Simviation Forums

So here's a question that one of my professors and subsequently flight test examiners ask. If you had two airplanes, let's say a Cessna 172, let's also say they are the exact same in every way, they burn the same amount of fuel, they create the same amount of drag, and so forth. Now in one plane, is one pilot. In the second plane are two people, making it heavier. Which one will glide further if the engine is turned off, provided human error and atmosphere is negligible and they both start at the same altitude?
Now... I've been told the answer quite a few times, it's completely illogical to me, but that might be because I have no idea why the answer is the way it is. My question is, how does this work?

Assuming neither plane is over gross, and both are trimmed for best glide, I would guess the difference would be negligible.

cheers,
Joe

More weight, means a shorter glide... just like more weight makes for a slower climb.


Edit:..

yeah... I've tossed this back and forth many times... and to me... logically, the heavier one would not be able to travel as far as the lighter one... but the answer dictates that they will travel the same distance... but why... why would they both travel the same distance? Is it that the heavier one is more flying than falling, and the lighter one is more falling than flying... and somehow that just works out to be the same rate of descent? Or maybe it's magic? haha... I have no idea on this...

I have trouble getting my head around it too..  And it's not a simple thing.. like as noted, at a certain weight you transition from flying to falling. Fill the plane with lead, and it drops straight down ..

I Think of it as a trick, question. By calculations you get one result, but reality is different. An army of physicists and engineers aren't gonna convince me that the C172 with just me in it, can't be glided further than the C172 with me and three friends in it. For one, there's always some sort of wind, and if you're aloft longer, the wind will have more of a say.

Actually... the only way for this to make any sort of sense to me at all is if the lighter one is the one falling... because the heavier one needs the faster airspeed... therefore it's nose is pointed down, and there's more air flowing over the wings... so that would be the one that's more flying than falling... whereas I'm starting to think that the lighter one appears to be more falling than flying, because it doesn't require as much of a nose down attitude as the heavier one. Kind of like they're both descending at the same rate... just with different attitudes. In either case though, the wind would definitely affect the gliding distance, and no two airplanes are the same, even if they are the same model or make. We as pilots have the ability to change things too, just because we can be consistent, does not mean that our actions will result in the same consequences, if all other variables are negligible. But yeah... this is quite a question, thank god that instructor never asks why they can both glide the same distance... haha

[quote]More weight, means a shorter glide... just like more weight makes for a slower climb.


Edit:..

A great question... let Professor Insomniac explain...

Maybe we should merge this thread and

To Professor Insomniac:

Gravity will act on both aircraft equally, so if weight was the only force acting on the aircraft with no air resistance or other forces, they would both hit the ground at the same time. But the wings act against weight and hold the aircraft in the air for longer.

More weight = a lower resultant force of the weight against the lift. Less lift = in the air a shorter time.

So I would say the lighter aircraft would be able to glide for longer.

[quote]


Psssst:

[quote]
Psssst:

Gravity will act on both aircraft equally, so if weight was the only force acting on the aircraft with no air resistance or other forces, they would both hit the ground at the same time. But the wings act against weight and hold the aircraft in the air for longer.

More weight = a lower resultant force of the weight against the lift. Less lift = in the air a shorter time.

So I would say the lighter aircraft would be able to glide for longer.

You're forgetting that we can change how the wing deals with gravity, by changing the A of A. The wing does not fight gravity by creating drag, like an old-fashioned round parachute, it does it by moving the air a certain way. That way is variable, thanks to the elevator.

  If the pilot of the heavier glider cannot change A of A and thus airspeed, yes, he will not cover the same distance from the same altitude. His A of a will be too high, to the point where the lift gained by more A of A will not overcome the lift lost by the lower airspeed (remember, speed and angle work together to produce lift). Pitching up more will just make things worse, just as they do if you try to maintain a climb with pitch only in any aircraft. So he has to lower the nose- just a little- to pick up a little more speed. The vertical speed will increase, but it will go farther in the time it takes to run out of altitude, because it is moving forward faster, too.


On the other hand, let's say the competition is to stay aloft the longest period of time, in still air (no lifting air of any kind). This is completely different from trying to achieve a given distance from a given altitude!! Best-glide speed is not the same as minimum sink speed!!
As long as the heavier glider can adjust its airspeed (by changing a of A), it can maintain the same vertical speed as the lighter glider, and stay up just as long.

Keep adding weight to it, and yes, eventually the wing won't lift so well regardless of A of A, but with a 2-seater, designed so that the same rate of vertical speed can be achieved with one or two people aboard, if the weight is within limits, the same endurance can be achieved... IF the airspeed is changed. The heavy glider, gliding at a higher indicated airspeed, will not necessarily be sinking faster, IF that indicated airspeed is yielding "minimum sink".  Minimum sink is achieved by assigning a specific A of A, and thus airspeed, to the wing... it is a V-speed. And V-speeds change with load factor, including load increased because of a higher payload.

Here's another question that I asked the same professor today, and he didn't exactly give me a straight answer. The thing with him is that he's quite a bit older of a fellow and he's got tonnes of experience, he's got every sort of license under the sun including an aerobatic's instructor rating so I asked him how to recover from a tail stall, the only thing he really told me was to pull back on the stick instead of forward, which didn't make a whole lot of sense, I then asked him about power settings for such a recovery and he wasn't too sure as the last time he had looked at such a procedure was at least three years ago... and the likelihood of this scenario ever happening at my school is slim to none as we are not allowed to fly in known icing conditions, both single engines and multi-engines owned by the school simply aren't rated for such conditions. Being a curious person who wants to know everything about everything, what is the procedure for such a scenario?