Factors affecting aircraft glide ratio

What factors affect the glide ratio of an aircraft designed to be flown under power? I’m not talking about ultralights or powered gliders, etc., but civilian commercial and amateur aircraft and military aircraft. Do designers worry about glide ratio, or is it a by-product of desired performance characteristics?

Some background:

A few days ago I happened to catch part of a Discovery Channel documentary about British Airways Flight 9, but I didn’t get to see the denouement. To satisfy my curiosity, I looked up BA 9 in Wikipedia, and discovered the amazing truth. I recommend the article.

That article led me to the Gimli Glider and other accounts of commercial airliners that have glided to a landing.

The BA 009 article states (essentially) that the glide ratio of a 747 is roughly 15 to 1. I tried finding a more accurate cite, and by Googling came up with a figure of 17.2:1. The Gimli Glider exhibited a ratio of about 12:1 during its descent.

When I mentioned these figures to a friend who is studying for her pilot’s license, she looked at me as if I were mad. She said that her Cessna only had a ratio of 7:1. I had to show her the evidence to convince her I wasn’t making it all up!

I see plenty of threadness about airliners running out of fuel and gliding, but nothing about their glide ratios. Thus my curiosity.

Drag, drag and drag. The less drag a plane has at a speed above stall speed, the better it will glide. There are no other factors.

And lift, lift, and lift. The more lift a wing generates the better it’ll glide for a given value of drag.

Bear in mind your pilot friend’s cessna has a big windmilling propeller acting as an airbrake when the throttle is closed not to mention fixed landing gear, by comparison a jet is sleek and clean and has efficient wings.

By the way, these factors are no different from those affecting a dedicated glider, it’s just that a glider is optimised for gliding.

An aircraft’s glide ratio is the same as its lift/drag ratio.
Airplanes like Cessnas and ultralights have low glide ratios because they’re draggy. Lots of exposed struts, landing gear, propellers, and in the case of ultralights brace wires, exposed control cables and airfoils optimized for lots of lift at the expense of drag. An airliner with gear and flaps up is very clean, aerodynamically.
Interestingly, glide ratio isn’t affected by weight much. The heavier plane just proceeds along the same slope at a higher rate of speed. It gets to the same spot on the ground sooner.

The free body diagram of an aircraft in steady powered flight includes equal and opposite horizontal components of thrust and drag, as well as equal and opposite vertical components of lift and gravity. Once thrust is removed, the drag component slows the plane, and forward momentum (i.e. kinetic energy) must be converted from the plane’s potential energy, sacrificing altitude for speed. If there was no friction due to air, the plane wouldn’t drop in altitude.

Stopping the propeller on a light aircraft like a single engine Cessna will greatly improve the glide ratio.

Amazing … a thread about airplanes and no one’s brought up the dreaded T-word.

‘Dreaded’? I kind of like a little turbulence.

Hmm, seems to me that making the propeller go really fast would give a better glide ratio. :wink:

But seriously, is it because there’s less eddy currents compared to a propeller that’s providing less forward velocity than the plane is already moving?

Well, for this discussion, lets assume that the device that makes the propeller go round real fast is broke. Heaven forbid that the nut behind the stick did not bring along enough fuel for that device.

Even if all you are doing is to pull the engine back to full idle, what you have is about about a 6’ diameter expanded steel plate in front of the airplane. Put an outboard propeller on a rope and drag it behind the boat. It will cause a tremendous amount of drag even if it is on s swivel. Same thing applies to an propeller on an aircraft. the flatter the pitch and the faster through the air you are moving, the more drag you have. So an adjustable pitch propeller that can be put into a high pitch condition so it rotates slower will cause lest drag than a fixed pitch propeller.

Also, on worn out training planes quite often the compression is rather low in comparison to higher powered engines and getting the plane slow enough that the compression will actually stop the prop is sometimes difficult unless you really work at it. An larger engine with higher compression like in an Lycoming IO-540 can be shut down with the prop in high pitch and a gentle stall with full flaps will allow the compression to stop the prop quite nicely. also, an increase in airspeed is less likely to start it windmilling again.

I heard a story one time about an Cessna-180 that was working at 10,000 feet above ground level and need just about all the fuel to get the job done. As it was well within gliding distance, they stayed up to finish the job. As soon as it was over, the prop control; was pulled to low RPM = high pitch position, the mixture was cut , full flaps applied and the aircraft gently stalled to stop the prop and then they played glider for a while and circled well into the traffic pattern at the airport of choice. On final the prop was moved to high RPM or flattest pitch position and the starter bumped to get it to spin enough for the air to keep it moving, mixture in and the mags hot and so there was enough fuel to taxi tot he fuel pumps.

The feds get cranky if you run out of fuel before you get to the pumps. Just a story I heard.

The prop going around dragging the engine along is really just a little below the RPM of a normal full idle position with the engine actually running. ( depends somewhat on the airplane and engine combination and a few other factors) but the propeller acts almost like a solid disk when spinning as far as the air is concerned. It is constantly deflecting the air passing through it all the way around the rotation area of the area the prop covers as it was a solid plate.

There was a “Beta” prop made that could be put into a truly flat position in flight and revers when on the ground. when it was in the ‘beta’ position and the engine RPM kept up to normal cruise RPM, like say 2400 RPM. it produced so much drag that in something like a Cessna-206, the airplane would have to be kept in a near vertical dive if any flaps were also extended to keep the airspeed high enough to actually control the aircraft.

It took a lot of training to make pilots able to use this feature, it was expensive, and for the size of planes it was designed for, it was not a really good fit ofr a lot of reasons.

Watch a C-130 being put through it paces and you will see what flat props and reversing props can do with a good, well trained pilot at the controls.

The P&W turboprop engines automatically go to a totally feathered position with engine shut down. (Think Beech KingAir) This is both the blessing and the curse of that engine design, great for in flight glides with a total power failure but not so good for getting into small places with the engine out.

Interesting stuff to mess with when learning what all a particular aircraft can do. It is a shame that more advanced training still does not teach a lot of the things that can make or break your life.

Wow, Gus, thanks for that!

To the OP: It is all about drag. Due to the high airspeed, commercial airliners need to be pretty clean. Drag force increases with the square of airspeed, so going twice as fast requires 4 times the force and 8 times the power (power = force X speed). Thus a 600kt airplane (airliner) must be MUCH cleaner than a 100kt airplane (Cessna).

Cessnas are pretty dirty, but are really clean compared to biplanes and other wire braced designs.

As a sailplane and hang glider pilot, I can offer the following approximate data points:

Hang glider…<10
SGS2-33 (two place training glider)…~20
SGS 1-26 (metal/fabric low
perforforance single place glider)…~25
Composite sailplane, 15m sport…~36
Composite sailplane, 15m racing…~40
Composite sailplane open (~20m span).~50

The L/D of the higher performance sailplanes will degrade noticeably if flown through a swarm of gnats. To achieve such performance, surface finish is critical to maintain laminar flow over at least 2/3 of the chord. The L/D also goes in the shitter in the rain.

Per Sky King’s post upthread, I will confirm that L/D is not degraded by increasing weight. Higher performance gliders are equipped with water ballast tanks to allow increased speed and distance when flying in conditions with strong lift.

It takes a fair amount a power to turn a dead engine. That power can only come from the potential energy stored as altitude of the airplane. Thus power used to spin the engine must come from the airplane decending faster.

Perfectly said.

The situation is analgous to coasting downhill in a car. With the clutch & transmission engaged gravity has to turn the engine. With the clutch depressed or the trans in neutral, that load is removed & the car will accelerate to a higher speed. Same concept.

Surprisingly, fighters have a pretty good glide ratio too. The overall design (once all the external stores are jettisoned) is very clean. The bad news is the best glide speed is very high. Without digging out my books I recall we flew simulated engine out patterns at ~250 knots, slowing to ~150 at touchdown in a shallow (and unsustainable) glide at the end.

So as between us & a Cessna, we’d outglide them in distance, but arrive at the ground a lot sooner in time.