Best glide speeds?

[damo1089]

As far as I know, the 'best glide speed' of an aircraft is the speed at which the plane can cover the most distance, any higher and the maximum distance it could have achieved would have been shorter and any slower it would have been shorter.

My question is: Is the best glide speed is also the safest glide speed? For a big beast of a plane that falls quite rapidly even at it's best glide speed, will the 'best glide speed' also equal 'the best speed for elevator effectiveness to slow the descent, flare and land the aircraft safely?

An approach that is just making the landing area would generally be quite flat, and with no power on, it will be falling quite fast.

If the best glide speed gets the aircraft to cover the most distance, will it aerodynamically provide enough elevator effectiveness to land safely, or are there some cases where gliding faster will result in a safe landing even if at the expense of potential distance?

[Old Fella]

DAMO1089, the best glide speed for any aeroplane is that speed which gives the best lift/drag ratio. In light aircraft the best glide speed stated will be for the aircraft at maximum weight and because the weight change in flight will generally be insignificant the same speed will be used for all weights. In large aircraft the best glide speed for maximum weight will be a bit higher than for lesser weights. You have assumed that because an aeroplane is large it will not have as good a glide range as a small aircraft. This is not always so. A Cessna 152 for instance has a glide ratio of about 10:1, in other wods when flown at the correct glide speed it will descend 1000 feet for every 10000 feet it travels over the surface. I seem to remember that a B747-200 has a glide ratio of around 18:1 (I stand to be corrected) and thus will glide further from the same initial height than the C152 would. The approach will not be 'flat' unless you misjudge your aim point. The glide speed will be greater than the touchdown speed and thus you should not experience loss of aerodynamic effectiveness of the elevator.

[damo1089]

Cool. Thanks for the reply oldfella.

I was asking because today I was investigating the glide characteristics of the cessna 210. Extracting the glide speed for the weight I was at made it about 5 knots above where the stall warning would start sounding, which yeilded a descent rate of above 1000ft per minute.

That being said however, it was a clean glide speed, but I had the undercarriage extended. I could not find figures for flaps up and wheels out.

Due to the rapid descent rate, I had serious doubts as to whether the aircraft would have a safe amount of elevator effectiveness to flare and land without damaging the aircraft or it's occupants. Hence the question.

[sevenstrokeroll]

glide speed...harumph

sounds like you need to read "stick and rudder"...by wolfgang langweische

a thorough discussion of glide speed

you may want to increase your speed in some situations

[Runaway Gun]

At a clean glide of 85kts, there is plenty of Vee-squares (airspeed) in reserve for the flare etc at 69kts clean.

[TheRedBaron01]

Even in light airplanes, the glide speed can vary significantly based on weight. A Cessna 206's Vg at max weight is 75 KIAS and decreases to 65 KIAS at light weights.

You are correct to be concerned about energy management when doing a power off landing, because like you said, the difference between best glide and best flaring speed with no power (I made that up) might not be much. In the 206 I fly, when doing power off practice from altitude, I hold the best glide until 1000' AGL, then execute a power off 180 to a spot landing (-0, +200 feet). During the power off 180 maneuver I'm no longer concerned about "stretching the glide", but I'll generally go to 5 knots below best glide on short final, and maybe 10 knots, which for a 206 is typically 65-70 knots at gross weight. I would NOT let it get below 65 knots until very short final (this is with full flaps at this point) or else you won't have enough energy left to flare like you said. It's all about energy management. But I would have no reservation flying best glide all the way to the ground if I needed to. A 210 will fly very similar to a 206, especially with the gear out.