The explanation could get rather technical, but the simple way to say it is that the glide ratio is the ratio of lift to drag, and when air is less dense, both lift and drag decrease, in the same way (and for the same reason – fewer air molecules per cubic meter). So the glide ratio changes very little with altitude.
It would be slightly more accurate to say that if you want the same lift – usually, the weight of the aircraft – then when the air is less dense you must fly at a higher true speed to get it, other things being equal. And when you fly faster, drag increases back to what it was when lower and slower, generating the same lift. A consequence of having to fly at a higher true speed is the need for longer runways at high/hot locations, as you note.
It’s incorrect to say that glide ratio doesn’t change at all with air density, because of effects like changing Reynolds numbers. But those are minor.
I think “over 150 mph” is about right for the touchdown speed of a 7x7.
Assuming that without power you can cause the aircraft to arrive at the right spot, the actual touchdown should not be much different with or without engines. Large aircraft have emergency systems to power their hydraulics in case of engine failure, so the controls (ailerons, elevators, flaps) should work about as normal. The plane is certainly huge, but its control surfaces are sized accordingly – a near-normal touchdown should be possible.
You are quite correct that for any hope of a decent landing, a big aircraft requires a runway (long enough and rated for the aircraft’s weight).
I’m impressed that you were able to land a C-150 in a “backyard” – I suspect that few power pilots could do this on the first try (it was your first try, wasn’t it?).
About right. The range is from around 18:1 (60-year-old open-cockpit antiques) to 70:1 (The Eta, a 101-ft-wingspan super-exotic). Most modern designs are around 40 - 45:1.
Depends on the circumstances. The Gimli Glider, for instance, was flown to a landing in a slip - apparently there wasn’t enough power to deploy everything and they decided getting the landing gear down took precedence over the flaps.
The size required for control surfaces is, in part, dependent on speed. A C150 has a huge rudder in relation to the rest of it when compared to the rudder and fuselage of a 7x7. Sure, the Boeing has a bigger asolute rudder, but porportionally the Boeing rudder is smaller. The higher the airspeed the less control surface needed. The only problem is if you have a big, high speed airplane that utilizes various deployable devices for (relatively) slow-speed control but can’t deploy them. This requires the pilot to land at a higher speed than normal. Which can complicate things considerably.
Weight is a factor - a landing C150 and a landing 7x7 may both have a sink rate of 500 feet per minute, but fer darn sure the Boeing is going to thump the ground harder due to momentum= mass x speed. Which is why they have those big honkin’ landing gear, of course.
On that afternoon? Yes, it was my first try - and I almost didn’t get that one. And certainly my first in a Cessna. (So far have not had that honor in a Piper)
It was not, however, my first off-field landing. Back in the my ultralight days we used to do that sort of thing for fun (we knew the fields we were landing in - they weren’t randomly chosen). So when the Big Moment came I knew what I getting into and there weren’t too many surprises, even if I was in an aircraft twice as fast and three times heavier than what I had attempted that sort of thing in before.
I should probably mention that that “backyard” landing (actually, a small hayfield) was under power - it was weather forcing me down, not lack of power. A bit of different circumstance, really, but it does make the point that you can land an airplane in places you wouldn’t expect to be able to. In fact, that’s what the property owner said to me - “I wouldn’t have guessed you could land an airplane there.” Truth is, I had my doubts, too, but I figured slamming into a garage had a higher likelihood of me walking away than spinning into the ground under IMC spatial disorientation so there ya go. In the end - no damage to me, the airplane, or anything else (except my pride).
It did delay my private pilot checkride a bit, though, and I got a talking to by the Authorities, but in the end they just seemed happy not to have to do any paperwork on the matter.
I’m a lot more respectful of the weather since then.
Just an aside speaking of the Air Canada 767 that ran out of fuel due to a miscalculation by the FO…The 767 has a wind driven turbine that can be deployed and it provides enough power to assist with the flight controls and power cockpit displays.
Remember as in the Sioux city etc… the DC-10, 747, 777 the actual control surfaces are in fact very large and require a lot of force to be manually handled handled thus the hydraulics…
The answer to the question is they glide just fine.
You are correct that temperature is a big factor. a hot dry day like you said, takes a lot of runway to get off the ground. Aircraft fly best at cooler temperatures when the air is more dense. The only difference altitude makes is less time available to find a place to land I.E. USAirways landing in the Hudson.
I think you’re right the 777 is what like 17 to 1
There are lots of examples of aircraft gliding just fine…A great one is the Space Shuttle, Not quite as large as a 747 or 777 but, heavy and quite a good glider.
The only drawback of the ram air turbine is that it will slow down as the aircraft slows…This can make the aircraft harder to control as less power is provided.
Also the deployment of flaps and or gear will slow the aircraft and make it harder to control.
jetset707, you may not have noticed that this thread has been gliding since 2003. Just your luck, though – most posters are still around, but not the OP, who may have crashed.
That Reader’s Digest story of a 747 whose four engines turned off because it flew right into the ash column of an erupting volcano over Indonesia (“Mayday, we’ve lost all four engines!”) The pilot reckoned he can glide 175 miles from an altitude of 40,000 feet. That’s 22 miles for every 1 mile of altitude lost, about the same performance as a well-trimmed sporting glider.
The problem with “gliding” is that when the fuel runs out the engines don’t shut down at the same time. One does, and then the other. Unless the plane was being controlled or the engines were shut down by the pilots, the plane would bank in the direction of the engine which is off and then crash land in that direction.
BA 9. Captain Moody delivered one of the coolest lines in history, “Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress.”