Best Glide Speed And Bank

[Adrock]

This question came up while considering Steep Spirals: Does best glide speed increase in a bank? Given this fact: The heavier the plane the greater the glide speed required to result in the same best AOA for LDmax. Following that logic when bank increases the load that the wing supports increases and if the best glide speed were kept constant in a steep bank the AOA would exceed the best glide AOA wouldn't it? So to remedy this it seems like best glide speed would increase in a bank? So the steeper the bank the greater the best glide speed?

 

[Adrock]

Ithink I just figured out why this is not true: In a glide what determines speed is AOA i.e. the drag created by Differing AOAs; therefore, to maintain the specified glide speed with a bank, the rate of descent will increases but the AOA and corresponding drag that determines the speed will remain unchanged. In otherwords it takes a steeper descent in a gliding turn to maintain the same AOA and drag profile but the airspeed will still indicate the correct glide AOA.

 

[subpilot]

The increased wing load would only result when maintaining altitude in a turn. If you are performing a steep spiral then the wings are not being loaded since you are pitching for best glide.

 

[Guy]

The increased wing load would only result when maintaining altitude in a turn. If you are performing a steep spiral then the wings are not being loaded since you are pitching for best glide.

Roll an airplane over into a 60 degree bank and spiral towards the ground and tell us if you don't feel the G Force.

 

[subpilot]

Roll an airplane over into a 60 degree bank and spiral towards the ground and tell us if you don't feel the G Force.

Why stop at 60 degrees? A properly executed aileron roll is only performed at 1G because the wings never get loaded. I always set up my students with unusual attitude recovery by rolling into 45-60 degrees bank and they never feel it because I don't load the wings.

 

[ryan1234]

Why are you worrying about best glide when doing steep spirals?

I don't see how they could both be used together? The objective of best glide is usually to gain the best lateral distance per altitude lost. Best sink would be the most beneficial if the objective was to stay aloft for the longest period of time. Obviously, steep spirals and/or steep turns would not be the best way to accomplish either of these.

 

[USMCmech]

Roll an airplane over into a 60 degree bank and spiral towards the ground and tell us if you don't feel the G Force.

We've had this argument before. The areodynamic experts say physics dictate 60 deg bank creates 2G load factor regardless of pitch. Those of us who have tried it say that there is a dramatic difference between 60 deg in a level turn vs 60 degrees nose down in a steep spiral.

 

[subpilot]

It's not a factor of what pitch attitude you have as I agree that holding a -10 degree pitch will still create a 2G load. The trick to no loading is to let the nose fall (no elevator input). With this said, there would be loading in a steep spiral because you are going to be manipulating the elevator to maintain airspeed.

 

[Guy]

Why are you worrying about best glide when doing steep spirals?

I don't see how they could both be used together? The objective of best glide is usually to gain the best lateral distance per altitude lost. Best sink would be the most beneficial if the objective was to stay aloft for the longest period of time. Obviously, steep spirals and/or steep turns would not be the best way to accomplish either of these.

I had the same thought at first, but there is one time I can think of when such knowledge could be beneficial. If one were trying to make "the impossible turn" following an engine failure on takeoff. Gliders do it all the time as a training maneuver from 200' AGL, but it varies with aircraft and airplanes need a lot more altitude to make that turn.

The aerodynamically best angle of bank for making a gliding turn with the least loss of altitude is 45 degrees. However, there are a lot of other factors that come into play including the offset from the runway one will have after completing a 180 degree turn. The stalling speed goes up by about 20% in a 45 degree bank and 40% in a sixty degree bank. The G Force in a 45 degree bank is the square root of two, or approximately 1.414. To get the best glide speed for a 45 degree bank turn, it would take data for 1.414 times the aircraft's weight and most likely that would be something over the MGW and not published.

However, exactly as you noted, there isn't a practical application for using either minimum sink speed or best glide speed in a steep spiral maneuver.

 

[drunkenbeagle]

Gliders in fact do it all the time, I demonstrated a 60 deg bank 180 at 200 AGL to a student today, who didn't think it was possible until he saw it.

In a turn, we care about minimum sink, not best glide. This will be just above stall speed for that angle of bank. Since you can not calculate it for every possible scenario, you need to use cues like a wing buffet or stall horn in the real world.

 

[Pilotforhire587]

Why stop at 60 degrees? A properly executed aileron roll is only performed at 1G because the wings never get loaded. I always set up my students with unusual attitude recovery by rolling into 45-60 degrees bank and they never feel it because I don't load the wings.

This is true, when you roll 60 degrees the only way you will pull 2G's is if you hold you altitude. In a steep spiral you are obviously not holding altitude. And Ryan, the reason why you would hold best glide in a steep spiral is because the PTS calls for it...

 

[pwapilot]

This question came up while considering Steep Spirals: Does best glide speed increase in a bank? Given this fact: The heavier the plane the greater the glide speed required to result in the same best AOA for LDmax. Following that logic when bank increases the load that the wing supports increases and if the best glide speed were kept constant in a steep bank the AOA would exceed the best glide AOA wouldn't it? So to remedy this it seems like best glide speed would increase in a bank? So the steeper the bank the greater the best glide speed?

Adrock,

You initially had the correct reasoning. The bank requires an increase in lift assuming you want to maintain steady state flight. With no configuration changes, the only ways to increase lift are to increase AOA or increase speed. Since L/D_max for a given configuration is only a function of AOA, you can't change AOA and still keep the "best glide" condition. As such, speed has to be increased in order to make a best glide turn.

Note that the steady state condition does NOT require you to maintain altitude as a few have mentioned. It does require that you are not accelerating vertically – that is, your vertical speed is constant.

Mike

 

[Guy]

the reason why you would hold best glide in a steep spiral is because the PTS calls for it...

Which PTS calls for holding best glide in a steep spiral?

 

[ryan1234]

And Ryan, the reason why you would hold best glide in a steep spiral is because the PTS calls for it...

I can't really make sense of why it would call for it. Even as beagle pointed out that minimum sink would be more appropriate for the "impossible turn". But even that math game is a function of turn rate/radius and minimal sink rate as well as energy state/instantaneous turn performance. I don't know the right answers

My initial reaction would be to gently turn and climb a little, 1st notch of flaps, and then press into a 45-50 degree bank descending turn... feeling for the tickle. I don't know if that's the right way to do it or not.

 

[subpilot]

The best way to perform the impossible turn is to not do it. There is a reason it's called the impossible turn.

 

[ryan1234]

The best way to perform the impossible turn is to not do it. There is a reason it's called the impossible turn.

Valid verbiage... I guess glider pilots should call it the "possible turn"

 

[drunkenbeagle]

Valid verbiage... I guess glider pilots should call it the "possible turn"

That's the one from 100' for us... We do have the benefit of that typically being runway straight ahead though.

Depends on the aircraft, depends on the field, there is no one right answer.

 

[nosehair]

there isn't a practical application for using either minimum sink speed or best glide speed in a steep spiral maneuver.

If your engine quits, and you see a really good flat field down to your left front between the few cumulus you were flying over, having the knowledge and skill to fly either best rate or best sink, as the situation dictates, would be a very attractive practical application. To me.

 

[Adrock]

Adrock,

You initially had the correct reasoning. The bank requires an increase in lift assuming you want to maintain steady state flight. With no configuration changes, the only ways to increase lift are to increase AOA or increase speed. Since L/D_max for a given configuration is only a function of AOA, you can't change AOA and still keep the "best glide" condition. As such, speed has to be increased in order to make a best glide turn.

Note that the steady state condition does NOT require you to maintain altitude as a few have mentioned. It does require that you are not accelerating vertically – that is, your vertical speed is constant.

Mike

The Airplane Flying Handbook uses best glide while the PTS just says "specified airspeed." I have always felt g-loading during a steep spiral so im sure there is an increased load on the wing. I cant find information published anywhere to confirm that best glide speed increases in a turn. Flight-Sim has 300 extra pull 2-gs during a gliding Steep Spiral too--not that flight sim is perfect but the physics and numbers are usually correct. It is possible to turn to any bank-angle and not load the wings but it takes an acceleration toward gravity (or away from the direction of turn) to offset the acceleration of the turn. With a constant descent rate, as is such in an established steep spiral there is no acceleration to offset gravity so the plane has to support the normal 1g and support the acceleration of the turn.

 

[drunkenbeagle]

However, exactly as you noted, there isn't a practical application for using either minimum sink speed or best glide speed in a steep spiral maneuver.

By definition, the steep spiral will be at a speed above best glide or minimum sink. Otherwise it would simply be a steep turn.

In airplanes, the purpose of a steep spiral is to intentionally lose altitude in a minimum radius turn over a fixed point. It is done at a specified speed that will give sufficient sink.

In gliders, the spiral dive happens when the nose is too low in a steep turn, typically in a thermal. The glider will build excessive airspeed and sink, and the wings are not sufficiently loaded, resulting in the airspeed buildup.

In both cases, the recovery is to level the wings in a coordinated fashion, and then pitch up to slow the aircraft. (It is entirely possible to overstress the airframe trying to slow it to best glide/min sink first, which is why the wings should be levelled first.)

But to answer the original question, best glide (and more appropriately minimum sink) both increase in a bank. For a typical training glider with MCA at 35kts, thermalling at 60 deg bank the MCA would then be 50kts.

One more thing - in a steep bank, not all aircraft have sufficient elevator authority to stall past 60 degrees of bank. This is true of almost all gliders - past 60 degrees bank, with full back stick, airspeed will increase as the nose drops, resulting in a spiral dive. This is the primary reason that gliders tend to fly a tighter traffic pattern with much steeper bank than airplanes do.