Post by Tom Goodrick on Aug 23, 2008 21:03:48 GMT -5
Cruising might seem to be the simplest part of flying. In many ways it is. You can turn on the autopilot and lean back. Many pilots write magazine articles on their laptops while cruising. But you do have to pay a little attention to whether the aircraft is working right and where it is going. You'd better know when to quit cruising and descend for the approach. You must also give some thought to several aspects of cruising before takeoff. It is very helpful to do things in FS9 that you would do if you had access to a plane and had a pilot's license. This helps you understand how useful airplanes can be in your business or in your personal life. If you have to travel a fair amount in your job, with a plane and a license you could avoid the trouble of an airline flight. Even flying a lowly Cessna can beat the airlines in convenience and can come close in total time away from home.
PLANNING A FLIGHT
First thing to do is to find a course that will get you to your destination in a reasonably direct way. Set up a flight plan listing your departure airport and your destination airport. Select "Direct GPS" at first to get an idea of the shortest route. Display the route and look at it in various levels of zoom. Zoom out to see the entire route. You'll often see that it has a significant curvature. It is a Great Circle route which is a circle with a center at the center of the Earth. The longer the distance, the more the curvature as seen on the surface map. Zoom in a bit until you can see the markings indicating restricted airspace and busy traffic areas around airports. In FS, you are free to do anything you like. Just get in the plane and fly a direct GPS route without worrying about airspace restrictions. I have done it many times. But you can also get realistic and set waypoints that will keep you out of places you'd rather not go in real life. Just use the mouse to "grab" the red path line at any point and drag it to an airport or navaid that will keep you out of a restricted area. Click on the point and it becomes a waypoint in the list connecting the departure point to the destination. The advantage to avoiding waypoints is that during cruise a glance at the bottom of the GPS tells you how long you have remaining in the flight. If you have waypoints, you get only time and distance to the next waypoint. When you have a good plan, save it and prepare to fly.
When you have a flight plan, look at NAVLOG. It will tell you the total distance and the distance for each leg between waypoints. But disregard everything else it shows you. It bases the time calculations on one airspeed value set in the aircraft.cfg file. But no aircraft has only one airspeed for cruise. The cruise speed in KTAS at a given power setting will change depending on altitude and load - both payload and fuel load. The groundspeed is what determines actual flight time. It is the airspeed minus the windspeed. It will vary from day to day and with altitude. For all this NAVLOG shows you only garbage.
HOW HIGH TO FLY
When you make a flight plan, it will show the recommended minimum altitude. This is the minimum safe altitude for the exact route. If you are flying a plane without turbocharging that does poorly above 10,000 ft, you might be able to shave some altitude if you are going to fly in VFR conditions where you can see what's coming at you. Also you can select waypoints that will keep you low in the valleys between mountain peaks. There is a discussion on this Forum about "Optimimum Altitude." If you are flying 100 nm and there is a 6,000 ft ridge halfway there, your optimum altitude for that trip is going to be at least 7,000 ft!
The capabilities of your airplane to climb, cruise and descend in reasonable fashion will dictate how high you fly. It takes time and burns more fuel to climb than to cruise. Also the same amount of time has to be spent coming down on the other end of the trip. So for short trips, it does not make sense to go too high.
The aircraft flies best at a particular indicated airspeed where the forces and moments acting on the aircraft balance well and allow good efficiency. The ratio of lift to drag, an indicator of aerodynamic efficiency, depends on indicated airspeed. Many aspects of flight including climb ability and power required are improved with a high lift/drag ratio. You should always cruise near this indicated airspeed which will be mentioned in the checklist or reference text. As you fly higher at this same indicated airspeed, your true airspeed - the actual speed relative to the air, will increase as the density decreases.
LEGAL ALTITUDES
Only certain altitudes are legal. First, below 18,000 ft, the barometric setting for the altimeter must be the pressure at sea level below you. On the ground just set field elevation and the pressure will be set right. Any time ATC gives you a baro setting, tap B to set your atimeter correctly. When flying above 18,000 ft, just tap B and your altimeter baro setting will become 29.92 which is standard and mandated by law at the high altitudes. It assures that aircraft passing near each other will be at different altitudes if their pilots are trying to fly different altitudes.
All pilots flying easterly (heading 001 to 180) will fly at odd thousands of feet. Those flying westerly courses (heading 181 to 360) will fly at even thousands of feet.
If flying on IFR, pilots will fly exactly on thousand foot intervals. If on VFR, pilots will fly at thousand foot intervals plus 500 ft. Thes rules help keep converging aircraft at different altitudes.
ALTITUDE DEPENDS ON ENGINE TYPE, CABIN EQUIPMENT AND WINDS
If you do not have a pressurized aircraft, you will climb not much faster than 500 ft per minute and will descend at exactly 500 ft per min to avoid harming the ears of yourself and your passengers. You will need your hearing to talk to the tower and ground control. That is the main reason you will not often fly turbocharged aircraft like the Mooney Bravo higher than 12,000 ft. It takes two minutes for each 1000 ft to descend. Also everybody in a turbocharged but unpressurized airplane needs to suck oxygen from a bottle above 12,000 ft. This can get uncomfortable and should not be done for long periods. It becomes unsafe at some level above 20,000 ft due to leaks.
Pressurization is another thing that keeps cruise altitudes down to 12,000 ft for many piston aircraft. Few are pressurized. I have two that I developed, the Cessna 340 and the Cessna 414 on my site. Those and the turboprops and jets can cruise much higher. The piston aircraft can go as high as 22,000 ft with pressurization, the turboprops can go to 30,000 ft and the jets can go to 41,000 ft and maybe 51,000 ft with cabins that feel to the passengers as though they are at 8,000 ft.
We have not even mentioned the business of power, economy and true airspeed yet. Many of these considerations are discussed in another article on the "Optimum Cruise Altitude." When the considerations mentioned above permit, each type of aircraft has a general preferred cruising altitude. But a another factor that is very big is the wind. It will generally become stronger as you climb. If it is a headwind, you will want to fly lower than planned. With a modern panel you will always know the wind. In FS you must have one of my wind gauges on the panel. In the real world your Garmin 430 or 530 would tell you.
A normally aspirated piston plane does best at about 6,000 ft. That is where full throttle gives 75% power which is considered the max continuous power for cruise. It is hig enough so you get some benefit from reduced density with a true airspeed 9.4% higher than the indicated airspeed.
A turbocharged but unpressurized piston aircraft does best at 18,000 ft to 22,000 ft where the cabin reaches 8,000 ft. Pressurization comes from excess pressure developed by the turbochargers which is only enough to use these lower altitudes along with reduced shell weight compared to jets. True airspeed is 32% to 42% greater than indicated airspeed. They can descend at 1500 fpm without bothering the passengers.
A turboprop aircraft gets plenty of pressure from the bleed air developed by its jet engines. Its cabin is enclosed in a strong pressure shell that permits it to fly in the low 30,000 ft altitude regime. Engine power does decay with altitude so they seldom fly much above 30,000 ft. For considerations of relative speed and air traffic, they generally stay in the mid to upper 20,000 ft regime. They can descend at 2000 fpm without bothering the passengers.
A jet aircraft normally has to fly above 30,000 ft where the fuel flow, power and true airspeed all combine to make an optimum cruise condition. The process of determining the best cruise altitude for a jet is very complex. payload and fuel weight play a part. Quite often when starting a long trip, a jet can only climb efficiently to a medium altitude. Then as fuel is burned off, the aircraft can climb to more efficient altitudes. These are also complex to fly. Below 30,000 ftlike most aother aircraft there is a speed limit based on indicated airspeed that must be observed to avoid structural problems if a gust is encountered. Above 30,000 ft the limit is based on Mach number which affects stability and controllability. Many jets are prohibited from flying above 30,000 ft without a working autopilot.
TEST YOUR AIRCRAFT
Choose a favorite aircraft, or perhaps one of each type. Set the weather at Clear. This sets no wind and a standard temperature profile. Set aside several pages in a notebook for notes on your observations. Load the aircraft as you normally would for payload and with as much fuel as you can load and stay under max gross weight. Take off and climb to your lowest cruise altitude and establish cruise at nominal cruise power. Use the autopilot to keep it steady and wait long enough for speed changes to stop. Note the speed and fuel flow. Repeat this and 1000 ft intervals through the altitude regime you expect to use in cruise. Next repeat the process with a very light weight - just a pilot onboard. If your aircraft carries over 100 gallons of fuel, repeat the test with half the fuel. This should show where true airspeed peaks and where fuel flow is low. Nautical miles per gallon or nmpg can be calculated as KTAS/GPH. This is a good figure to calculate for each altitude you try. It will show you the "optimum altitude."
PLANNING A FLIGHT
First thing to do is to find a course that will get you to your destination in a reasonably direct way. Set up a flight plan listing your departure airport and your destination airport. Select "Direct GPS" at first to get an idea of the shortest route. Display the route and look at it in various levels of zoom. Zoom out to see the entire route. You'll often see that it has a significant curvature. It is a Great Circle route which is a circle with a center at the center of the Earth. The longer the distance, the more the curvature as seen on the surface map. Zoom in a bit until you can see the markings indicating restricted airspace and busy traffic areas around airports. In FS, you are free to do anything you like. Just get in the plane and fly a direct GPS route without worrying about airspace restrictions. I have done it many times. But you can also get realistic and set waypoints that will keep you out of places you'd rather not go in real life. Just use the mouse to "grab" the red path line at any point and drag it to an airport or navaid that will keep you out of a restricted area. Click on the point and it becomes a waypoint in the list connecting the departure point to the destination. The advantage to avoiding waypoints is that during cruise a glance at the bottom of the GPS tells you how long you have remaining in the flight. If you have waypoints, you get only time and distance to the next waypoint. When you have a good plan, save it and prepare to fly.
When you have a flight plan, look at NAVLOG. It will tell you the total distance and the distance for each leg between waypoints. But disregard everything else it shows you. It bases the time calculations on one airspeed value set in the aircraft.cfg file. But no aircraft has only one airspeed for cruise. The cruise speed in KTAS at a given power setting will change depending on altitude and load - both payload and fuel load. The groundspeed is what determines actual flight time. It is the airspeed minus the windspeed. It will vary from day to day and with altitude. For all this NAVLOG shows you only garbage.
HOW HIGH TO FLY
When you make a flight plan, it will show the recommended minimum altitude. This is the minimum safe altitude for the exact route. If you are flying a plane without turbocharging that does poorly above 10,000 ft, you might be able to shave some altitude if you are going to fly in VFR conditions where you can see what's coming at you. Also you can select waypoints that will keep you low in the valleys between mountain peaks. There is a discussion on this Forum about "Optimimum Altitude." If you are flying 100 nm and there is a 6,000 ft ridge halfway there, your optimum altitude for that trip is going to be at least 7,000 ft!
The capabilities of your airplane to climb, cruise and descend in reasonable fashion will dictate how high you fly. It takes time and burns more fuel to climb than to cruise. Also the same amount of time has to be spent coming down on the other end of the trip. So for short trips, it does not make sense to go too high.
The aircraft flies best at a particular indicated airspeed where the forces and moments acting on the aircraft balance well and allow good efficiency. The ratio of lift to drag, an indicator of aerodynamic efficiency, depends on indicated airspeed. Many aspects of flight including climb ability and power required are improved with a high lift/drag ratio. You should always cruise near this indicated airspeed which will be mentioned in the checklist or reference text. As you fly higher at this same indicated airspeed, your true airspeed - the actual speed relative to the air, will increase as the density decreases.
LEGAL ALTITUDES
Only certain altitudes are legal. First, below 18,000 ft, the barometric setting for the altimeter must be the pressure at sea level below you. On the ground just set field elevation and the pressure will be set right. Any time ATC gives you a baro setting, tap B to set your atimeter correctly. When flying above 18,000 ft, just tap B and your altimeter baro setting will become 29.92 which is standard and mandated by law at the high altitudes. It assures that aircraft passing near each other will be at different altitudes if their pilots are trying to fly different altitudes.
All pilots flying easterly (heading 001 to 180) will fly at odd thousands of feet. Those flying westerly courses (heading 181 to 360) will fly at even thousands of feet.
If flying on IFR, pilots will fly exactly on thousand foot intervals. If on VFR, pilots will fly at thousand foot intervals plus 500 ft. Thes rules help keep converging aircraft at different altitudes.
ALTITUDE DEPENDS ON ENGINE TYPE, CABIN EQUIPMENT AND WINDS
If you do not have a pressurized aircraft, you will climb not much faster than 500 ft per minute and will descend at exactly 500 ft per min to avoid harming the ears of yourself and your passengers. You will need your hearing to talk to the tower and ground control. That is the main reason you will not often fly turbocharged aircraft like the Mooney Bravo higher than 12,000 ft. It takes two minutes for each 1000 ft to descend. Also everybody in a turbocharged but unpressurized airplane needs to suck oxygen from a bottle above 12,000 ft. This can get uncomfortable and should not be done for long periods. It becomes unsafe at some level above 20,000 ft due to leaks.
Pressurization is another thing that keeps cruise altitudes down to 12,000 ft for many piston aircraft. Few are pressurized. I have two that I developed, the Cessna 340 and the Cessna 414 on my site. Those and the turboprops and jets can cruise much higher. The piston aircraft can go as high as 22,000 ft with pressurization, the turboprops can go to 30,000 ft and the jets can go to 41,000 ft and maybe 51,000 ft with cabins that feel to the passengers as though they are at 8,000 ft.
We have not even mentioned the business of power, economy and true airspeed yet. Many of these considerations are discussed in another article on the "Optimum Cruise Altitude." When the considerations mentioned above permit, each type of aircraft has a general preferred cruising altitude. But a another factor that is very big is the wind. It will generally become stronger as you climb. If it is a headwind, you will want to fly lower than planned. With a modern panel you will always know the wind. In FS you must have one of my wind gauges on the panel. In the real world your Garmin 430 or 530 would tell you.
A normally aspirated piston plane does best at about 6,000 ft. That is where full throttle gives 75% power which is considered the max continuous power for cruise. It is hig enough so you get some benefit from reduced density with a true airspeed 9.4% higher than the indicated airspeed.
A turbocharged but unpressurized piston aircraft does best at 18,000 ft to 22,000 ft where the cabin reaches 8,000 ft. Pressurization comes from excess pressure developed by the turbochargers which is only enough to use these lower altitudes along with reduced shell weight compared to jets. True airspeed is 32% to 42% greater than indicated airspeed. They can descend at 1500 fpm without bothering the passengers.
A turboprop aircraft gets plenty of pressure from the bleed air developed by its jet engines. Its cabin is enclosed in a strong pressure shell that permits it to fly in the low 30,000 ft altitude regime. Engine power does decay with altitude so they seldom fly much above 30,000 ft. For considerations of relative speed and air traffic, they generally stay in the mid to upper 20,000 ft regime. They can descend at 2000 fpm without bothering the passengers.
A jet aircraft normally has to fly above 30,000 ft where the fuel flow, power and true airspeed all combine to make an optimum cruise condition. The process of determining the best cruise altitude for a jet is very complex. payload and fuel weight play a part. Quite often when starting a long trip, a jet can only climb efficiently to a medium altitude. Then as fuel is burned off, the aircraft can climb to more efficient altitudes. These are also complex to fly. Below 30,000 ftlike most aother aircraft there is a speed limit based on indicated airspeed that must be observed to avoid structural problems if a gust is encountered. Above 30,000 ft the limit is based on Mach number which affects stability and controllability. Many jets are prohibited from flying above 30,000 ft without a working autopilot.
TEST YOUR AIRCRAFT
Choose a favorite aircraft, or perhaps one of each type. Set the weather at Clear. This sets no wind and a standard temperature profile. Set aside several pages in a notebook for notes on your observations. Load the aircraft as you normally would for payload and with as much fuel as you can load and stay under max gross weight. Take off and climb to your lowest cruise altitude and establish cruise at nominal cruise power. Use the autopilot to keep it steady and wait long enough for speed changes to stop. Note the speed and fuel flow. Repeat this and 1000 ft intervals through the altitude regime you expect to use in cruise. Next repeat the process with a very light weight - just a pilot onboard. If your aircraft carries over 100 gallons of fuel, repeat the test with half the fuel. This should show where true airspeed peaks and where fuel flow is low. Nautical miles per gallon or nmpg can be calculated as KTAS/GPH. This is a good figure to calculate for each altitude you try. It will show you the "optimum altitude."