Range and Endurance lesson

[trey kule]

There is a rule of thumb that says add half the headwind to the still-air best range airspeed. So in the case of a 15k headwind best range speed might be 7kts faster than the usual 70-whatever knots. That’s still slower than a typical cruise speed. I haven’t any theoretical analysis or justification for that rule to offer.

Ah, photo. What happens if you are already flying at best range speed? According to your example you would have to increase your speed by 7 knots. You are assuming that you are not already flying at best range speed.

speeding up into a head wind will sometimes help range, as the headwind compnent becomes smaller. That being said, after a certain speed adding extra power does not significantly increase the speed proportionally. I think that is where you came up with the add 1/2 the headwind to the best range speed. But again, in saying always slow down you are assuming you are already well above best range speed.

Your “rule” about the headwind, at least as far as fuel planning goes is to use 150% of the forecast headwind , and 50% of forecast tailwind, of course once you are in cruise you can compare forecast to actual. If one is even the least bit nervous and has limited options, one should already be at best range speed....

[AuxBatOn]

Because in a simple model of a turbojet, thrust, not power, is proportional to fuel flow.

I know...

But having done actual cruise performance flight test (W/d) and compared to the a simple theoratical model of the engines and a flight test-derived aircraft model, the results matched very closely...

[AuxBatOn]

In both propeller and turbojet, level flight at minimum drag means the minimum amount of thrust is required. For the jet only, that means minimum fuel flow, which means maximum endurance.

No. It is always the minimum fuel flow. For jets, fuel flow is proportional to thrust (or drag, for balanced flight) and for prop, fuel flow is proportional to power (thrust * V)

So max E is mim thrust for Jets and min power for aircraft.

[photofly]

Ah, photo. What happens if you are already flying at best range speed? According to your example you would have to increase your speed by 7 knots. You are assuming that you are not already flying at best range speed.

Yes. Let's examine what I first wrote on the subject:
"To my mind there's one inportant take-away from the lesson on range and endurance. That is, if you ever find yourself in the air and uncomfortable about your fuel situation, slow down. Best range and best endurance speeds are slower than the range of typical cruising speeds"

If you are already flying at best range speed, then by definition, either speeding up or slowing down would diminish your range. Else you wouldn't be at best range speed.

But again, in saying always slow down you are assuming you are already well above best range speed.

Yes, that's right. Everyone cruises a C172 at an airspeed significantly above the speed at which it achieves its best range. We accept the reduced miles-per-gallon of a typical cruise speed in exchange for reaching the destination significantly earlier than the speed at which we get get best mpg. Nobody bothers to fly a 172 at highway traffic speeds. Until they start to get uncomfortable about their fuel situation.

If one is even the least bit nervous and has limited options, one should already be at best range speed....

Yes, again. That's my point. At the point at which one realizes there is a fuel situation, that is when one should slow down from a more typical cruising speed to best range speed, which will be slower than one was going before one realizes there's a fuel situtation. Can't put it simpler: When you realize you're in a fuel situation, slow down. I think that matches what I first said on the subject.

One reason it bears repeating is because being nervous about the amount of fuel on board is an unpleasant situation and there's a very natural inclination to bring this situation to a quicker conclusion by speeding up to get the destination faster. That's the wrong tendency. Unfortunately a better idea is slow down. You'll be shitting bricks for longer, but you'll go further before you run out of fuel.

[photofly]

In both propeller and turbojet, level flight at minimum drag means the minimum amount of thrust is required. For the jet only, that means minimum fuel flow, which means maximum endurance.

No. It is always the minimum fuel flow. For jets, fuel flow is proportional to thrust (or drag, for balanced flight) and for prop, fuel flow is proportional to power (thrust * V)

So max E is mim thrust for Jets and min power for aircraft.

You're correct. But in your correction of what I wrote, I think you misread it.
For the jet, minimum drag is minimum thrust, is min fuel flow, is max endurance.
For the prop, minimum drag is minimum thrust and therefore max range. It's not the minimum fuel flow, and it's not maximum endurance.

Therefore (and it's late) I still believe this statement is accurate:
In both propeller and turbojet, level flight at minimum drag means the minimum amount of thrust is required. For the jet only, that means minimum fuel flow, which means maximum endurance.

[Big Pistons Forever]

The subject is Range and Endurance for PPL training.

This should not be an aeronautical engineering degree lesson.

For PPL’s I think it is very important to keep things practical, so I like Photo’s observation that in general when fuel is tight slowing down will extend your range. This is the kind of simple practical lesson which should be part of basic flying instruction. The truth of it is easily demonstrated by reviewing the POH range charts.

The only thing I would add is to guard against the tendency to teach exercises in isolation. How not to get tight on fuel in the first place needs to be included in the discussion. So part of the practical “range” lesson means when you are teaching Nav you need to talk about how to decide how much contingency fuel should be added and emphasize the importance of monitoring your enroute fuel state so if things start looking tight you already have a plan B.

My personal opinion is that in general flight schools don’t do a good job of this because training flights are dispatched with much more fuel than they need so a student never has to worry about whether they have enough fuel to finish the lesson with sufficient reserves.

[AuxBatOn]

I'd argue that to be a good instuctor, you need to know more than the mere facts you are teaching: you need to understand the why and the how.

In the case of maximum endurance, the fact that propeller driven aircraft and jet aircraft have different relationships between fuel flow/thrust/power is the reason why your maximum endurance airspeed aerodynamic definition differs. I'd argue this is fairly important to understand as an instructor.

[Big Pistons Forever]

Aux

I agree the why and how matter, but the OP is someone working on his instructor rating so there is a lot of why and how he needs to know. Studying up on jet vs prop theory for range and endurance should only happen after he had mastered the basic theory and even then given the amount of knowledge you need to be an effective instructor, I don’t think it is the best way to spend the study time.

For this lesson “basic” theory means understanding and be able to explain in about 3 mins
the total drag and power required graphs in the FTM.

As a general rule flight training, especially initial flight training should be concentrated on the practical. If the information can’t be used in a practical way then it should probably not be part of the lesson

Most of the aerodynamic theory falls into this category, so I spend very little time on it, but I spend a lot of time on explaining how the aircraft systems work. Not understanding what the gauges are saying and what it means to the proper functioning of the system, not understanding what is happening when the knob is pulled or the switch flicked has real practical consequences.

BTW, I said in an earlier thread I had never flown a GA aircraft at endurance speed for real which is true. I did once fly a 96,000 lb transport category aircraft at max endurance. We had a sudden need to loiter for about 40 mins and not a lot of fuel. The solution was to go to the AFM and look up the max endurance power setting. That power setting was applied and resulted in us conserving enough fuel to finish the job

Now I am sure there was a lot of aeronautical engineering behind how that power setting was derived but I sure wasn’t going to over think it.