Testing the Impossible Turn

[lgtech]

The problem with this so-called experiment is that it is not a valid experiment.
They were attempting to determine the altitude loss in a 360 degree gliding turn.
In a gliding turn, there are 4 independent parameters that can be varied. They are
(1) air density - altitude
(2) Aircraft weight
(3) Bank angle
(4) Angle-of-attack
The purpose of the experiment was to determine the effect of the bank angle on the altitude loss. If you perform that type of experiment you must keep the remaining 3 parameters constant, otherwise the experiment is flawed.
If you work through the 4 parameters and carefully review the video, you will note that:

(1) The initial altitude varied by a few hundred feet - air density is essentially constant
(2) The weight of the aircraft was essentially constant- slight variation in weight due to fuel burn (not much, so weight is essentially constant)
(3) The bank angle utilized was 15, 30, 45 and 60 deg. This was the experiment.
(4) The angle-of-attack was not fixed. In order to fix the angle-of-attack for each bank angle, you must fly at a speed that is a specified percent above the accelerated stall speed for that bank angle. They flew almost the same speed for all four bank angles. That means the angle-of-attack could not be the same, which means there were two parameter varying and not one.
Thus, the conclusions that are drawn from this experiment are flawed.

One really needs to understand basic aerodynamics of the turnback maneuver if you want to perform a valid flight experiment.
Sorry to burst your bubble.

 

[denverpilot]

Maybe because they don't want to be doing agressive maneuvers too close to the ground to recover? I wouldn't want to be in the right seat with a student pulling 45* banks below 500 feet (or 1500 feet for that matter).

The 'engine out over the practice area' allows level of near level flight when near the ground. Sorta 'stabilized'

The altitude is debatable at how high you'd have to do it, of course.

Go up to Boulder sometime and watch the kids prep for the Commercial Glider checkride and you'll start to wonder why powered folk can't do it, say perhaps in the Commercial Powered curriculum instead of the Private.

Arguably might save more lives than lazy eights...

You'll also wonder how they're getting around the "30 degrees in the pattern" aerobatic rules, 'cause the glider kids crank it around a lot steeper than that...

 

[jpower]

I just skimmed through the thread, so this might be totally irrelevant, but a friend of one of my instructors was a professor of aerospace engineering at the Naval Academy (and chief pilot for the Naval Academy's flight test engineeing course). He put together a bunch of documents on the Impossible Turn, including some original research in his E33A Bonanza (which I've flown in!). Scroll to the very bottom of this page.

 

[RoscoeT]

You'll also wonder how they're getting around the "30 degrees in the pattern" aerobatic rules, 'cause the glider kids crank it around a lot steeper than that...

But there's no such rule about 30 degree bank angle limits in the pattern. Aerobatics is not defined by any bank angle. It's just that at a certain point, a fed could deem it to be an "unusual attitude", which is part of the definition. It's all totally subjective. The only time bank (or pitch) angle limits come into play is on the subject of when chutes must be worn by all occupants if passengers are involved. But nowhere in the FARs are chutes and aerobatics mentioned together.

 

[lgtech]

If you remember the old adage : :We have met the enemy and it is us"

This is the real problem with CFI's teaching turnback maneuvers without really understanding the basic aerodynamics of the turnback maneuver. As both a aerodynamicist and flight instuctor for more years than I would like to admit, I attended Barry Schiff FAASTeam seminar in southern california to hear about his ideas on the turnback maneuver. After listening to the seminar, I concluded that there was very little analysis backing up many of his statements and some issues were left open that if one drew the wrong conclusion, attempting a turnback maneuver could be potentially fatal. A few months later I developed a 160 slide presentation called the "Anatomy of a Turnback Maneuver", in which I disected the turnback maneuver from both an geometrical point of view and an aerodynamics point of view. Basic aerodynamics is what was used (no different that what Rogers presented in his paper). I presented it as a FAASTeam seminar to over 200 pilots in four different locations. However, the folks who preach the best way to fly the turnback maneuver never showed up (including Barry Schiff). If CFI's are going to teach the turnback maneuver, they really need to understand the basic aerodynamics of the turnback maneuver before they start teaching it. Although the teardrop turnback maneuver is geometrically simple, there are pitfalls in excuting the maneuver that all instructors need to be aware of before teaching it to pilots.

 

[PilotAlan]

Go up to Boulder sometime and watch the kids prep for the Commercial Glider checkride and you'll start to wonder why powered folk can't do it, say perhaps in the Commercial Powered curriculum instead of the Private.

Serious question, how susceptible are gliders to stall/spin? Does the loooooong wingspan change the probability of an mistake ending up in a stall/spin?

 

[eetrojan]

other stuff...

Sorry to burst your bubble.

No bubble to burst. It wasn't my experiment.

Not sure, however, why you found it necessary to use such a demeaning phrase to end your post. Would you say that if we were talking in person?

 

[Dead Stick]

Serious question, how susceptible are gliders to stall/spin? Does the loooooong wingspan change the probability of an mistake ending up in a stall/spin?

Good question. They are no different than powered aircraft - some are very docile others, not so much. Most of the popular training gliders are comparatively benign, some of the high performance ships are much more demanding - just like powered aircraft.

 

[Dave Krall CFII]

(1) I am curious to understand your definition of "academics"
(2) What exactly is your definition of "practice"? What are you teaching your students in regard to the turnback maneuver?
(3) Prior to my posting of the information on the basic aerodynamics of the turnback maneuver, what were you teaching your students in regard to how one would determine the critical altitude for the turnback. In addition,what information were you providing to your students in regard how to scale your results for the critical altitude from one set of conditions to another?

With my flight students in the air, we look at the VSI on power off glides. As previously stated, we practice at altitude and then do the power off turn back maneuver at a wide open (prefer untowered). I'll let them initiate at lower altitude with each repetition, until the trees alongside the airport boundaries make the "wide open " airport look not so great near the end of the glide and we'll either land or power out of it.

For some academics we'll review glide distances and descent rates in different conditions and also I'll refer them to overview the Nall Report and mention a few pertinent cases. We'll talk about the usefulness of coordinated turns in glide eficiency. We'll talk about reaction time variables and agree to add a healthy amount of altitude in determining a personal critical altitude.

As flight lessons progress and confidence builds on previous successful completion or powering out, I'll pull power at different altitudes on the upwind to see if the pilot makes a valid decision to either land off-airport or attempt the turn around.

I have access to a short strip, which has much open water and some grass stretches. Of course, I will save this unique place until they're ready and pull power just below what they now have determined to be their critical altitude. They have thus far always headed for the grass and they are repeatedly instructed to "just land it", which they have done well, on what is actually a private grass strip surrounded by green grass.

It becomes patently obvious to them that 500 AGL may not be doable except under ideal conditions and that 800 is much better, with sometimes 1000 AGL or more needed.

 

[lgtech]

I spent many years flight instructing out of SNA operating out of the GAT building not far from Sunrise Aviation. I assumed your post was to pitch Church's experiment as something all pilots performing a turnback maneuver should be using as valid information. There was no demeaning intent on my part at all.
By the way, Church did not show up either at the FAASTeam seminar I gave at the Orange County Pilots Association sponsored by Robert Baker.
Again, no demeaning intent on my part!

 

[Dave Krall CFII]

How do you recommend assessing the difference between the climb angle and the descent angle under the prevailing conditions?

By the methods I stated above.

 

[lgtech]

I am not sure you read my earlier long post on the aerodynamics of the turnback maneuver. However, just to clarfiy the point:
(1) The teardrop turnback maneuver contains two different types of segments, one a gliding turn and one a wings-level glide. The goal in either of these types of segments has nothing to do with the aircraft vertical speed(VSI) alone. The goal of the gliding turn is to find the optimum angle-of-attack and bank angle to give a minimum amount of altitude loss per degree of turn. One is trying to minimize the ratio of the rate of descent (VSI) divided by the rate of turn. The wings- level segment is trying to minimize the glide path angle (not the VSI) by flying at the attitude for the maximum L/D ratio. This so-called critical altitude for the turnback maneuver depends on the distance from the departure end of the runway where the turnback will be initiated. The altitude loss in the two turning segments are a function of the aircraft weight, air density, bank angle and angle-of-attack. Although this provides the altitude loss per degree of turn, one still need to know the number of degrees the aircraft will need to turn in order to point it at the departure end of the runway under no wind conditions. This intercept angle is only a function of the distance from the runway where the turnback is initated divided by the radius of the turn in the initial turn back toward the runway. This radius depends on the bank angle and true airspeed in this intial turn. The altitude loss in the wings-level glide is given by the distance from the departure end of the runway divided by the maximum L/D ratio (given in the POH). The altitude loss in the turning segments will depend on the total number of degrees of turn in the first turn plus that in the second turn (which would always be less than 360 degrees)
Therefore, the critical altitude for a "potentially successful" turnback will depend on how far from the runway one initiates the turnback and how close to optimum the pilot flies the maneuver. The important point here is that the pilot first needs to know what the aircraft can potentially do in the turnback, and then how to correct this for pilot technique. One can also account for the wind to correct the no wind critical altitude.
Hope this information helps.

 

[lgtech]

If you are referring to the effect of the wind on the climb/glide path angle, then one can utilize elementary trigonometry to obtain the answers. If the climb/glide path angle is less than 10-15 degrees, a good approximation is

VTAS*Sin(gam1)= ground speed *Sin (gam2)

Here VTAS is the true airspeed
gam1 is the climb/glide angle without a wind
gam2 is the climb/glide angle with a wind

Therefore, if one knows the true airspeed and the climb/glide angle under a no wind condition, then the climb/glide angle with a wind can be determined if one knows the ground speed (i.e. true airspeed plus/minus the wind speed)

Hope this information helps

 

[gismo]

You'll also wonder how they're getting around the "30 degrees in the pattern" aerobatic rules, 'cause the glider kids crank it around a lot steeper than that...

I'm not aware of any 30 degrees in the pattern aerobatic rules, sounds more like a guideline. Of course there is a rule prohibiting aerobatics in the pattern of an airport but the bank limit for that is 60° not 30.

 

[lgtech]

The Rogers papers are one way of viewing the turnback maneuver and his last paper was countering the pundits (i.e. Schiff) view of the turnback maneuver. After I heard Barry Schiff's presentation, I took a somewhat different approach to the turnback maneuever. The basic aerodynamics is very similar to what Rogers utilized, however, I was interested in generating a chart that a pilot could view prior to take-off which would tell him when "never to attempt a turn-back" maneuver, or when the envelope for a "potentially successful" turnback maneuver would be narrow. I utilized an inverse approach wherein you specify a distance from the departure end of the runway at which the turnback is initiated, and then determine the required altitude loss for the teardrop turnback maneuver. Knowing the required altitude at that distance from the departure end of the runway, you then specify a particular climb profile. This could be 1.05Vx as in Rogers' paper or some more conservative profile like Schiff's. However, the desired result was to determine the minimum runway length necessary for a "potentially successful" turnback maneuver as a function of distance from the departure end of the runway where the turnback is initiated. In this manner, the pilot does not have to make the decision at the time of the engine failure whether he has enough altitude for a "potentially successful" turnback. He can determine that prior to takeoff and therefore, the risk reduction is performed on the ground rather than in the air. As an example, in a C-172 at sea level, standard temperature, gross weight, flying 10% above the accelerated stall speed in the turns, the minimum runway required was about 5300 feet. Note that this was based on a climb profile which used a take-off over a 50 ft obstacle followed by accelerating to a speed of a few knot below Vy (a somewhat conservative approach), and then maintaining that climb angle until the engine failure. It also included a 5 sec delay in initiating the turnback (i.e. the human factor region) Also note that I included the final turn segment between the wings level glide and the final runway alignment, whereas Rogers excluded that segment. Rogers also stated that the intitial turn ended about 210 degrees (30 intercept to the runway centerline). However, what was not in the Rogers paper's, where the fact that under no wind conditions, the intercept angle depends only on the ratio of the distance from the departure end of the runway to the point where the turnback maneuver is initiated and the radius of the first turn segment. Depending on the distance from the departure end of the runway, this angle could be considerly larger than 30 degrees.
Just some food for thought.

 

[Palmpilot]

By the methods I stated above.

Can you be more specific? I've looked back through your posts in this thread, and I see where you mentioned doing calculations on the ground to determine the critical altitude, but that doesn't address the problem of how to determine whether the glide angle exceeds the climb angle.

If the glide angle exceeds the climb angle, then one can be above the critical altitude and still not be able to glide to the airport. In that circumstance, more altitude hurts instead of helping, because it eventually means that you will be too far from the airport to glide back to it.

 

[denverpilot]

I'm not aware of any 30 degrees in the pattern aerobatic rules, sounds more like a guideline. Of course there is a rule prohibiting aerobatics in the pattern of an airport but the bank limit for that is 60° not 30.

True, but you won't see any powered CFIs teaching any students the bank angle necessary to make that turn back, and yet you will see the glider community doing it, was my point...

 

[lgtech]

I did not include the answer to your question in my previous posts, so here is the answer to your question.

In the wing-level glide, the glide path angle is given by
Tan (gam)=1/(L/D)max
where gam is the glide path angle.
Here, the L/D is the maximim L/D.
In a gliding turn, the glide path angle is given by

Tan (gam)= n/(L/D)
where n is the load factor in the turn and the L/D is the lift to drag ratio corresponding to the angle-of-attack in the gliding turn. Since you are trying to minimize the altitude loss psr degree of turn, the aircraft will not be flown at the max L/D but something less than that, since one is flying close to the stall angle-of-attack.

In regard to the climb angle, the climb angle is given by

Sin (gam)=(T-D)/W

where T is the thrust, D is the drag and W is the weight of the aircraft.
In order to determine the thrut versus true airspeed, one will need the equivalent of the drag polar for the aircraft, and that is called the propeller polar.
It gives the thrust coefficient of the propeller for a given propeller advance ratio (J=V*rpm/D, where V is the true airspeed, rpm is the propeller rpm and D is the propeller diameter).

Therefore, knowing the thrust,the drag and the weight gives one the climb angle at any airspeed.

The formula I provided ealier allows one to take the no wind climb and glide angles and correct them for a wind. Note the plus/minus wind was the headwind/tailwind component.
Hope this answers your question.

 

[Palmpilot]

...Hope this answers your question.

I was curious about what Dave Krall is teaching his students, but I appreciate your input as well.

Have you ever attended any of Steve Philipson's lectures on the subject? He teaches at PAO, and has worked out a practical method of calculating on the ground what aircraft performance the pilot needs to see in order for a turnback to have a chance of being successful.

 

[bbchien]

True, but you won't see any powered CFIs teaching any students the bank angle necessary to make that turn back, and yet you will see the glider community doing it, was my point...

Well, that depends on if you look or not, Nate.....I just taught a trio of them last weekend. 45 degrees, he flies, I watch airspeed and ball 90% of the seconds in the turn....

And the reason you teach the first one from a 1000 foot floor, tur nback at 2,000, is so that you learn if the guy is going to kill you or not.

 

[RoscoeT]

True, but you won't see any powered CFIs teaching any students the bank angle necessary to make that turn back, and yet you will see the glider community doing it, was my point...

Not all pattern ops are student pilot training flights.

 

[Seanaldinho]

Serious question, how susceptible are gliders to stall/spin? Does the loooooong wingspan change the probability of an mistake ending up in a stall/spin?

It is easy to stall and spin them. BUT you know well in advance when it will stall. You can feel it in the wings and then it buffets and breaks but its as easy as putting the nose down. In a thermal you try to get as slow as possible to stay in the lift and will experience a lot of very small stalls.

Spinning is easy because they have a lot of rudder authority but its just as easy to prevent it by kicking the other direction.

In the pattern it would be hard to stall in my opinion. Because in the pattern youre supposed to be flying well above the stall (i.e. my glider stalls around 40 knots and i fly the pattern at 55 knots in calm wind) and with some speedbrakes out. So if you get slow you can put the brakes in or push the nose down.

 

[RoscoeT]

It is easy to stall and spin them. BUT you know well in advance when it will stall. You can feel it in the wings and then it buffets and breaks but its as easy as putting the nose down.

Yep, sounds a lot and airplane.

 

[Seanaldinho]

Yep, sounds a lot and airplane.

And they are extremely similar but in my opinion stalls in a powered plane are a bit more volatile and there is less warning before you feel the wing start to stall.

 

[denverpilot]

Well, that depends on if you look or not, Nate.....I just taught a trio of them last weekend. 45 degrees, he flies, I watch airspeed and ball 90% of the seconds in the turn....

And the reason you teach the first one from a 1000 foot floor, tur nback at 2,000, is so that you learn if the guy is going to kill you or not.

Good! I should have said "many CFIs" not "any". There's a few of you still willing to teach airmanship to a higher standard.

I've been exceedingly lucky or blessed to have flown with a few. Especially early on.

It's exceedingly uncommon to see anyone practicing it around here.

 

[lgtech]

I am located in the southern california area so I have seen any of Steve's lectures on the turnback maneuver. If you have something of his that I can review just send it to lgtech@roadrunner.com and I can look it over.

An interesting note on developing a minimum runway length for a "potentially successful" turnback. If you develop it for a zero wind case, that will be the most conservative one (as long as you do not depart on a tailwind). If we want something more accurate, you will then need overlay curves for various wind speeds and wind directions relative to the runway heading. However, if you look at the case of a wind and just consider the return flight, the wind blowing 45 degrees to the rundway heading is the one that requires the minimum altitude for the turnback. This is somewhat intuitive since the wind blows the aircraft toward the centerline and toward the departure end of the runway. However, you bought a round-trip E-ticket for the turnback, which you you must get out to the point where the engine fails. If you consider the round trip case, one finds that the headwind case is the best one for the minimum runway length required. For example for a C-172 in a 10 Kt headwind case, the required runway length actually decreases as you initiate the turnback further from the runway. The reason is that the climb angle with the headwind outbound is greater than the glide angle back with the tailwind. Therefore as one proceeds further and further out before the engine failure, the airplane is storing excess altitude that one would need to dissipate prior to landing. So if you want to have zero excess altitude at touchdown, you need to reduce the required runway length.
Basic aerodynamics provides a wealth of knowlege to both the CFI and the pilot.
However, todays standards for aerodynamic knowlege for certifying the CFI is really below par. There are many controversial piloting issues out there and much of it is due to the true lack of understanding of the problem. If everyone is on the same page with the correct understanding of the basics, much of the controversy disappears.

 

[Dave Krall CFII]

Can you be more specific? I've looked back through your posts in this thread, and I see where you mentioned doing calculations on the ground to determine the critical altitude, but that doesn't address the problem of how to determine whether the glide angle exceeds the climb angle.

If the glide angle exceeds the climb angle, then one can be above the critical altitude and still not be able to glide to the airport. In that circumstance, more altitude hurts instead of helping, because it eventually means that you will be too far from the airport to glide back to it.

Those specific calculations typically go like this:
If you're heavy, add 300 feet plus if it's hot, add another 300 feet, if you're feeling less than lightning fast on your reflexes, figure an additional amount to add, then add some more. Add another 10% to total for fudge factor.

Your analysis of climb and glide angles is a valid point. Our practice of the turn back maneuver has always been done at or between Vx and Vy on climb out.

Flight students are taught to fly the wing, with or without power. None of this pulling nose up because they see the ground approaching is allowed. Nose attitude to attain and maintain best glide with which they are already practiced. Customary pattern bank or a little more if they're comfortable.

One advantage of a crash right at many airports is better availability of aid services as opposed to a mile or more out.

After my above flight training, including successful turn arounds to landings, all primary students have thus far stated they would most likely choose NOT to try it and choose the best spot more or less forward.

 

[Palmpilot]

I am located in the southern california area so I have [not] seen any of Steve's lectures on the turnback maneuver...

Sorry, my eyes saw "Woodland Hills," but my brain saw "Woodside."

Steve's method eliminates the need to predict the aircraft's performance. It involves calculating what climb performance is required to make the maneuver possible, and then watching to see if that performance is achieved during takeoff.

Others in the thread have talked about determining a minimum safe turnaround height. If the airplane has reached that height by the end of the runway, then a successful turnaround may be possible, but as previously discussed, the climb angle also enters into it.

In order to know whether the climb angle is sufficient, the rate of climb needed, in feet per minute, is about 100 times the ground speed in knots divided by the glide ratio. If you don't see that climb rate during takeoff, then you know that continuing to climb straight out is eventually going to make a successful return to the airport impossible. One can consider climbing in the pattern in that case.

Ground speed can be predicted prior to takeoff by converting the planned climb speed to true airspeed and subtracting the headwind component.

Glide ratio can be calculated from data in the POH. For example, in a 1979 C-172N manual, the glide graph shows a range of 15 nm for 10,000 feet. There are about 6,076 feet in a nautical mile, so the glide ratio is 6,076 x 15/10,000, or about nine to one.

A headwind increases the effective glide ratio, because it becomes a tailwind after the turnaround. If one calculates the headwind component as a percentage of the best glide speed, the effective glide ratio will be increased by that percentage.

I'd like to emphasize that I am not taking a position on whether anyone should or should not attempt a turn back to the runway. I'm just concerned that people are oversimplifying the problem by using altitude alone as the criterion for whether the maneuver is possible, without taking the climb and descent angles into account. It appears that you are concerned about this as well.

 

[Palmpilot]

After my above flight training, including successful turn arounds to landings, all primary students have thus far stated they would most likely choose NOT to try it and choose the best spot more or less forward.

Most of the people that Steve Philipson has given turnback training to have come to a similar conclusion.

 

[lgtech]

The point of using the minimum required runway length chart is that it encompasses both the climb performance and the glide performance including the effect of the wind. When one determines the minimum runway length as a function of the distance from the departure end of the runway for a "potentially successful" turnback, this is how the aircraft should perform. One can look at the chart and say that if I want to consider making a turnback within a certain distance of the departure end of the runway, I will need a certain minimun runway length. If the aircraft is flown exactly as how the aerodynamic/wind performance is indicated, then one should be successful in the turnback, provided the pilot is aware of all the gotcha's that may come into play during the turnback. Note that the aircraft vertical speed has nothing to do with the performance chart. It is all about climb angles, glide path angles, and minimum altitude loss per degree of turn. However, here is the kicker. How does the pilot know the aircraft is operating according to the POH. If the pilot puts all his eggs in one basket saying that I have the minimum runway for a turnback, he could be in for a surprise. Therefore, If based on the minimum runway chart, the pilot is going to commit to a turnback within a certain distance from the departure end of the runway, one needs to be able to check the aircraft performance during the departure. There are a number of possible checks
(1) If starting off with a take-off over a 50 foot obstacle procedure, the actual runway used for the TOD over a 50 ft obstacle, first the initial distance for liftoff and then runway distance utilized to climb to the 50 ft elevation.
(2) The altitude above the runway at the departure end of the runway. This number is always available when using the minimum runway requirement chart.
If the aircraft has not reached this minimum elevation at the departure end of the runway, the turnback is a no-go. It may be that the engine is not performing properly or the pilot is not flying the climb profile correctly. Again, either one is a no-go.
The reason why this will work is that if the engine fails prior to the departure end of the runway, the teardrop maneuver will not be utilized and a racetrack/keyhole pattern in general cannot be used due to the considerably more runway length required than for the teardrop pattern. Thus the pilot is commited to a landing maneuver other than a turnback.
Finally, there are many "Rules of thumb" that folks come up with in connection with the turnback maneuver. A " Rule of thumb" is something that people use in getting a quick answer to a problem. The important point is that when you use a "Rule of Thumb", one must always specify the limitations under which the "Rule of Thumb" provides a reasonably accurate answer. Without that limitation, the use of a "Rule of Thumb" can be potentially fatal. The beauty of the aerodynamic analysis that goes into determining the minimum required runway length is that it tells one whether the "Rule of Thumb" being used is valid for the present conditions.
Finally, I do not want to give the impression that I am pushing pilots to use the turnback maneuver. What I am providing is a valid aerodynamic analysis which can tell the pilot when "never to attempt" a turnback maneuver, or if there exists a narrow envelope for a "potentially successful" turnback. If the conditions allow for a "potentially successful" turnback, then each pilot must make their own decision on whether they believe their skills are good enough to complete a successful turnback.
These skills can be properly honed provided they understand all the potential pitfalls in attempting the maneuver. Note that each different type of turnback maneuver can have its own set of pitfalls. Therefore, the CFI's need to fully understand the aerodynamics and pitfalls of the turnback maneuver before they start teaching pilots in the proper technique.

 

[jesse]

The altitude is debatable at how high you'd have to do it, of course.

Go up to Boulder sometime and watch the kids prep for the Commercial Glider checkride and you'll start to wonder why powered folk can't do it, say perhaps in the Commercial Powered curriculum instead of the Private.

Arguably might save more lives than lazy eights...

You'll also wonder how they're getting around the "30 degrees in the pattern" aerobatic rules, 'cause the glider kids crank it around a lot steeper than that...

FWIW the bank is 60 degrees that you're thinking of and it relates to parachute requirements.

 

[lgtech]

In a previous reply to one of your post's I gave you a formula for how to determine the dlimb/glide angle with a wind. In your post you talk about going for a vertical speed based on the ground speed and the climb angle in a wind. The formula I gave you shows that when the consider only horizontal winds (i.e. no updrafts or downdrafts), the vertical speed is the same with or without a wind. That formula is the statement that the vertical speed is the same with or without a wind. So if one knows the true airspeed during the climb and the climb angle without a wind, you can determine a target VSI for the climb out. Again, that formula would be accurate for climb/glide path angles less than 10-15 degrees. How about the accuracy of the VSI?

Again, the metric for a "potentially sucessful" turnback maneuver is a minimum altitude versus distance from the departure end of the runway. That is the only parameter that can tell you whether you have a chance of sucess. In fact, when one generates the minimun runway requirment for a "potentially sucessful" turnback maneuver, one needs to pad that distance. The prediction is based on never encountering a downdraft during the climbout to the point where the aircraft initiates the turnback. If you do encounter a downdraft even for a few seconds, if you had exactly the minimum runway requirment you would not be at the required altitude, you would be below the required minimum altitude.

Finally, even if the pilot based his decision on the required minimum atltitude at a given distance from the departure end of the runway, how many pilots could accurately judge the distance from the departure end of the runway (maybe within 1/16th of a mile) , and the altitude above the departure end of the runway(50-100 feet). So one has the pad the minimum required altitude just for uncertainty in altitude above the departure end of the runway (not the altitude below the aircraft at the point of the turnback), and the distance from the departure end of the runway.

Then there is a pad for the imperfect aerodynamics of theaircraft (i.e. aircraft down not perform exactly as in the POH),and the imperfect pilot skills.

How about all those uncertainties that you never hear about during discussions on the turnback maneuver?

Bottom line is one really needs to understand the complexity of this maneuver before attempting a turnback maneuver.

 

[bbchien]

Those of us who teach it are not simpletons les....It becomes calculus-free when you measure by vertical descent rate at best glide, and measure vertical descent rate in Vy+5 45 degree banked turn.

The 60 degree banked turn yields less time in the air- and nets about 100 feet compared to 45, but few GA pilots can execute that after a period of "no practice". Plus, then he has to go to >1.414 x Clean stall, which is another pilot competency problem.

The effect of headwind is NOT to be underestimated, nor is the effect of making a bad decision and starting the turn downwind instead of into the lateral component of wind.

PART of every departure, MEI or SE is to verbalize the wind and the emergency procedure. "Failure above X,000 feet will result in a right turn (right crosswind) at Yv+5, and emergency return....Failure below X,000 feet will result in landing in the river...." etc.

Teaching only: And woe be unto the MEI pilot who does not verbalize V1 vs V2. That gets rewarded with my hands holding the throttles closed to pre-emt departure.

 

[denverpilot]

FWIW the bank is 60 degrees that you're thinking of and it relates to parachute requirements.

Correct and preciously noted. I've definitely seen greater than 45 at Boulder. I wouldn't guess at whether they're up to 60. Hard to judge from outside on the ground, but it looks like a helluva fun ride. The large wingspan if the Schweitzer 2-32 that was doing it that day probably exaggerates what it looks like from the ground.

 

[lgtech]

Did not mean to imply you were a simpleton. The problem is most CFI's are not aerodynamicists. I happen to be one of those that wear two hats. Both a professional aerodynamicist and a CFI.

In regard to your statement about the best glide speed and the 60 degree bank. In a wings-level glide the key parameter is to fly the aircraft is at the angle-of-attack for max L/D, i.e. best glide speed. In the gliding portion of the turnback one is trying to miminize the altitude loss per degree of turn. In order to do this you do not fly at the angle-of-attack for max L/D, the aircraft is flown at the maximum value of the lift coefficient times the L/D ratio. This occurs at the max lift coefficient, i.e., just before the aircraft stalls.

In regard to the bank angle, I was not clear if you were stating that the 60 degree bank angle gives a reduced altitude loss compared to the 45. If one flies at the same angle-of-attack at 45 degrees and 60 degrees, it is easy to show the 45 degree bank angle will always give lower altitude loss per degree of turn. On can show that the minimum altitude loss per degree of turn will occur somewhere between 45 and 46 degrees ( with a slight dependence on the L/D ratio).
Finally is your training your pilots to take into account the wind, you may be very much surprised to find that when you initiate your turnback the wind just died. A conservative approach should always be taken by at least using a no-wind minimum altitude versus distance from the departure end of the runway.