Maximum Endurance vs Maximum Range

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Horia1337
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Maximum Endurance vs Maximum Range

Post by Horia1337 » Sat Jun 01, 2019 10:05 am

Hello everyone i have a little question and doubt about Maximum Endurance and Maximum Range because apparently i was wrong for a little while

It has been established that maximum range, as far as the aerodynamics of the airplane is concerned, is a function of a particular coefficient of lift (Cl), the one at which the lift-to-drag ratio is at a maximum. So then if it is like this why cant i draw a graph of Drag vs Velocity on the white board and explain that my MAX range is at my best L;D ration the lowest point on the total drag curve?? Because after my book i have to draw a Horsepower required vs CAS and show where is my best RANGE SPEED. I am a little confused on this one :prayer: :prayer:
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digits_
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Re: Maximum Endurance vs Maximum Range

Post by digits_ » Sat Jun 01, 2019 10:09 am

Because a you state, you have to maximize L/D, not just minimize D.
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Horia1337
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Re: Maximum Endurance vs Maximum Range

Post by Horia1337 » Sat Jun 01, 2019 10:19 am

Thanks for the quick reply :) but i am beginner can you please elaborate more for me please because i understand your statement of you have to maximize not just minimize. But how a bigger like me can minimize that :)
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digits_
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Re: Maximum Endurance vs Maximum Range

Post by digits_ » Sat Jun 01, 2019 11:00 am

Look at an L/D graph. Those usually have a speed on the horizontal axis. Randomly pick a certain speed and read the value of L and D. If there are no values on your graph, choose or estimate a value that looks right.

Now calculate L/D with your estimated values for that speed.

Repeat the above for different speeds. You will notice that the maximum L/D value is not achieved at the speed that gives you minimum D.
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photofly
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Re: Maximum Endurance vs Maximum Range

Post by photofly » Sat Jun 01, 2019 4:19 pm

Horia1337 wrote:
Sat Jun 01, 2019 10:05 am
Hello everyone i have a little question and doubt about Maximum Endurance and Maximum Range because apparently i was wrong for a little while

It has been established that maximum range, as far as the aerodynamics of the airplane is concerned, is a function of a particular coefficient of lift (Cl), the one at which the lift-to-drag ratio is at a maximum. So then if it is like this why cant i draw a graph of Drag vs Velocity on the white board and explain that my MAX range is at my best L;D ration the lowest point on the total drag curve??
You can, and it is.

The “power required for level flight” curve is more useful for consideration of best endurance, but if you want to find “best range” speed on that curve, it’s where the tangent passes through the origin.
digits_ wrote:
Sat Jun 01, 2019 11:00 am
Repeat the above for different speeds. You will notice that the maximum L/D value is not achieved at the speed that gives you minimum D.
On the understanding that weight stays the same, and that lift is equal to weight, and that therefore in the flight in question the lift needed to maintain horizontal flight is fixed, then yes, best L/D is achieved at the speed where the drag is least.
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Horia1337
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Re: Maximum Endurance vs Maximum Range

Post by Horia1337 » Sat Jun 01, 2019 6:05 pm

You made it more easy and clear for me to understand i appreciate sir. Thanks you
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digits_
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Re: Maximum Endurance vs Maximum Range

Post by digits_ » Sat Jun 01, 2019 7:00 pm

photofly wrote:
Sat Jun 01, 2019 4:19 pm
...that lift is equal to weight, and that therefore in the flight in question the lift needed to maintain horizontal flight is fixed, then yes, best L/D is achieved at the speed where the drag is least.
Well that was a whole bunch of gibberish that I wrote.

You are of course correct.

Should have known better than to try and beat you to an answer on questions like these!
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AuxBatOn
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Re: Maximum Endurance vs Maximum Range

Post by AuxBatOn » Sat Jun 01, 2019 7:39 pm

Don't forget that power is not a force but a force x velocity. In this case, you'd have to plot the Drag x Velocity vs Velocity to find your Max E and Max R speeds. Velocities should be in KTAS which can easily be converted in KCAS at a given pressure altitude, What photofly said about Max R is true in still air. With a headwind or a tailwind, shift your origin by the amount of head/tailwind (in the negative for tailwind, positive for headwind).
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photofly
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Re: Maximum Endurance vs Maximum Range

Post by photofly » Sat Jun 01, 2019 7:49 pm

digits_ wrote:
Sat Jun 01, 2019 7:00 pm
Should have known better than to try and beat you to an answer on questions like these!
It's very easy to get turned around on this topic!

In addition to what AuxBaton added about the wind, a real consideration of best range (or best endurance) has to take into account propeller and engine efficiency, or equivalently the amount of thrust (or power) that the engine or propellor generates for a given fuel flow and at a given airspeed.

Going a little bit faster may result in more drag, but if the prop gains efficiency at the new higher airspeed this could more than make up for it: your fuel flow will increase less than your speed increases, and presto-hey, your real best range speed is now actually faster than the speed at which you get best L/D.
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Big Pistons Forever
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Re: Maximum Endurance vs Maximum Range

Post by Big Pistons Forever » Sun Jun 02, 2019 10:40 am

I think it is important that while it is good to understand the theoretical concepts around max range there are a host of practical factors that also apply.

Any discussion about situations where you are trying to get the most range out of the fuel you have, must start with pilot decision making including

- How confident am I in knowing exactly how much fuel I have on-board
- How confident am I in the weather, especially if you are trying to take advantage of a tailwind
- What is my plan B if it looks like I can’t make my destination
- Best Range for piston engines is predicated on optimal learning, a skill that doesn’t seem to be widely understood in newer pilots

The only time I ever actually flew at a max range airspeed was preceded by several really bad decisions and resulted in me landing with about 10 mins of fuel left

Finally an easy real world approximation of best range speed in a piston single is Vy. Flying that speed in no wind/tailwind conditions will get you pretty close to the theoretical least pounds of fuel per nm
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youhavecontrol
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Re: Maximum Endurance vs Maximum Range

Post by youhavecontrol » Mon Jun 03, 2019 3:13 am

...all of the above and don't forget to lean the mixture!
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Big Pistons Forever
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Re: Maximum Endurance vs Maximum Range

Post by Big Pistons Forever » Mon Jun 03, 2019 11:23 am

I hate auto correct. My last bullet got auto corrected from “leaning” to “learning” ......although in retrospect I guess both apply

Bottom line is you should get in the habit of leaning the mixture for cruise at every altitude on every flight, and the much quoted FTU-ism that you should not lean in cruise below 5000 needs to be stamped out. It is completely totally wrong.
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