I seem to be motoring thru flight training at a pretty good pace. I only did one flight today (I normally try for two): Flight for Range and Endurance.
We started off as usual with a short ground brief to make sure that I read the FTM section and that I have some understanding of what we would be practicing. It seemed pretty straight forward.
The purpose of the exercise is to set up the airplane for to fly for its' maximum distance given the amount of the fuel it has on board. This is Flight for RANGE (FFR). One important point is that this does not necessarily mean that you will be airborne for the longest time, just that you will fly the farthest in this configuration. It is also important to note that it does not mean that you will have the fastest ground speed. Setting up the airplane for the maximum time in the air is called Flight for ENDURANCE (FFE). FFE does not maximize ground speed nor fuel efficiency, just time aloft. Think glider...with an engine providing minimal power.
As explained to me, it is important to know how and when to use each of these types of configuration. For example, you would want to set up the airplane for maximum range (FFR) when your are on a long distance flight and you want to minimize the number of fueling stops. Fueling takes a lot of time especially when the airport is busy. You might even have to hold or divert depending on circumstances at that airport you planned to stop at. Landings and take-offs also put you in slow flight near the ground which is a very undesirable state of flight (though safe when trained as I will find out!).
Now we all know that fuel is very expensive, so when do we ever want to fly in a less than ideal state of fuel efficiency such as FFE? Say for example, you are approaching an airport on you cross country trip that you intend to make a fuel stop at. You arrive in the area with about an hour of fuel in reserve as planned. As you contact ATC to get clearance to the circuit for landing, they put you in a hold for some reason (high traffic, VIPs, deer on the runway, an accident on runway, whatever). As a pilot, you have a decision to make: hold or divert. ATC will not make that decision for you. If it is a matter of traffic, you probably can make a pretty good estimation of the time needed to fit you in. Sometimes it is not so clear. The problem becomes critical if you do not have enough fuel to make it to the next airport or if you do and that airport is closed due to weather. This is when FFE becomes your configuration of choice. This flight configuration maximizes the amount of time you can remain in the air for the fuel you have on board. FFE is also the slowest you can fly the airplane before you enter SLOW FLIGHT (which I will discuss in another post).
So how do you achieve them? That was the point of the lesson today. Sometimes, an airplane will have a table, but the point today was to learn how to figure it out by experiment in flight.
We took off an headed out to the practice area at 3,500. We accelerated up to cruise at 105 MPH at 2,300 RPM. The rule of thumb is that for each reduction of 100 RPM, we should see a drop of 5 MPH of airspeed while holding altitude. We keep dropping the RPM by 100 increments until we see a drop in airspeed larger than the 5 MPH linear drop. In our case this was about 1,800 RPM. The speed dropped about 8-10 MPH. We note that RPM and speed on our kneeboard and put the RPM back up to 2,300 and accelerate back up near cruise and then reduce RPM back to the last RPM that gave us the steady linear 5 MPH to 100 RPM drop and add 100 RPM. In our case 1,900 RPM at an airspeed of 85 MPH. We adjusted the mixture which is important for fuel economy. This is FFR. If we flew at 85 MPH, at 1,900 RPM at 3,500 we would fly for the maximum range that we could achieve with the fuel on board. If we know the fuel on board and the fuel flow, we can calculate how long we could fly at that airspeed. Using wind information, we could calculate the ground speed and we would know how far we could fly. It is important to note that we would have to find a new FFR if we were at a different altitude because the engine runs at a different efficiency at different altitudes. Generally, flying higher gives greater range (up to a point).
Now to find FFE, we use a similar procedure with a subtle difference. We again start off at level flight 3,500 MSL at cruise speed with 2,300 RPM. We begin reducing the throttle by 100 RPM however this time, we are paying attention to altimeter. For each RPM drop, I kept changing the attitude to maintain straight and level flight at 3,500. The lowest RPM that I was able to maintain altitude was noted. It was just under 1,900 RPM...1825 RPM. A tiny difference in this plane. We accelerated back up to cruise and reduced RPM back down to the noted 1,825 RPM and we settled in at our FFE airspeed and attitude. This was the configuration that would allow us to remain airborne for the greatest possible TIME.
To finish off, we descended to 2,500 and found that the lower the altitude, the greater the endurance time we could achieve. This is an important point: when FFE is required, you should fly at the lowest safe altitude possible. Things like obstacles, buildings, ATC, traffic and turbulence should be taken into consideration. Of all considerations, turbulence is the most problematic generally. Because, in FFE, you are operating at the lowest power setting possible to maintain level flight, turbulence can cause changes in drag and more power will be required to maintain altitude. This results in constant changes in throttle and higher fuel flow which will decrease your endurance. As I learned, in this case, you should either change altitude or fly at a slightly higher power setting which will not require constant throttle changes which would be less efficient than a slightly higher constant power setting.
To be honest, I really enjoyed this lesson. For the first time, I really felt in control and confident in flying the airplane. Unfortunately, I accidentally set my GoPro Hero 3+ Black to still image instead of video mode so I all I found at the end of the flight was a picture of my hand pressing the record button.
Tomorrow, I have slow flight and I will be sure to get a video up on that one.
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