. The thrust curve may be either an inclined straight line for a Jet or an inverted U for a propeller. There should be two points where the two curves cross. The low end is Vs (Really Vmin). The high end is Vmax.

At points in between, the thrust curve will be above the drag curve. The speed at which the greatest difference is found will be Vy.

So far, I am sure you are in agreement. Imagine now that we adjust the thrust curve a bit. Reduce the low end and decrease slope of the curve (because of a high minimum pitch setting) and increase the peak value because of higher horsepoer (and thrust). Now the greatest difference in thrust will be at a much higher speed.

In case you are thinking this has no place in the real world, consider that a Spitfire Mk.I or Mk.II had 1030 hp. The Spitfire Mk.IX in its final versions had over 2000 hp. There was some additional weight but the form of the airframe didn't change much. Consider the Me 109F to Me 109K transformation. Again, in form, the airframe didn't change much, but power was much increased.

I am guessing that Vx should occur at the AoA of CLmax but I am not sure. If this is the case, then consider what differences would be if the aircraft had a LOT of power as versus not enough power to match drag at CLmax. I believe an example of not enough power would be a powered Sailplane.

BTW, FWIW, all of the flight models I have put together have been based on the Stock CFS1 P-51D. They all were extensively modified though. Now as far as not touching the actual AIR file and doing all changes in the CFG file, note that there isn't one with Combat Flight Simulator. Also the Propeller tables have a couple hiccups (discontinuities) in the P-51D.

I don't believe the version of AIR file that is used in CFS1 has an option for Turboprops at all. I believe that perhaps you can simulate their effect by adjusting the propeller tables though....
;-)

Also, did you notice that my A6M2 power curve falls off with altitude more quickly than the one from IIS 85? Perhaps that explains the difference in service ceilings.

What is your protocol for testing climb rate? I know the FAA methodology which obviously works, but I don't have the patience for it.

- Ivan.

 

[Brett_Henderson]

I don't believe I agree with you that Vy is a constant AoA no matter what the power is. Vx I believe won't vary as much as Vy, but isn't constant either. The reasoning is thus: Drag (Parasitic + Induced) is a U shaped curve plotted against velocity. At the low speed end, it rises because Induced Drag rises even though Parasitic drag drops. At the high speed end, it rises because although Induced drag is dropping, Parasitic drag rises as to V^2. The thrust curve may be either an inclined straight line for a Jet or an inverted U for a propeller. There should be two points where the two curves cross. The low end is Vs (Really Vmin). The high end is Vmax.

At points in between, the thrust curve will be above the drag curve. The speed at which the greatest difference is found will be Vy.

Of course the drag curves by airspeed.. that's why Vy is a constant airspeed. You pitch so that the AoA (airspeed) is constant. Different power settings will result in different angles of incidence, because you chang the pitch to counter the power change and maintain a Vy AoA (airspeed). The wings don't know what the angle of incidence might be.. all that they see is the angle in which they're passing through the air.

See attached image: The available and required power lines are curved as a result of drag at increasing airspeed. Changing power would result in moving the entire purple line up or down. The point at which the greatest distance between the two power curves stays the same.. it's Vy.

 

[Ivan]

Hi Brett_Henderson,

What happens when the power curve changes to the one shown in Red?
Seems to me that Vy would be higher.

Also, changing power settings I believe would do more than just move the entire power available curve up and down. It would change the shape of it as well.

- Ivan.

 

[fliger747]

To give an example, as one approaches the absolute ceiling (for aircraft in the very low sub sonic range) power decreases, finally to a point where excess power exists only in the bottom of the "bucket". As you can see from the above examples the point of excess power (think thrust) has to move a bit to the left.

For early jets for which pure thrust is related to airspeed, the shift and effects are considerable more pronounced!

Cheers: T

 

[Brett_Henderson]

What happens when the power curve changes to the one shown in Red?
Seems to me that Vy would be higher.

The power curves in that image are bent due to drag changing by airspeed. That red curve would be for a completely different airframe, with different drag characteristics. The available power curve does not represent a change in the power being generated.. it represents how much of that power is useable due to the effect of drag as airspeed changes.

Also, changing power settings I believe would do more than just move the entire power available curve up and down. It would change the shape of it as well.

I'm sure it would change a little bit, but not enough to matter for a flight-sim model or even in a real airplane. That curve represents how drag-by-airspeed will "bend" a fixed amount of power.

Vy is Vy, regardless of power (where you set the throttle, or how you tune the engine). More power will result in a higher angle of incidence, hence higher rate of climb, but the highest rate of climb for any power setting is the same airspeed... Vy.

Think about a sustained climb. The power literally DOES change with altitude. When I climb into a real airplane, I pitch for Vy as soon as a climb is established.. and continue pitching for Vy through the climb. As power lessens due to altitude, the pitch angle needed to maintain Vy becomes lower.. the angle of incidence lowers.. the rate of climb lowers.. but the BEST rate of climb always happens at Vy.. and it stays the same.

 

[Brett_Henderson]

Try looking at it from this angle:

If you walk out onto the ramp to your little Piper Warrior, and cut the plug-wires to one of the cylinders, you've got a completely different engine, with a completely different power curve.. right ?

HOWever (if you're goofy enough to fly this airplane), as you roll down the runway, the airspeed at which you can rotate hasn't change at all.. nor has the airspeed where you'll get the best rate of climb.

It will take longer to reach rotation airspeed (and use more runway), and your actual vertical-speed will be less, but the vertical speed will be at its highest, at the same airspeed it would be at its highest, if you were using all four cylinders.

 

[Ivan]

Hi Fliger747,
Seems like we are in agreement here.

Hello Brett_Henderson,
The Green curve represents Airframe Parasitic + Induced Drag.
The Purple curve represents Thrust output from your engine, not drag.
The Red curve isn't a new airframe, it is just an engine with a different power curve.

This MUST be the case because where the two curves cross represent points where the thrust and drag are equal. The two points are minimum and maximum speed.

Regarding your example with the Piper Warrier:
Your Power Off stall speed is unchanged.
Your Power ON stall speed is higher and closer to your power off stall.

(I believe fliger747 was also trying to state this.)
Your Vy speed is now totally dependent on your new power curve. There are a couple possibilities:
1. You have plenty of power but your Vy speed goes down because there isn't enough power at higher airspeeds for the max surplus power to be at the same airspeed as before.
2. There is so little power that you can't even REACH your former Vy speed, so you now have a much lower Vy speed. (More specific example of Case 1.)
3. There is so little power that you can't get airborne in which case Vy is Zero.

- Ivan.

 

[Brett_Henderson]

I think I see how you're getting confused...

(Fliger747, you're welcome to chime in any time)

The Purple curve represents Thrust output from your engine, not drag.

No... The thrust (power) is not a variable on that graph. The purple line does not represent a changing power.. not by power-setting, nor RPM, nor altitude, nor anything. The purple line represents how much of a CONSTANT (non-changing) amount of power is left available as airspeed (drag) changes. If an engine's ouput were a variable, it would have to be on a 'Z' axis. The graph we're seeing would be a cross-section of that 3D graph.. Any cross-section would be representing power as a constant.. different cross-sections would have the entire purple line at different levels.

The Red curve isn't a new airframe, it is just an engine with a different power curve.

Again.. the engine's power curve is not in play in that graph. Anywhere along the purple line, as it goes up or down.. the engine power doesn't change.. the modulation in that line represents the power remaning after airpeed-drag takes its toll. .. i.e.. 'Available Power'

This MUST be the case because where the two curves cross represent points where the thrust and drag are equal. The two points are minimum and maximum speed.

You're agreeing with me here. Yes, they are max/min airspeed for a CONSTANT power. Think about it... if the engine's power output is a variable on that graph, the max airspeed would vary too. Max airspeed is a set point in that graph, because engine output is a constant. Think about my example of how the purple line would be at different levels on differnt cross-sections. A cross-section with the purple line elevated would indeed extend the max-airspeed intersection out to a higher airspeed. More power, higher max airspeed.

Regarding your example with the Piper Warrier:
Your Power Off stall speed is unchanged.
Your Power ON stall speed is higher and closer to your power off stall.

Yes.. the stall speeds differ by power (the power-on speed would be lower, not higher), because at the very edge where a wing stops flying, airflow generated by the prop becomes proportionally significant enought to alter the AoA, and at a high pitch angle, thrust itself becomes a vector that counters gravity. That's why I didn't mention stall speeds.. it's a different discussion. And again, it's moot. Different engine power does not come into play on our Vy graph. In the Warrior scenario, Vy does not change by power. Vertical speed will change AT Vy, but not Vy itself.

To stay one step ahead of you.. I'll ask (and answer) why the thrust vector doesn't change VY, especially on high power-to-weight airplanes. If you're using thrust as lift, you're venturing into Vx territory... a better climb angle.

(I believe fliger747 was also trying to state this.)
Your Vy speed is now totally dependent on your new power curve. There are a couple possibilities:
1. You have plenty of power but your Vy speed goes down because there isn't enough power at higher airspeeds for the max surplus power to be at the same airspeed as before.
2. There is so little power that you can't even REACH your former Vy speed, so you now have a much lower Vy speed. (More specific example of Case 1.)
3. There is so little power that you can't get airborne in which case Vy is Zero.

(1 & 3): Again.. Vy does not change. The rate-of-climb is dependent on available power, and will go down for the cut-wire Warrior, but Vy reamains the same. The cut-wire scenario would be no different that taking off with the throttle set lower than takeoff power, on an engine using all four cylinders (you still pitch for the same Vy climbing out at the reduced power setting).. or even like taking off at a high-density-altitude airport. Vy is still Vy.

(2): The lessened power will give me a lower VERTICAL speed at Vy, but it does not change Vy

 

[Ivan]

http://www.genebenson.com/Training Pvt - Comm/drag_clip/drag_wrapper.htm
http://www.slkelectronics.com/help/Eca00020.htm


Stall speed with power off is higher than stall speed with power ON.
With a smaller amount of power available, the NEW power ON stall speed will be closer to stall speed with power OFF and therefore a higher not lower speed. There are other factors such as increasing airflow over parts of the wing and tail, but the most simple factor is that the thrust line is inclined upward. Thus if the AoA is 15 degrees (stall in level flight), there is a force of Thrust * sin (15 degrees) offsetting weight.

I still believe you are interpreting the graphs incorrectly. Note that the parenthesis shows Vy at the maximum of the DIFFERENCE between thrust and drag. If the Purple line represented thrust minus drag, then the maximum excess thrust would be at the PEAK of the Purple line. As you can see in the graph, the Vy velocity is NOT at the peak of the purple line.

If the Purple line represents thrust minus drag, then why is the intersection of the two lines significant? It is significant because the Purple line represents thrust and when it meets the drag curve, the aircraft has reached its maximum level speed.

Perhaps you are getting the posted graphs confused with the graph at the bottom of this page?

http://www.allstar.fiu.edu/aero/BA-Form&gra.htm


- Ivan.

 

[Brett_Henderson]

You explained properly (as did I), that the thrust vector effects power-on stall speed.. but you've still got it backwards. The lifting thrust would allow you to fly SLOWER before a stall sets in...

The other charts you've indicated introduce another variable (altitude)... we have to come to grips with this graph first.. LOL : )

Now.. if the purple line is the engine's power curve, and engine power is a variable.. you'll have to explain to me why the max airspeed is a fixed point (or more importantly how it would occur at less than max thrust). I won't mind at all being proved wrong here, because that means I've learned something..

ANYway.. I can't keep repeating myself, so I'll wait for your interpretaion of why max airspeed is a fixed point. (in the morning) ..


Edit: Did you edit the line in your last post about power-on airspeed.. or am I just that tired ?

 

[Brett_Henderson]

OK.. I got a good night's sleep.

We're in agreement that the amount of power effects the power-on stall speed, and why.. so we're done with that topic.

Now.. If the purple line represents an engine's power curve; by what influence is it curved ?

Typically, a piston engine power curve is graphed across RPM (throttle is the influence).

For an airplane (with a fixed throttle setting), the curve can be graphed across altitude (atmospheric/manifold-pressure is the influence).

No matter what the influence might be; if the line represents the engine's power, then that graph suggests that the airplane will continue gaining airspeed toward max-airspeed well past max power WHILE drag continues increasing. That aint possible.

Further.. if the purple line DID represent an engine power curve, the max airspeed would have to be a curved line too. More power (by RPM, altitude, or any influence), would result in a higher, max-airspeed.

If, as I'm suggesting, the power is a constant, and the curve represents the power that is 'available' for climbing as airpeed increases (the purpose of the graph is to show where/why VY occurs)... a fixed max-airspeed makes sense.

Also, consider this. If the amount of actual power (not available power) DID influence Vy.. Vy would have to be a curve too.. but in this graph, it's a fixed point on the purple curve.

To step away from graphs and theory.. allow me to use real-world data. If I were to climb into a 300HP C206, with no passengers or payload, and a light fuel load; I'm going to takeoff and climb with manifold pressure well under maximum (to save engine wear and fuel). The airspeed at which I have enough lift for rotation is the same as it would be at full power. The airspeed at which I'll get the best rate-of-climb is the same as it would be at full power. Full power will get me to those speeds more quickly (and generate a higher vertical speed at Vy), but those airspeeds stay the same.

 

[Brett_Henderson]

I neglected to respond specifically to a couple of yourt points.. I'll do it now:

If the Purple line represents thrust minus drag, then why is the intersection of the two lines significant? It is significant because the Purple line represents thrust and when it meets the drag curve, the aircraft has reached its maximum level speed.

It's not just thrust minus drag; it's how much thrust is available for climbing (or accelerating) as airpseed changes.

Remember, this is a graph to define Vy. By definition power has to be a constant (i.e.. climb power). With power set to a fixed amount, how does a pilot control airspeed ? By pitch, of course.

So the airspeed changes on that graph, as we pitch for different airspeeds.

Pitching for max level airspeed (zero rate of climb), the amount of power 'available' for climbing decreases as airpseed increases toward the point where 'available' power meets 'required' power.. a FIXED max airspeed, where there's no power 'available' for climbing (with more 'actual' power, max airspeed would be higher).

Conversely, pitching for a lower (climbing) airspeed moves us leftward on that graph, and the power that is 'available' for climbing increases. The point where power 'available' for climbing has the greatest seperation from the power 'required' for climbing, is obviously where the rate-of climb will be the highest.

 

[fliger747]

To add a little confusion.... the drag vrs airspeed curve is only applicable for a given weight.... An interesting fallout of this is that the maximum distance an aircraft will glide without power does not vary with weight, just the optimum speed varies. So the drag curve moves to the right at higher weights.

A very good explination of all of these factors is contained in "Aerodynamics For Naval Aviators"... essentially aerodynamics for the rest of us. Available in most pilot shops as a reprint.

The reason that one cannot reach the ultimate ceiling in stable flight at stall is one enters the area of reverse command where drag increases with decreasing speed. Any manuver will increase drag beyond the power available and speed will further decrease and drag will further increase. Level flight cannot be maintained and descent will occur, one way or another.

Cheers: T

 

[Ivan]

Hello Again Brett_Henderson,
Let's see if I can address each point in turn:

Now.. If the purple line represents an engine's power curve; by what influence is it curved ?

The Y-axis in this graph is force (either thrust or drag)
The X-axis in this graph is velocity.
The Thrust of a propeller changes because the combination of propeller pitch angle and advance ratio change as the aircraft changes speed. The efficiency at low speeds is fairly poor if the propeller cannot reach the fine pitch that would be optimal. The efficiency at high speeds is poor because the propeller blade at coarse pitch is spinning the air stream probably more than it is moving air backward. Also, as the propeller blade tips get closer to the speed of sound, efficiency drops.

No matter what the influence might be; if the line represents the engine's power, then that graph suggests that the airplane will continue gaining airspeed toward max-airspeed well past max power WHILE drag continues increasing. That aint possible.

Between the minimum speed and maximum speed, there is more thrust than drag which is why the aircraft can accelerate. If thrust is higher than drag, the aircraft can accelerate (or climb) and the AMOUNT of excess thrust determines how quickly this can be done. As the aircraft continues to accelerate toward max speed, the thrust drops and drag increases until they are equal at maximum speed.

Further.. if the purple line DID represent an engine power curve, the max airspeed would have to be a curved line too. More power (by RPM, altitude, or any influence), would result in a higher, max-airspeed.

The maximum speed is a point along the X-axis at which point thrust and drag are equal. We are not plotting max speed against altitude thus there is no curve here. Using THIS particular graph, if the throttle setting is changed or the altitude is changed, there would be another power curve entirely shaped differently than the Purple line.

- Ivan.

 

[Ivan]

It's not just thrust minus drag; it's how much thrust is available for climbing (or accelerating) as airpseed changes.

Total thrust minus total drag IS the amount of thrust available for climbing (or accelerating).

Remember, this is a graph to define Vy. By definition power has to be a constant (i.e.. climb power). With power set to a fixed amount, how does a pilot control airspeed ? By pitch, of course.

The throttle setting may be a constant, but the amount of thrust changes with airspeed. Even though the engine's horsepower may remain constant, the thrust changes depending on propeller pitch and advance ratio.... which determines efficiency.

- Ivan.

 

[Brett_Henderson]

The X axis is velocity.. we agree..

The Y axis is AVAILABLE thrust. It is consumed as airspeed increases, so less is available for climbing..OR the inverse is true too. As you use the thrust for climbing, airspeed goes down. Perfectly logical, and displayed perfectly on the graph.

You are correct in that propeller properties play a roll in how it's consumed as airspeed changes, just like airframe drag (primarlily induce as we change pitch) plays a roll... those are what curve the constant power into the AVAILABLE thrust curve.

Go back and watch the slide-show link you provided. It states clearly that power is a constant in this graph. We can assume that it's climb power (most likely full power) since it is designed to show the give-n-take between level-flight max-speed, and the climbing speeds, controled by pitch; specifically Vy. Consequently, they are both fixed points.

A graph that would show actual engine power curves would have to have something like manifold-pressure, RPM, or altitude as one of the axes.

Your link also mentions that it's for a fixed-pitch prop. But that's really of no consequence in our discussion. A constant-speed prop would probably flatten out the purple line a bit, as that prop's goal is to apply all power via a prop that maintains a constant RPM. How much the varying blade-pitch would curve the line, depends on a few more variables. But again, of no concern when you're simply trying to graph the compromises between level flight and climbing, for a PREDETERMINED amount of power

I'm confident this will click for you eventually.. If not, take the meat of this thread and submit it to others for discussion. I'll be happy to explain myself if confusion continues.. I've been discussing graphs like this many times over my 30+ years of piloting.

 

[Brett_Henderson]

Just for reference..let's introduce the constant-speed prop, because it indeed gives us the ability vary the whole power-source.


Back to my C206... without getting silly, we could set prop-rpms, and manifold pressure to any number of combinations.. but the airspeed where it rotates stays the same, and Vy stays the same.


EDIT: But the rate-of-climb, and max-speed WILL vary..

 

[Brett_Henderson]

OK.. let's try this from another angle..

This graph we've been working with, is a generic power-curve. It's purpose, as mentioned, is to show the compromises involved between level-flight, and climbing. Or it allows you to visualize being in front of, or behind, the power curve.

Every point on the airspeed axis is a pitch-controlled airspeed, and is a steady-state. There is no 'delta time'... There is no acceleration happening.

I've modified the image with relative airspeed lines that for our purposes will be: Vy(60)Cruise-climb(100)Max-airspeed(120)

At any one of those lines, we're locked into a thrust, pitch, RoC, and atmospheric-pressure. In other words, at 60, we aren't accelerating on our way to 100. And at 100 we aren't accelerating on our way to 120. Any point along the airpseed axis represents an equilibrium.

There is no axis/variable (i.e. time) that would allow for representing an actual thrust curve. The only way for thrust to be represented on this graph, is as a potential, or 'available' amount of an assumed constant., at different states of equilibrium.

If we WANTED a graph were were time was a variable. We could manipulate one where airspeed and time can co-exist on the X axis. If that assumption is applied to the current graph, we've got the physical impossibility of an airplane that continues accelerating while thrust decreases, and drag increases.