Vmax range vs Vmax endurance

[Mr Smiley]

I wonder if I could ask you more knowledgeable ladies and gentlemen out there a question I was recently asked in an interview, and one which I could not honestly answer? The question is, why is Vmax range greater than Vmax endurance?
I am sure there must be a simple answer (apologies in advance if the answer is so simple that it borders on complete stupidity to even ask such a question!!) but I have been through my old CPL/ATPL notes and even asked a couple of pilot friends of mine and have yet to find a convincing answer.
I would be most grateful for any feedback or helpful references.
Many thanks in advance,
Mr Smiley

[barit1]

It's similar (in a way) to the difference between V maxROC and V obstacle clearance.

In one case, you are merely trying to reduce fuel flow (kg/hr) to minimum so you can last a max. number of hours. You don't care how fast you travel; you are simply very near the min. drag speed.

In the other case you are shooting for the best distance traveled per kg of fuel. Speed obviously enters into the equation; you want the quantity

Knots (NM/hour) / Fuel flow (kg/hr)

...to be at a maximum.

[Rivet gun]

It's really easiest to answer this question if you can draw the graphs on a whiteboard. Most of the graphs you find in textbooks make the assumption that for prop aircraft fuel flow is proportional to power and for jet aircraft fuel flow is proportional to thrust. Of course neither assumption is quite correct and modern turbofans fall somewhere between the two assumptions.

If the candidate answers the question along the right lines, you can really test his understanding by asking him to explain how headwind / tailwind affects the best range speed, or how the range and endurance speeds are affected by weight.

[camlobe]

Mr Smiley, if we accept that VMAX range is the speed selected for the maximum distance possible (with or without diversion fuel) attainable for a given fuel load, and that VMAX endurance is the speed selected for the maximum duration possible (with or without diversion fuel) attainable for the same given fuel load, those speeds are a reflection of fuel flow (thrust selected) and drag. Both of these are already mentioned by barit 1.

In a basic answer to your question 'why is VMAX range greater than VMAX endurance' if you are aiming to stay aloft for as long as possible, you will be set up at a higher angle of attack than if you went for furthest distance. Although you have set a lower fuel burn for endurance, this is offset by the higher drag penalty and you will travel a shorter distance for the same fuel burnt.

Hope this helps.

[Mr Smiley]

Thanks very much for all your responses it is greatly appreciated...
...and a special thanks to you Camlobe, that was a spot on explanation... just what I was looking for.
Thanks again

[rhovsquared]

theoretically, max endurance (ignoring altitude and winds) is at Vmd. thrust = drag; drag low=thrust low=FF low, 'best' speed is the speed obtained by drawing a tangent line from Vmd and seeing where it touches the drag curve=lowest drag for highest speed.

[balsa model]

why, oh, why..

Let me have a go at question why:

First, lets bring into picture drag vs. speed curve.
It looks like a bowl, with a round bottom. That's the key.
As was reiterated here multiple times, best endurance is where the graph reaches its lowest point: least power is needed to counter the drag, so the fuel flow will be the lowest, and the tank will last the longest.
We gonna get some decent range at this speed, too.
However, if we increase the speed a tad, you'll notice that the drag will hardly change, since the curve's bottom is smooth and near its best drag point it actually has a flat slope. So increasing speed by 1% will pretty much get you almost 1% rise in range. The actual number will depend on the aircraft of course, but the key is: to find the max range speed you gotta go hunting to the right of the curve.

Is this along the lines of an explanation/proof that speaks to you?

[wondering]

Actually, Vmax endurance is somewhat lower than Vmd for a jet aircraft since fuel flow lines projected onto the drag curve have a negative slope. The opposite is true for specific range lines which have a positive slope. Hence, Vmax endurance is lower than Vmax range.

If someone could explain how to insert graphs, IŽd be more than happy to post one showing the relationships.

[enicalyth]

try this url

http://www.professionalpilot.ca/aero.../range_jet.htm

[Charles Darwin]

Actually, the Vmax´s range isn´t all that great, and it has no endurance at all. Only forward speed and great acceleration.

[archae86]

Originally Posted by wondering

Actually, Vmax endurance is somewhat lower than Vmd for a jet aircraft since fuel flow lines projected onto the drag curve have a negative slope. The opposite is true for specific range lines which have a positive slope. Hence, Vmax endurance is lower than Vmax range.
If someone could explain how to insert graphs, I´d be more than happy to post one showing the relationships.

Responding privately to the quoted post, I offered wondering posting service. I downsized and compressed his high-quality scans considerably hoping to make them suitable for in-line posting here. To see them in much better resolution see the linked copies.


full resolution graphs 16 and 17

full resolution graph 18

{edited to add a source reference: Jet Airplane Performance by
Lufthansa Consulting, First Edition, July 1988}

[bubbers44]

Simply stated long range cruise gets you the most distance, max endurance such in holding, keeps you in the air longer. In the 727 if you were at high altitude, max indurance holding speed meant descending to FL250 or so to save fuel and maximize endurance. LRC required you to stay at optimum altitude and LRC.