Can any plane glide unpowered? (unless it is a brick of course)

[Charzy]

No it does not. Ultralights can too glide, they are literally built out of gliders. They will glide, especially since, at least from when I last checked, the average ultralight glide ratio is something like 8:1 which isnt actually all that bad. All planes can glide, not necessarily glide well, but they can glide.

The one I showed you will glide 7.9:1 under the absolute best circumstances.

Ultralight trikes cannot glide enough to make any landing safe. The very best you can hope for in the event of an engine failure is that you don't die and only escape with broken bones.

[Rokker]

The one I showed you will glide 7.9:1 under the absolute best circumstances.

Ultralight trikes cannot glide enough to make any landing safe. The very best you can hope for in the event of an engine failure is that you don't die and only escape with broken bones.

You sure? Just about anything can land while gliding if you flare hard enough.

[luizopiloto]

You sure? Just about anything can land while gliding if you flare hard enough.

some trikes have a puller to release the engine...

[mrfox]

The one I showed you will glide 7.9:1 under the absolute best circumstances.

Ultralight trikes cannot glide enough to make any landing safe. The very best you can hope for in the event of an engine failure is that you don't die and only escape with broken bones.

The space shuttle has a subsonic glide ratio of 4.5. It glides and lands just fine.

[Charzy]

It also has a huge amount of energy. A microlight trike goes at around 90 km/h most of the time.

[mrfox]

Which is plenty of energy for a trike, considering it also has potential energy of altitude to trade off. But dont take my word for it.

raanz.org.nz/training/manual.pdf‎

Page 45

[KerikBalm]

The ability to glide doesn't mean the ability to land safely...

It basically means the ability to counteract the force of gravity and exert control over your trajectory by aerodynamic means.

Case in point: wingsuits -> they simply glide too fast, with their sink rate being too high, to land safely under *most* circumstances.

The "pilots" are not in freefall/accelerating at 9.8 m/s, but can maintain a constant velocity, and change their velocity vector

That trike can glide, it may not be able to land very well (although I would find that surprising, I basically fly those things sans engines).

Basically anything that can be aerodynamicaly stable while maintaining a non-zero angle of attack, can glide.

[WestAir]

The ability of an airplane to glide is defined by the ratio of its lift coefficient to its drag coefficient, called the glide ratio. Both of these are a function of air speed and air density. The best glide speed is usually equal to the airplane's V_Y speed, which also gives best rate of climb. So if you are trying to land an airplane unpowered, you shoot for V_Y. Airplanes with good glide ratio are relatively easy to land with no engines. A typical light airplane will have a glide ratio in the 8-10 range and unpowered landing is often part of standard training. High performance gliders can have glide ratio of over 60. These can stay in the air pretty much indefinitely with a good pilot, thanks to thermals. An airliner, typically, has much lower glide ratio. A 777, as an extreme case, has a glide ratio of only 4. (Actually, a 777 has a glide ratio of almost 10 at cruise altitude, but that's due to it being designed for flight at low density and high speeds. At low altitudes, it's only marginally better than a brick.) A wing suit has glide ratio of about 2.5-3. Helicopter in auto-rotation is also about 3, but helicopter can flare a lot better than an airplane can due to energy stored in the rotor, so they can do a soft landing unpowered, despite being very bad at "gliding". A 777 with all four engines dead is typically considered incapable of unpowered landing. It needs at least 1 engine running. Though, under ideal conditions, a survivable landing is possible with that glide ratio.

Technically, so does the brick. If you add a small tale to keep brick's orientation stable, it can do better than 1.

Two corrections: A 777 only has two engines, not four. In a glide the landing would be survivable because the pilot would flare it before landing to soften the impact. (Like the miracle landing in the Hudson). Also, I thought Vx (best angle of climb) was the unit closest to Vg (best glide), as opposed to Vy (best rate)? In the planes I fly Vx is closer to Vg than Vy. Even in the trainers I fly (Cessna 172SP) Vg Best Glide is 65, Vx Best Angle is 62, Vy Best Rate is 74.

I'd imagine if you've lost thrust, the time (rate) it takes to return to Earth isn't quite as important as the distance (angle) it takes. Unless you're flying a seaplane over the ocean. Then I guess you'd pitch for Vy.

(EDIT) As an additional note, a coworker of mine who says he used to fly for United told me a few months back that he wouldn't pitch for best glide if his 777 lost both engines. He'd shoot for something like 300 knots so he could try to relight the engines. Thought that was interesting.

Edited April 20, 2014 by WestAir

[PakledHostage]

An airliner, typically, has much lower glide ratio. A 777, as an extreme case, has a glide ratio of only 4. (Actually, a 777 has a glide ratio of almost 10 at cruise altitude, but that's due to it being designed for flight at low density and high speeds. At low altitudes, it's only marginally better than a brick.)

[citation needed]

While a B777 is not an A330 or a B767, this contrasts sharply with two well known dead stick landings carried out in those two aircraft types. Aerodynamically, the twin engined B777 isn't that much different than the twin engined B767 and A330.

The B767 dead stick landing was, of course, the Gimli Glider. I wasn't able to find the official accident report online but the Wikipedia article and many other sources describe the engines flaming out at 41000 feet over Red Lake, Ontario. Red Lake to Gimli is 122 nautical miles. Even so, the flight crew arrived at Gimli with TOO MUCH energy and famously had to side slip the aircraft to scrub speed before landing.

Similarly, page 8 of the official Portuguese GABINETE DE PREVENÇÃO E INVESTIGAÇÃO DE ACIDENTES COM AERONAVES report into the dead stick landing of Air Transat flight 236 in Terceira describes the flight crew performing a 360 degree turn to bleed excess altitude during their approach, despite having to glide for 65 nautical miles from where the second engine flamed out to Lajes airforce base in the Azores.

You may be thinking of the glide performance of British Airway's flight 38 (a B777-200ER) following the failure of both engines while on approach to Heathrow airport, but that isn't a fair comparison because the aircraft was flying slowly, close to the ground and in landing configuration when the engines quit.

[K^2]

I don't know why I wrote 777 in place of 747. The 777 is a much better glider, yeah. Pretty much any twin is designed to be landed with dead engines for safety reasons. With four engines, assumption that at least one engine is running is a common one.

Sorry about confusion with the numbers.

Glide ratio is constant as long as the aircraft is properly trimmed for gliding - it does not change as a function of altitude, it only varies as a function of angle of attack.

This is not even true for 2D air foil. Glide ratio is a function of a Reynolds number. Here is an analysis of a typical foil used by Boeing. 747 uses several variations of this foil along the length of the wing. Take a look at the Cl/Cd polar. The slope of the tangent will give you best glide ratio. 747 has MAC of 327.8" and V_Y of around 180KIAS at maximum landing weight. That gives me R = 50x10⁶ at sea level. At cruise altitude, air density is about a third, so under ideal circumstance, the velocity needs to increase by sqrt(3) to maintain lift at the same glide slope. That puts Reynolds number at less than 30x10⁶. I don't have polars for Reynolds numbers that high, but if the trend holds, this would actually reduce glide ratios.

However, real wings aren't constant 2D cross-sections of infinite span. The above is only useful to demonstrate the fact that the statement that glide ratio does not depend on altitude is wrong at a very fundamental level. For a real airplane, things are far, far more complicated. I'm going to skip all the factors related to the way the wing profile changes along its length, and the fact that body contribution is very different at different air speeds. What's critical for a 747 is actually the wing loading and finite wing span. That's what kills the 747 performance at low air speeds and, consequently, high air densities. Without getting into all complexities, lift depends on transverse circulation induced by an airfoil. Unfortunately, due to finite span, that tends to induce longitudinal circulation. Id est, tip vortices. While circulation is actually higher at cruise altitude, the low density and high air speed means that it is distributed over much larger volume of air. That results in much gentler wing tip vortices as well. At low speeds, the wing tip vortex is much more compact. It has a smaller radius, and the air flow past the wing is much slower, all while the impulse transfered to the air remains the same. This results in much higher air speed in the wing tip vortex, so more energy has to be transferred to air. More power lost to wing tip vortices at lower air speed means dramatically higher drag.

This also brings up another interesting aspect. For a 2D air foil, wing loading isn't even a factor in the performance. You have a fixed glide ratio, and it depends on qualities of the wing only. This is absolutely not the case for a real airplane. A heavily loaded airplane does not glide as well as an empty one. This is pretty intuitive, but another thing you don't seem to account for in your statement. The reason is exactly the same. Higher wing loading means stronger wingtip vortex. That means more energy lost to the vortex, and that means higher drag.

But, I believe, people will still ask me for references. Here is the manual. On pages 42-43 you can find the climb rate for maximum takeoff weight to be 2000FPM at 210KIAS with 880,000 lbs of weight and about 250,000lbs of thrust. That's a climb at a slope of +9%. That indicates an excess thrust of 9% of weight, meaning drag of 160,000lbs, indicating a glide ratio of 5.5. That's a bit higher than ratio of 4 I've mentioned earlier, but this is clean configuration. Add a bit of flaps to try to bring the landing speed down, and you're right back to 4 or bellow.

[Runboy]

The aerodynamic forces affect everything,even a brick as you said.Some objects may have a higher lift ratio than others,you can even make a brick fly if you shape it in a certain way or attach wings to it.

[DaveofDefeat]

(EDIT) As an additional note, a coworker of mine who says he used to fly for United told me a few months back that he wouldn't pitch for best glide if his 777 lost both engines. He'd shoot for something like 300 knots so he could try to relight the engines. Thought that was interesting.

You have to keep your airspeed up to prevent core lock.

[WestAir]

The aerodynamic forces affect everything,even a brick as you said.Some objects may have a higher lift ratio than others,you can even make a brick fly if you shape it in a certain way or attach wings to it.

Query: Would that brick glide over someone's head?

[PakledHostage]

[...] indicating a glide ratio of 5.5. That's a bit higher than ratio of 4 I've mentioned earlier, but this is clean configuration. Add a bit of flaps to try to bring the landing speed down, and you're right back to 4 or bellow.

You've given a "high level" description of the physical mechanism of induced drag, but I still take issue with your final sentence. As you've explained, increased circulation about a 3D wing results in increased induced drag, and circulation about a wing at high angle of attack with flaps deployed is a lot higher than it is about a clean wing at a low angle of attack. Fair enough, but it is an apples and oranges comparison. To focus on the relatively poor glide performance of a B747 (or any other aircraft) in landing configuration in comparison to another aircraft type in clean configuration is pointless.

[K^2]

You've given a "high level" description of the physical mechanism of induced drag, but I still take issue with your final sentence. As you've explained, increased circulation about a 3D wing results in increased induced drag, and circulation about a wing at high angle of attack with flaps deployed is a lot higher than it is about a clean wing at a low angle of attack. Fair enough, but it is an apples and oranges comparison. To focus on the relatively poor glide performance of a B747 (or any other aircraft) in landing configuration in comparison to another aircraft type in clean configuration is pointless.

The computation yielding 5.5 is for clean config. I've made no assumption of configuration or AoA in qualitative explanation, either. Given same on both, higher density, and consequently, lower air speed, results in much higher drag induced by wingtip vortices. How much of a difference that's going to make is going to depend on wing loading, among other things. 747 is an example of an airplane that can only glide in low air density.

[Guest]

The biggest problem with gliding in a 747 would be loss of hydraulic pressure in control surfaces, since the engines provide that. A glide ratio between 5 and 4 can be survivable, though you need to come in at high speed and angle, then flare very hard, somewhat like the Shuttle does. For actual gliding, l/d ratio is the most important, but even in a very bad glider you can, briefly, increase your lift to weight ratio enough to reduce vertical speed to something survivable. Not stalling the aircraft while doing this isn't a trivial matter, but it should be possible.

As for the general idea, even an actual building brick can glide if you throw it fast enough, at proper AOA. Case in a point, Apollo CM. It "skipped" the atmosphere a few times, meaning it managed a l/d ratio above one. It just took speeds around 11km/s to do that. Unless perfectly symmetrical, and with COM precisely on the axis, any reentering body will generate some lift. As long as it doesn't burn up, and it's oriented so that the lift vector points up, it can glide for a while, or even skip right back into space.

[WestAir]

The 747 is controllable with a quad engine failure. Between the RAT and the APU you will have viable use of most the control surfaces and essential avianics.

[K^2]

Dragon, I suggest you try this with a decent simulator. MS FSX or X-Flight 9 should suffice in this case. I suspect, you'll find that flaring a 747 on final with dead engines is a lot harder than it sounds. But as I've said originally, under ideal situation, it is possible to land. Real world rarely provides these, however.

[WestAir]

MS FSX or X-Flight 9 should suffice in this case. I suspect, you'll find that flaring a 747 on final with dead engines is a lot harder than it sounds.

K^2. You are correct. For those questioning it, a 1:4 glide ratio at 200 knots would yield you a 5,063 ft/min descent rate if my math is right. [50 miles down for every 200 miles forward. 50 nautical miles is 303,806 feet / hour. Divide by 60 to make that 5,063 feet per minute] That is disastrous. To put that in perspective, if this 747-400 were to do a typical ILS landing, the formula for it's approach descent rate is [Descent Rate (feet/min) = Ground Speed (kt) x glideslope %]. Because a 747's approach speed can vary more than 60 knots, I'll put it in the middle and use a generic 150 knots. Assuming the winds are zero, the groundspeed is then 150 knots, the typical glide slope is 3° which gives us 5%, so: Rate of Descent = 150 x 5 = 750 ft/min. A far, far cry from the 5,063 ft/min glide.

K^2 mentioned flight simulator; About a half-decade ago I flew a 747 deadstick into a waterlanding. The video is still on YouTube (skip to 6 minutes to see it):

Real world rarely provides these, however.

Thank god. In February I had the wonderful close-call of flying a small C172 on the flight before it suffered an engine failure. The pilots were able to restart it in the air. Like the long-retired 777 Captain I mentioned in an earlier post, they never pitched for best glide. According to the guys at the flight school, the extra speed allowed them to restart the prop.

Had it been me, I would have pitched for best glide and been in the back page of the Las Vegas Review Journal somewhere. Thankfully I've never had to deal with such a situation out of a sim.

[K^2]

It's been a few years, but from what I remember from the class, the procedure for a single-engine is to try a restart right away. If you fail after a couple of attempts, then you are in situation where you start trading altitude for air speed, so you need to have an idea of where you are going to land if your engine doesn't start. From there on, if you have altitude/range for it, you keep trying to maintain speed and trying to restart the engine. If not, you switch to best glide. On a twin, you just keep flying level, unless second engine quits. But I don't know how much of this is applicable to jet turbines. I also don't know how much of that I'd remember if I was actually flying an airplane that suffered a failure. Our CFI used to say, "Propeller is like a fan. When it stops spinning, you start sweating." And I was sweating enough just landing a 172 with an instructor next to me.

I do recall that landing was done with almost no throttle, though. So while I'm sure engine failure would cause me some new gray hairs, it's not something I'd worry about getting into the plane. I really need to get back to that once I have a paying job.

[MrZayas1]

Well, your claim falling like a brick does have its instances. But everything glides, even if it is the smallest amount. This is why large planes have big wings, because it needs that lift to get off the ground. But gliding depends on the shape and surface are. My dad flies helicopters, this one to be exact. He had his own helicopter tours and his helicopter failed multiple times, twice. So, even though this helicopter design isn't very aerodynamic, it still allowed him to perform an autorotation to land using the little lift he had. Gliding a plane is much easier, because the wings add lift, which allow it to catch the air and fly further.

But to be specific, everything has gliding capability, no matter how small, or how great. Even a brick, has some gliding capability, if little at all. But a brick's mass to lift ratio is extremely large on the mass side, so it does not glide. But a peice of paper, as you know is very light. If you try to drop one, it drops and floats a little bit because of the cushion of air all around you.

[PakledHostage]

The 747 is controllable with a quad engine failure. Between the RAT and the APU you will have viable use of most the control surfaces and essential avianics.

For what it is worth, most B747s don't have a RAT. Only the newest B747 type (the B747-8) has a RAT. What's more, the APU on the most common B747 model currently in service, the B747-400, cannot even be started in flight. The aircraft type relies on windmilling engines to provide limited hydraulics and batteries to provide electrical power in the event that all four engines flame out. The B747-8 only has a RAT because of its increased reliance on electrical power.

[WestAir]

For what it is worth, most B747s don't have a RAT. Only the newest B747 type (the B747-8) has a RAT. What's more, the APU on the most common B747 model currently in service, the B747-400, cannot even be started in flight. The aircraft type relies on windmilling engines to provide limited hydraulics and batteries to provide electrical power in the event that all four engines flame out. The B747-8 only has a RAT because of its increased reliance on electrical power.

Pakled,

I had no idea, that's really cool. I took a look around the internet and you're completely right. Boeing is usually several miles ahead when it comes to systems design, so I wonder why the APU isn't designated for inflight restart. It seems like in the event of fuel starvation or birdstrike the crew would be S.O.L. [Well, more SOL than they would be in an A320 that hit a bunch of birds, anyways]

[GoDores]

Thank god. In February I had the wonderful close-call of flying a small C172 on the flight before it suffered an engine failure. The pilots were able to restart it in the air. Like the long-retired 777 Captain I mentioned in an earlier post, they never pitched for best glide. According to the guys at the flight school, the extra speed allowed them to restart the prop.

Had it been me, I would have pitched for best glide and been in the back page of the Las Vegas Review Journal somewhere. Thankfully I've never had to deal with such a situation out of a sim.

The pilots were either pulling your leg or need some remedial training. You have to slow a single-engine piston airplane almost to a stall, and keep it there, to get the prop to stop windmilling. Step 1 in any engine failure checklist in a single is pitch for best glide. If the engine won't restart at that speed, it won't restart at any other speed.

Jets are different because it takes much more energy to get the turbine to windmill, and they're operated much higher where the air is thinner. There can be both a minimum speed and a maximum altitude to attempt to restart a windmilling engine, and the minimum speed probably is above best-glide speed.

[WestAir]

The pilots were either pulling your leg or need some remedial training.

GoDores,

I've learned over the years that no two pilots fly the same. If you've ever been to aviation forums like Pprune, you'd be well aware that pilots with 10k hours will endlessly argue over even the most basic of procedures; One will quote the manufacturer handbook, the other will quote their company policy, another pilot will quote what they did in the military and the last guy will quote the procedure his instructor taught him back in 1970. Take a walk into the little room all the standby pilots are always waiting in and ask "Do you pitch for airspeed or altitude?" and get ready to never hear the end of it.

My point is that I no longer argue with pilots when they say they've done something or tell me what way is the right way. Pitching for 120 knots worked for those guys to restart their prop and who am I to argue? I would have pitched for best glide.