What is best endurance speed?

[height & heading]

My understanding is that best endurance speed is the speed that I can maintain straight & level flight with the lowest power setting, there by giving me the least fuel consumption and therefore the longest time in the air (lowest point on the power curve).

I have tried this in 2 separate Cessna 150 aircraft and found that at 1900 RPM (with carb heat hot) I could maintain straight & level at 55 Kias, any lower and I started to descend, but when I consulted my aircraft POH it says my best endurance speed should be 45% power & at 2000ft on an ISA day this equates to 83 Ktas (Ktas & Kias at 2000ft have maybe a 1kt difference) & the aircraft was at Max weight 1600Lbs.

Can anybody explain why there is a 28 Kt difference between the manual and reality? What am I missing?

[The Greaser]

Sounds like you are flying on the 'back side' of the power curve. In essence there are two speeds (two identical values of drag) that you could be flying at with a given power setting. What you are doing is letting the aircraft slow down so much (maybe by cutting power to quickly) that you are flying near Vmca. In my experience you should be able to fly a 152 in S+L at 1900 rpm greater than 50 odd knots.
Remember that max. endurance in a piston is Vimd ie. the same as that for maximum glide.

[djpil]

In simple theory, per Kermode, for prop airplanes:
best range = minimum drag = best glide
best endurance = minimum power
= at CL for max value of ((CL)^3/2)/CD
then modify things for propulsion efficiencies

Or:
at best glide - profile drag = induced drag
at best endurance - profile drag = one third of induced drag

I don't have my 150 POH handy but from memory best glide is 60 kts. I'd guess that propulsion efficiencies would push the best range speed above that. You can do a simple plot of fuel flow and range vs airspeed and I'd guess you'd see that 45% power is way above the peak of both curves.

[I Fly]

I aggree with djpil. The speed should be 60 kt. I suppose Cessna is just lazy by not giving us graphs where you can see where it's best and where it's getting worse. There could be other reasons. Perhaps at 60 kt you have an attitude where you can't get the last drop of fuel out. In an ideal world, they would tell us. You could e-mail Cessna and enlighten us all.

[juggernaut]

Max endurance speed is slightly below the best glide speed, the Cessna 152 flight manual states that with propeller windmilling, the best glide speed in still air is 60 knots giving a best glide angle of 10/1, it would be greater with the propeller stopped, the max endurance is the minimum power speed and gives the best fuel consumption with minimum power set, this is effectively the lowest power setting to keep the aircraft straight and level, in the Cessna there are two airspeeds for the same power setting which you can demonstrate in slow flight teaching, for the Cessna it is around 58 knots depending on a few variables.
If you have a look in a few technical manuals the best glide is the point that the glide angle strikes the polar curve, the best endurance is the minimum sink speed and is the point on the polar curve where the least sink rate occurs, at a lower airspeed, I hope this is helps !

[height & heading]

Quite right - there are 2 speeds for every power setting (one in the normal flight range & one in the slow flight range)with one exception - bottom of the power curve, minimum power required to maintain straight & level powered flight, which is best endurance speed (least power, least fuel consumed - more left, more time or endurance) i.e. the point where i reduce the power to the minimum required to maintain level flight and DO NOT add power back in which is what you must do in the slow flight speed range.
My question was why does the manual say 45% power (83kts) & when i did this and found that my C150 could fly at 55Kts @1900 RPM (Carb heat warm - outside green arch!)

Slow flight of 48kts (1 kt above the clean stall can be achieved by pitching further up and adding power.

Any further ideas ?

[bookworm]

I find it very hard to believe that the best endurance speed of a C150 is 84 kt.

I've seen numbers from 60 kt (as reported by juggernaut) to 70 kt as the best glide speed, and the minimum sink rate glide speed will be that divided by about 1.3 (46 to 53 kt).

The best endurance speed will be close to, but perhaps a little higher than, minimum sink rate glide speed, as both represent maximum excess power. So your flight test looks to have a very sensible result.

Generally speaking, POHs only quote speed/power/alt for 45% to 75% power. Could it be that 84 kt is not, in fact "best endurance speed", but rather just the best endurance that appears in the tables as written?

[Checkboard]

Take the Piston Airplane Cruise Efficiency (max range & endurance) (http://www.db.erau.edu/research/cruise/) quiz, and tell us how you do! ;)

[bookworm]

But don't forget the discussion (http://www.pprune.org/forums/showthread.php?s=&threadid=49638) of that article on PPrune, in particular the excellent point made by Tinstaafl that the article takes no account of the variation of engine efficiency with altitude.

[Number1]

H and H
What you refer to as 'the lowest point on the power curve' must be refered to as 'MAXIMUM endurance speed'. Cessna's 'BEST endurance' refers to a less extreme value within the normal speed and power range.
When you use the experimental method to determine max. endurance, you are interested in the lowest power setting in order to obtain the lowest fuel burn and are not interested in a resulting range or speed.
Without the manual before me, best endurance could be descibed as the lowest normal flight condition that has been tested and used in the manual, much like the demonstrated crosswind component.