Best Glide
Other pilots reading this will recognize what I'm writing about here. It's something you have to learn.
When training for emergency landings, "engine out" is one of the situations that we train for. If the engine quits, pucker factor will increase - sharply. So, we practice what to do. One of the first things you learn is best glide speed. It's the speed, and rate of descent, where the plane will glide the furthest before contacting the ground. In the newer Cessna 172s that I started training in, it was 75mph.
When I bought the Ruptured Duck, I read the owner's manual cover to cover. Nowadays, they're called the Pilot's Operating Manual (POM), but in the early 60s, still thought of similar to cars, it was still known as an owner's manual. And, nowhere in there did the term "best glide speed" show up. So, I conferred with the FAA Designated Examiner. We decided that whatever it was, it would be pretty close to 75. Actually, I suspect that it is a smidgeon higher, because the newer plames have a slightly improved wing.
But, the Duck now has a much better wing than the new 172s. So, best glide should lie a bit south of 75. How much? Well, that's what I will do some test flights to determine. Terry, a friend of mine who is now retired from being a test pilot for Airbus, talked it over with me. There are at least two regimens for determination of best glide. I'll probably do both.
Method 1 is to Make a series of measured, timed descents, record the data (indicated airspeed [IAS], rate of descent, and time to descend at this constant rate through a 500' drop), draw a graph, and select the optimum point along the speed curve. Method 2 is to climb high, slow to, say, 80mph, start the descent, trim to maintain speed, look for the spot on the ground shead that "stands still". In a glide, the point ahead that stands still is your contact point. Nearer than that point, the ground appears to move down; you will overfly that. Beyond the still point the ground appears to move up; you will not get that far.
It's necessary to have a safety pilot, riding along with me to keep an eye outside, issue warnings, and record data. It's too easy to get either task saturation or target fascination, and an additional pilot will prevent that.
So, in a glide, stabilized at 80mph, locate the contact point. Run trim to raise the nose and slow to, say, 75mph. If the contact point moves further away good! Trim nose up to descend slower in small increments. If the contact point continues to extend, go slower yet. If it gets closer, trim nose down to speed the glide. At each speed, record indicated air speed, rate of descent, and "lift reserve".
This lift reserve is actually angle of attack (AOA), and I suspect that information will be more predictive than IAS. As a plane flies in different configurations and conditions, it requires more or less lift. That lift increases with airspeed and angle of attack - within limits. At a given airspeed, the wings can develop more lift (to hold up a heavier aircraft) by increasing angle of attack. At a given angle of attack, the wings develop more lift at a higher air speed. Thin air (high altitudes above sea level, or high density altitude) develops less lift than does thicker air (low density altitude, as at low levels and/or cold weather.
All this being said, I suspect that what I'm really seeking is the best angle of attack to achieve the longest glide distance. I expect AOA to vary less under widely changing conditions than will airspeed. We'll see. I'll do the tests at heavy weights, and then at light weights. Comparing the data will tell me what I want to know.
We'll see.
Other pilots reading this will recognize what I'm writing about here. It's something you have to learn.
When training for emergency landings, "engine out" is one of the situations that we train for. If the engine quits, pucker factor will increase - sharply. So, we practice what to do. One of the first things you learn is best glide speed. It's the speed, and rate of descent, where the plane will glide the furthest before contacting the ground. In the newer Cessna 172s that I started training in, it was 75mph.
When I bought the Ruptured Duck, I read the owner's manual cover to cover. Nowadays, they're called the Pilot's Operating Manual (POM), but in the early 60s, still thought of similar to cars, it was still known as an owner's manual. And, nowhere in there did the term "best glide speed" show up. So, I conferred with the FAA Designated Examiner. We decided that whatever it was, it would be pretty close to 75. Actually, I suspect that it is a smidgeon higher, because the newer plames have a slightly improved wing.
But, the Duck now has a much better wing than the new 172s. So, best glide should lie a bit south of 75. How much? Well, that's what I will do some test flights to determine. Terry, a friend of mine who is now retired from being a test pilot for Airbus, talked it over with me. There are at least two regimens for determination of best glide. I'll probably do both.
Method 1 is to Make a series of measured, timed descents, record the data (indicated airspeed [IAS], rate of descent, and time to descend at this constant rate through a 500' drop), draw a graph, and select the optimum point along the speed curve. Method 2 is to climb high, slow to, say, 80mph, start the descent, trim to maintain speed, look for the spot on the ground shead that "stands still". In a glide, the point ahead that stands still is your contact point. Nearer than that point, the ground appears to move down; you will overfly that. Beyond the still point the ground appears to move up; you will not get that far.
It's necessary to have a safety pilot, riding along with me to keep an eye outside, issue warnings, and record data. It's too easy to get either task saturation or target fascination, and an additional pilot will prevent that.
So, in a glide, stabilized at 80mph, locate the contact point. Run trim to raise the nose and slow to, say, 75mph. If the contact point moves further away good! Trim nose up to descend slower in small increments. If the contact point continues to extend, go slower yet. If it gets closer, trim nose down to speed the glide. At each speed, record indicated air speed, rate of descent, and "lift reserve".
This lift reserve is actually angle of attack (AOA), and I suspect that information will be more predictive than IAS. As a plane flies in different configurations and conditions, it requires more or less lift. That lift increases with airspeed and angle of attack - within limits. At a given airspeed, the wings can develop more lift (to hold up a heavier aircraft) by increasing angle of attack. At a given angle of attack, the wings develop more lift at a higher air speed. Thin air (high altitudes above sea level, or high density altitude) develops less lift than does thicker air (low density altitude, as at low levels and/or cold weather.
All this being said, I suspect that what I'm really seeking is the best angle of attack to achieve the longest glide distance. I expect AOA to vary less under widely changing conditions than will airspeed. We'll see. I'll do the tests at heavy weights, and then at light weights. Comparing the data will tell me what I want to know.
We'll see.