Best glide speed

[jrenwick]

Copied from The Hangar, topic "Angle of Attack indicator":

I think if properly installed, it would give you a good sense of your true best glide speed, which some day may help make a better glide should a glide be required. I like the benefit of knowing what the optimal angle of attack is regardless of weight, but I find it more beneficial in higher-performance aircraft.....

I just watched an old video from "Flying" magazine in which Barry Schiff talked about engine-out emergencies. "Best glide speed" is an indicated airspeed (or calibrated, if you're fussy). It assumes zero wind (very seldom the actual situation), and maximum gross weight. Schiff went through a series of flight tests to determine best glide speed for a 182 (70 knots). He pulled the throttle to idle at 4500' MSL, established a stable glide at a particular speed (I don't understand how some people can make the ASI appear to be broken, "stuck at" some particular speed), and used a stopwatch to time the descent from 4000' MSL to 3000'. Then with a quick conversion to TAS and some more math, he determined the glide ratio (12:1 for a stock 182 with the prop windmilling). Do this for several indicated airspeeds, and you can bracket the best glide speed.

For aircraft weights below maximum gross, which should be the case for any engine-out emergency at altitude, he used the simple formula of reducing the best glide speed by 5% for every 10% below maximum gross weight.

He also talked about the "minimum sink" speed, for when you're already over the emergency landing field, but not yet down to pattern altitude. For that, he uses something halfway between best glide and stall speed, as a rule of thumb.

He went on to demonstrate the improvement in glide ratio with the prop set to high pitch, and again with the prop stopped. Both made significant improvement in glide speeds, up to 20% with the prop stopped (assuming you've got time for that).

These experiments are all easy to do. All you need is a little math, a stopwatch, and a pilot who can make the gauges sit up and beg. 55+ years out of the factory, our airplanes might be all a little different, and it would be fun (and maybe even helpful) to know what the real numbers are for our own airplanes. I intend to try this pretty soon.

[voorheesh]

Best glide speed and minimum sink are terms used for gliders. Best Lift over Drag ratio L/D is a function of design and can be obtained at different indicated airspeeds. A glider usually has one value, like 23 to 1 in a trainer, that can be achieved at diferent weights and diferent airspeeds. The greater the weight, the higher the speed to obtain best L/D. That is why gliders carry ballast for added performance. When they are heavy, they take longer to takeoff and longer to climb in thermals but in between thermals, they lose less altitude per unit of distance covered because they are going faster. Best L/D produces the greatest forward distance for altitude lost. Minimum sink is a value that represents the least altitude lost as a function of time and the minimum sink speed is between best L/D and stall speed. We use minimum sink in thermals because you are looking to gain altitude, not cover distance. This speed is just above stall speed and glider pilots have to have good slow flight and stall recognition skills when thermalling. What may be of interest to power pilots is the soaring concept called "speed to fly". This is where you use the wind value or ground speed and the sink value, sometimes a downdraft, and vary your indicated airspeed to get best penetration or best performance given the existing conditions. If you are going into a headwind, or through sinking air, you are going to increase your airspeed to get through it faster. This is counter intuitive to power pilots who usually want to slow up in sinking air because they believe they will stay up longer.
When considering this subject with respect to powered aircraft, best glide speed is of course good to know but I wonder how usefull it really is. If you lose your engine you need to find the safest place to make a landing and a go around is not an option. Your ability to visually spot a suitable landing place usually means you are going to choose something well within your gliding range so your best L/D is not always going to be relevant. Some would even suggest that trying to "extend your glide" after an engine failure is a risky venture that could lead to less choices of suitable landing places. Some call it running out of airspeed and altitude at the wrong time. The only situation where it would really be important would be when you are over such a bad location (mountains at night) that a glide to safety is essential. Regarding going to minimum sink when you are over your emergency landing site, I would be carefull with that too. In his advice for pilots facing engine out emergencies, Wayne Handley stresses airspeed. Airspeed is control. Stalling is loss of control. When you are at minimum sink, you are so close to stall speed that a bank could easily result in a stall at a critically low altitude followed by an accident. For example, when we practice rope breaks in gliders from 200'AGL, we pitch to best L/D and a 30 to 45 degree bank. We do not go to minimum sink because we have to maintain a margin above stall speed.
I do not mean to put down Barry Schiff and his excellent analysis of all types of flying subjects. For those of you who have not tried soaring, check it out.

[jrenwick]

Most of us now fly with a GPS that can give us, almost instantly, the distance and heading to a few of the nearest airfields. The tape I watched was so old, it didn't take this factor into account. In an engine-out emergency, once you've turned toward the nearest acceptable airfield and established best glide speed, the GPS can give you ground speed. In theory, it could also estimate what your altitude would be once you reached the field, but I haven't seen any that can do that.

With a little more sophistication, they could even help you establish the best glide speed based on winds, but that's probably farther than the lawyers will allow the manufacturers to go. At a minimum, it would be nice to have a rule of thumb for best glide speed based on ground speed for your aircraft. That would take the winds into account. I don't know how to figure that out, but it might be fun to think about.

[voorheesh]

If the GPS can give you time to station, you could divide the time by the sink rate per minute and get a rough calculation of whether you could make it. Regarding head winds, if a glider pilot knows or can estimate the headwind value, we use that as the starting point for the speed to fly calculation. We go to the headwind value on the airspeed line of the graph and then draw a line from the headwind speed to the tangent on the "polar". Go up vertically and you get speed to fly. It is always greater than best L/D when flying into a headwind or in sinking air. Obviously that happens in preflight planning and not in the aircraft. Most gliders have a "Macready ring" on the variometer (similar to a VSI but an instrument that indicates vertical performance) that has a penetration airspeed that corresponds with indicated sink. Glider pilots add 1000' to destination field elevation in Xcountry flight planning and also subtract a percentage of planned glide performance (safety margin) before actually attempting to go from point A to point B. The point is you do not want to get yourself in the situation where you arrive at your planned destination with out sufficient altitude to safely maneuver for landing.
When an airplane loses the engine it becomes a glider but most have far less performance than even the most rudimentary sail planes (I understand that a B747 has a 20-1 glide ratio that is not too bad, all things considered). I think many airplanes have a published best glide speed and it usually is close to Vy. Finding the exact speed by flight test is certainly an interesting exercise and may actually help out if a real emergency happens.

[4-Shipp]

The anywhere Map software allows you to program glide performance for your airplane and then places circles around every airport on the map. They call them "cones of safety". The circle size is based on your gps derived altitude and glide performance. If you are inside a circle, you can glide to the airport based on your programed parameters and no wind conditions. Garbage in/garbage out, obviously but it can give you some quick SA on whether an airport is well within range ,well outside or on the edge. Not that useful when cruising around at 1-2 thousand AGL but pretty cool when up closer to 10 thousand.

It also has a VNAV function that allows you to program a desired altitude at a fix and then shows an arc at the range you will arrive at that altitude based on your GS and descent rate. Not familiar with other products but they likely have simialr functions.

[Bruce Fenstermacher]

All the Garmin products since at least the Pilot III have a VNAV feature. The key to using it in an emergency situation would be knowing how you have the VNAV set up prior to the emergency. For example you set your VNAV to put you over your destination at say 500 ft. Once the emergency happens hitting direct to your destination will give your the decent rate required to achieve the preset goal. If you are sinking faster than the rate given you won't make the target.

One again probably only good if your not "chasing snakes" (flying low) as my friends like to call the altitude I usually fly.

The Anywhere map cones of safety basically does this automatically and is a separate feature from the VNAV. I seem to remember seeing the cones of safety type feature on another unit but I am probably mistaken.