Range & Endurance

[mathewc]

I know there is the experimental method for finding the airspeed/power setting for best endurance on an aircraft but is there an experimental method for finding the airspeed/power setting for best range? I have a vague memory of this from a few years ago but can't remember specifically.

The method I remember being shown was to start at cruise power and reduce power by 100RPM increments and note the airspeed change while maintaining a constant altitude. The decrease in airspeed will come in fairly constant intervals until a point where it decreases significantly with the same power reduction. The power setting before the large airspeed drop was the best L/D ratio and therefore the airspeed for best range. Mixture should then be leaned and RPM adjusted accordingly.

Is this correct?

[Walker]

Yup!

that's the story in a nutshell...

[mathewc]

Thank you!

[Shiny Side Up]

Ugh. There is no experimental method for getting best range out of the aircraft, only for finding best endurance. Keep in mind that sometimes to get your best range might also be at a higher power setting than one would expect. The power one would use for best range factors most heavily on the wind.

Here's an example using the Cessna 172's performance numbers for 6000' pressure altitude. Say we're covering a distance of 120 miles. Say we had a strong upper wind of 40 Knots just to make the example more graphic.

At 47% power - about 2100 RPM your airspeed is 90 knots. With the headwind your groundspeed becomes 50 Knots, time takes 2.4 hours and @ 5.5 GPH we guzzle 13.2 gallons.

At 77% power - about 2600 RPM your airspeed is listed at 120 knots (going almost flat out for the poor bird) but your groundspeed becomes 80 knots and we cover the distance in 1.5 hours @ a greedy 8.6 GPH but we only then guzzle a total of 12.9 gallons.

Things change around of course when we have a tailwind.

It is unfortunate that the range excersise is so often misunderstood. Its more about judging the relationship between speed and angle of attack, angle of attack and drag. Drag of course directly affects the ammount of power required to overcome it, and the ammount of power we need to overcome it is directly proportional to the ammount of fuel we need to burn to make said power. The true purpose of the excersise is in demonstrating those relationships to the student and making the student proficient in manipulating the power controls.

This of course leads to endurance....

[AEROBAT]

I like Shiny Side Ups answer. As a rule of thumb your best L/D ratio airspeed is very close to your best rate of climb airspeed, with no flaps. With homebuilts you usually don't have any sort of POH to go by and of course a pilot wants to know your various numbers before you fly the plane a whole lot.

I have found best rate of climb is close to best endurance and best glide airspeeds on two different planes I have "tested" to discover the numbers. As Shiny Side Up pointed out best range is influenced by wind.

[MichaelP]

I like this exercise but IMHO it is done at the wrong time... There's only a few ace students who can become test pilots with about 6 hours in their books.
The real reason for the exercise at the time it is done is to work on S+L at Various Airspeeds.

IMHO this exercise needs to be done again around the cross country phase of a student's training where the information is invaluable.

You know that the lesson has been forgotten through the choice of power setting for the cross country portion of the flight test... In my mind a student who demonstrates to the examiner consideration for the wind in the choice of power setting is a student who got the message.
At this stage, sadly, most students do not consider why so many performance figures are published.

At the end of my lesson on Ranger and Endurance (you might have read I used the Zero over the Pacific illustration with the Catalina scraping home...), I go into other factors in the power decision.
Most people do not consider the maintenance cost and engine overhaul fund in their calculation.
It's foolish to spend two hours to save a couple of gallons when those two hours cost you $45 or more in maintenance and engine.

[Cat Driver]

(you might have read I used the Zero over the Pacific illustration with the Catalina scraping home...)

Did you take T effect into consideration with the Catalina?

[Shiny Side Up]

I like this exercise but IMHO it is done at the wrong time... There's only a few ace students who can become test pilots with about 6 hours in their books.
The real reason for the exercise at the time it is done is to work on S+L at Various Airspeeds.

IMHO this exercise needs to be done again around the cross country phase of a student's training where the information is invaluable.

I fully agree on that.

At the end of my lesson on Ranger and Endurance (you might have read I used the Zero over the Pacific illustration with the Catalina scraping home...), I go into other factors in the power decision.
Most people do not consider the maintenance cost and engine overhaul fund in their calculation.
It's foolish to spend two hours to save a couple of gallons when those two hours cost you $45 or more in maintenance and engine.

Its important to keep in mind that when teaching it that what the pros and cons of the power settings are. The power settings are there though to be used, and your example of the Zero and Catalina run contrary to your arguement of costing higher maintenance. I'm pretty sure that both the Catalina and Zero pilot both would shave a few hours of their engine's life off if doing so might save their own. If you treat the engine well during regular operation, it will put up with your treating it poorly when you absolutely have to.

One has to be careful with students these days though, most usually are clueless when I start talking about stuff like "Zeros" and "Catalinas", and the history lesson I have to give usually takes too long for many students for me to get the point across. Most pilot students I find aren't nearly as interested in the history of aviation as one would wish since there's a lot of lessons to be learned there.

I have found best rate of climb is close to best endurance and best glide airspeeds on two different planes I have "tested" to discover the numbers.

Its no coincidence that these numbers all fall in the same place on the power curve. The only difference between the numbers depends on how much thrust is being generated to counteract the drag at the angle of attack that gives the best lift over drag ratio. At best rate of climb you have full power, best endurance you are generating just enough power and best glide you are generating no power. Theoretically these numbers will be farther spaced the greater the power you have available, best rate of climb always being the highest, best glide the lowest of the three.

[Strega]

Theoretically these numbers will be farther spaced the greater the power you have available, best rate of climb always being the highest, best glide the lowest of the three.

Care to go into this in more detail? for the record Vy is not always higher than best glide speed.

[AEROBAT]

Shiny Side
The L/D ratio of a wing is independant of power. When you climb at 8000' the best rate of climb indicated airspeed does not change yet you only have 75% of sea level power.

[Strega]

What speed do you need for max endurance for a turbo jet/fan powered aiplane SSU?
What speed do you need for max range (ok zero wind here folks) for a turbo jet/fan powered airplane?

oh and for the record, the whole "my airplanes goes 120kts, and burns 10 gallons an hour, so if I have 40 gallons of fuel I will be able to go 480 nautical miles in still air" is WRONG!!!!!!!!!

Wheres Auxbat.. he "knows" what hes talking about

[Shiny Side Up]

The L/D ratio of a wing is independant of power. When you climb at 8000' the best rate of climb indicated airspeed does not change yet you only have 75% of sea level power.

Didn't say the L/D ratio was dependant on power, its based on AoA.

Care to go into this in more detail? for the record Vy is not always higher than best glide speed.

Ah, indeed you've caught me once again. I should have been more specific. For most conventionally designed light aircraft it will be. That is to say your typical Cessnas, Pipers, etc. I'll appologise again for offending your sensibilities as my mind is often stuck in the world of small boring aircraft. For the sake of arguement remember I'm also stuck in the realm of single engined aircraft. I'll ammend the previous to say that in most cases when dealing with most light conventionally designed aircraft the numbers will be within a reasonable realm. It changes if we start playing sufficiently with (but not limited to) the design of the wing, thrust lines, available power and weight. Since you clearly know more, I'd invite you to extrapolate here, or if you wish we can talk about it some time when you next drop by.

[Strega]

For most conventionally designed light aircraft it will be.

Once again, care to back that up?

[Big Pistons Forever]

I like this exercise but IMHO it is done at the wrong time... There's only a few ace students who can become test pilots with about 6 hours in their books.
The real reason for the exercise at the time it is done is to work on S+L at Various Airspeeds.

IMHO this exercise needs to be done again around the cross country phase of a student's training where the information is invaluable.

.

A big plus 1.

I personally gloss over the subject presolo. Endurance only gets mentioned as part of a broader discussion on the realtionship between airspeed and power required. Range gets a 1 min 26 sec defintion explanation so I can put an entry in the PTR . However after solo, Endurance gets fully dealt with as part of the PDM and then practice of precautionary landings and Range gets fully explored as part of the Nav lessons (ground and air). The bottom line message I want my students to take away is a practical concept of Range as combination of aircraft performance and environmental factors (wind) which results in an optimum power setting for any given set of conditions.

[Big Pistons Forever]

Ah, indeed you've caught me once again. I should have been more specific. For most conventionally designed light aircraft it will be. That is to say your typical Cessnas, Pipers, etc. I'll appologise again for offending your sensibilities as my mind is often stuck in the world of small boring aircraft. For the sake of arguement remember I'm also stuck in the realm of single engined aircraft. I'll ammend the previous to say that in most cases when dealing with most light conventionally designed aircraft the numbers will be within a reasonable realm. It changes if we start playing sufficiently with (but not limited to) the design of the wing, thrust lines, available power and weight. Since you clearly know more, I'd invite you to extrapolate here, or if you wish we can talk about it some time when you next drop by.

Shiny

Better step aside and let Strega pass on all of that wisdom he has. After all we actually go out and instruct real people, on a fairly regular basis, what could we possible know about ab intio instruction

[Strega]

what could we possible know about ab intio instruction

I'm going to go ahead and say the bare minimum.....


Why would you teach something that was not correct?

Just trying to get people thinking... something thats very much lacking in aviation today..

[Big Pistons Forever]

Sigh

Another thread on the way to be ruined by Stega, an individual that seems to be unable to contribute anything of practical value but is really good at insulting everyone.....

[Strega]

BFF,,

I dont think Ive insulted anyone.. Im merely asking for more information..

If I said the moon is made of cheese.. what would your response be?

[Cat Driver]

I thought it was only me who is like that.

[Shiny Side Up]

For most conventionally designed light aircraft it will be.

Once again, care to back that up?

Ok, then in my experience it is. That holds for all the single engine Cessna and Pipers - wait, I must remember to be specific. In some of those cases the number is the same. For sake of the arguement, lets say we're dealing with Indicated speeds as well - Indicated Vy for all the aircraft I'll mention decreases with altitude. Lets see - remember we're talking aircraft where Vy is higher than best glide, since that now has become the point of contention....

Wait, make sure you feel free to point out precicely where I'm wrong. I am interested in ammending my train of thought if I am indeed grossly incorrect.

So lets see. Vy is higher in the following aircraft, lets start with Cessna, the Cessna models 140, !50, 152, 170, 172, 180, 182, 185 and 206 - I'm only going to list the ones I have a reasonable ammount of experience with. Continuing with Piper, it holds for all the models based upon the Cherokee airframe including from the bottom the 140, 151, 160, 161, 180 (D), 200 (Arrow) the Cherokee Six and its variants the 6X and the Saratoga. Interestingly enough it also holds for the other use of the airframe - the Seneca - but to be fair I don't have experience with models beyond the Seneca III, only the I and II. It holds for the Traumahawk, it holds for the Commanche though once again in fairness I can only say that for certain with the 250 and 260 models - I cannot say for certain that it does for the 400 (though I suspect that it does). It holds for the Short-wing Pipers, the lineage extending from the PA-11, the Cubs and SuperCubs, Pacers, Tri-Pacers and Colts - though only marginally, for practical purposes the speeds are largely the same. Moving on, it also holds for the Beechcraft light planes, the Musketeer, Sundowner, and the straight tail Bonanzas - being specific once again, tell me if this doesn't hold true for those with V-tails. It holds true for the Grumman small planes, the Yankee, Traveller, Cheetah and Tiger - and their rare twin cousin the Cougar. It holds for the Citabria and Decathalon, it holds for the DA-20 and DA-40. Would that be considered "most"?

NOW, with all that to consider, we do indeed know that its not true all the time. For example, pulling out my book for the Cessna 414, Indeed best rate with both props turning is indeed lower than the listed best glide for the aircraft. Interestingly though those speeds become the closer with only one prop turning and best glide also shrinks below Vy the lighter the airplane gets. The Navajo paints a similar picture (though its a bit tougher to find, the one thing Cessna has seemingly always had over Piper is better POH editors). How have we changed the power curve diagram of these aircraft over the ones I've listed above?

I'll let Strega answer that, I have no doubts he - and many others here know. Its getting pretty far away from the level of the question and the training the opening poster has posed his question at. Realistically this discussion can be carried a long ways - most of us are somewhat aware after all to what happens with an airplane if its capable of reaching into the transonic speeds and beyond, but for many of us thats far and beyond our areas of expertise. I know its far above the realm that I like to think that I dwell in.

If I said the moon is made of cheese.. what would your response be?

If what I have attemtped to explain above is comparable to your above example claim, then yes, ridicule away. The point is I'm not on the level of making such a claim, I've only posted the simplified explanation for the benefit of the entry level pilot. What I've explained isn't false, it simply isn't the whole picture. For those of you interested in how this can still adhere to the Learning Factors (which some might well ridicule - I stand prepared) rest assured it does.

My mistake was being unspecific for the benefit of higher level pilots and the general Avcanada crowd.

[MichaelP]

I am interested in how people do this 'experimental' exercise in an aeroplane fitted with a CSU.

With the Katana we use the book figures, setting the power for 85% down to 55% using the Aircraft Flight Manual and then reducing the manifold pressure in steps while leaving the RPM set at 1,900.
From 'best endurance' the power is increased in stages in accord with the AFM and the figures logged both ways.

While this is a good exercise in straight and level at various airspeeds and learning power setting, I think the real lesson is in using the AFM to plan your cross country with consideraton of the conditions.

We never did this when I learned to fly, we simply did straight and level and I'm not sure I was worse off for it.
Learning the POH lead me to reading Pilot's Notes and manuals for all aircraft I subsequently flew, if such documents existed.
These should be consulted first and then you need to learn you specific aeroplane... Not all tachometers are created equally!

[Hedley]

Not all tachometers are created equally!

See CAR 625 App C Out Of Phase Task Listing

http://www.tc.gc.ca/civilaviation/regse ... /a625c.htm

8. Tachometers
The accuracy of mechanical drag cup type tachometers, for fixed wing propeller driven aircraft, shall be checked on site annually, and be accurate to within the tolerances established by the aircraft manufacturer or, where no tolerance has been specified by the aircraft manufacturer, to within +\- 4% of engine RPM at mid-point of the cruise range.

This applies to private and commercially registered aircraft.

[MichaelP]

Yes, 4% of 2,500 RPM is 100 RPM and that is significant.

I always teach people to fly within the smooth range.
Sometimes setting the power to the book leads to vibration from the engine and adding 50 RPM or so can really smooth it out.
A smooth running engine is an engine more likely to last.