Low altitude steep descending turns...

[dtuuri]

I bought a Cherokee 180 the other day. Yesterday my cfi and I were going thru the logs, afm, and owners manual. We found a page that listed exact stall speed at different bank angles. Of course these figures are based on full load and holding altitude. I believe I found out in this thread that the stall speed will be lower if descending while banking.

Whatdidyour CFI say? Ify ou believeit , how doyo upla ne to makeuse o f it?d
tu
uri

 

[dtuuri]

This is why when descend we have an increased stall margin, all else being equal.
Comments welcome.

You betcha. Youretalkingtoa' studentp ilotth ere. While what yousay istru e, it is trivia. Heresapostinanotherforu' m by apresumedly sharp engineer:

For a 45 degree bank, and a descent angle of say, 10 degrees, the load factor is
=cos(10)/cos(45)
=.985/.707
= 1.393
A level turn load factor for a 45 degree turn is
=cos(0)/cos(45)
=1/.707
=1.414
So the difference is 1.414 - 1.393 = .021. Not much.

Thus an aircraft with a wings-level stall speed of 50 knots will have a stall speed in a level steep turn of
new stall speed = sqrt(1.414) * 50
= 59 knots.
In a descending turn, the stall speed will be
new stall speed = sqrt(1.393)*50
= 59 knots
In other words, the small difference is lost during rounding.​

d
tuuri

 

[RotorDude]

You betcha. Youretalkingtoa' studentp ilotth ere. While what yousay istru e, it is trivia. Heresapostinanotherforu' m by apresumedly sharp engineer:

For a 45 degree bank, and a descent angle of say, 10 degrees, the load factor is
=cos(10)/cos(45)
=.985/.707
= 1.393
A level turn load factor for a 45 degree turn is
=cos(0)/cos(45)
=1/.707
=1.414
So the difference is 1.414 - 1.393 = .021. Not much.

Thus an aircraft with a wings-level stall speed of 50 knots will have a stall speed in a level steep turn of
new stall speed = sqrt(1.414) * 50
= 59 knots.
In a descending turn, the stall speed will be
new stall speed = sqrt(1.393)*50
= 59 knots
In other words, the small difference is lost during rounding.​

d
tuuri

As per my above post, a descending turn would have a lower stall speed. The new stall speed would always be lower (assuming all else the same), and could be a lot lower, depending on the angle of descent. So the critical parameter is the angle of descent (relative to the airmass), and you left that unspecified in your quoted message

Edit: I now see you did specify the descent angle as 10 degrees, and I agree that for that angle the reduction in stall speed would be tiny (the load factor goes with the cosine of the angle).

 

[Witmo]

So let's depict a possible student pilot scenario utilizing the many words of wisdom gleaned from this thread. Base to final turn, a bit high and overshooting final. No problem--just drop the nose more since you're high and steepen the bank; still overshooting so you need to pull harder but no problem since you're in a descent and your AOA must be lower you can pull harder since you're so much faster and you won't stall since your stall speed must be lower than "normal" . What were those trig formulas? Still not quite lined up so maybe a little more rudder, so what if the ball is a little displaced. Ground is really rushing up at me now so I need to pull harder and arrest the descent rate. Why am I upside down and dead? Total elapsed time give or take 15 seconds.

This thread has gone far beyond any helpful advice to a student pilot unless his airplane is equipped with an AOA indicator and his instructor takes the time to show its relationship to various phases of flight. We use airspeed instead of AOA because that's all we usually have, so saying airspeed is irrelevant is not only ridiculous it's dangerous. A student pilot should be concentrating on staying ahead of his aircraft, keeping his turns coordinated (</= 30 degrees) and not trying to salvage bad patterns by max performing the aircraft.

I'll leave you now to discuss how many angels/devils can dance on the head of a pin if the pin is attached to a student pilot's hat on his first solo in a descent vs climb.

 

[dtuuri]

Edit: I now see you did specify the descent angle as 10 degrees, and I agree that for that angle the reduction in stall speed would be tiny (the load factor goes with the cosine of the angle).

For the OPs' sake, that's more t han three t imes an IlS glide slope or VASI.

dtuuri

 

[RotorDude]

For the OPs' sake, that's more t han three t imes an IlS glide slope or VASI.

dtuuri

Sure, good point. In real life (assuming you don't fly F-15s for a living), unless you are flying very steep descents/climbs, you can just assume the same stall speed (in unaccelerated flight), and you'd be erring on the safe side.

 

[dtuuri]

Sure, good point. In real life (assuming you don't fly F-15s for a living), unless you are flying very steep descents/climbs, you can just assume the same stall speed (in unaccelerated flight), and you'd be erring on the safe side.

I doubt an F-15 pilot makes a steep enough approach to take advantage of this trivial factoid either.

dtuuri

 

[2Airtime2]

I like it, good read RotorDude.

 

[RotorDude]

I doubt an F-15 pilot makes a steep enough approach to take advantage of this trivial factoid either.

dtuuri

True, but the neat thing is that the same stall speed reduction also happens for a climb (the cosine function is symmetrical about zero). Which means that in a steep climb (at a steady speed) the stall speed is reduced, potentially all the way to zero when aiming straight up, as I noted above.

 

[tuwood]

I just googled a stall speed chart for Cessna and found this one:

You'll notice that once you get past 30 degree bank you have to be flying substantially faster to avoid stalling. I can't imagine ever trying to do a 60 degree bank in the pattern without dying.
These numbers also assume you're coordinated, so if you get out of coordination even a little bit you'll have one wing stall faster than the other. Then we'll all be reading about your base to final spin on an NTSB report.

I'm still pretty new to flying, but everything I've ever read or been told has been to never exceed 30 degrees in the pattern. When you look at the above chart you can kind of see why that's a good limit. Obviously more experienced pilots can be more aggressive, but just last month there was what appears to be a base to final spin by a very experienced pilot in a Cirrus in Ohio.

 

[RotorDude]

I just googled a stall speed chart for Cessna and found this one:

You'll notice that once you get past 30 degree bank you have to be flying substantially faster to avoid stalling. I can't imagine ever trying to do a 60 degree bank in the pattern without dying.
These numbers also assume you're coordinated, so if you get out of coordination even a little bit you'll have one wing stall faster than the other. Then we'll all be reading about your base to final spin on an NTSB report.

I'm still pretty new to flying, but everything I've ever read or been told has been to never exceed 30 degrees in the pattern. When you look at the above chart you can kind of see why that's a good limit. Obviously more experienced pilots can be more aggressive, but just last month there was what appears to be a base to final spin by a very experienced pilot in a Cirrus in Ohio.

Your graph assumes level flight, and for that the load factor is increased (assuming coordinated turns) as depicted.
And I fully agree that for student or new pilots (or even tired/distracted veteran pilots) keeping the bank angles to no more than around 30 degrees makes sense. But I assume some military (and military wannabe) pilots will tell you that in the "break" they routinely fly 60 degrees (2g) banks, and somehow still manage to live another day.

 

[tuwood]

Your graph assumes level flight, and for that the load factor is increased (assuming coordinated turns) as depicted.
And I fully agree that for student or new pilots (or even tired/distracted veteran pilots) keeping the bank angles to no more than around 30 degrees makes sense. But I assume some military (and military wannabe) pilots will tell you that in the "break" they routinely fly 60 degrees banks, and somehow still manage to live another day.

yeah, my context is 100% student/new pilot. I'll likely barrel roll out of the sky with more than a 10 degree turn.

I did read about not doing abrupt turns in the pattern as being a good practice. Then, even if you do get over a little far over there's plenty of warning to get the nose down or level out.

 

[Everskyward]

As long as the student doesn't get so hung up on the 30 degree bank limitation and either decides to, or unconsciously, cheats with bottom rudder.

 

[exncsurfer]

I'd like to take a stab at this starting from the top:

I'm training in a c172 and am in the process of buying a Cherokee 180.

When turning in the pattern, usually to final, sometimes the wind is such that I want/need to turn steeper. When around 75 kts how close are you getting to a stall condition at 30 degrees, at 45 degrees, at 60 degrees?

The cross wind doesn't have to require a steeper turn, for example with a tail wind on base, you'd just need to start your turn to final early/sooner and turn till you're crabbing into the wind towards the runway. If you overshoot a little just hold the coordinated turn till you are back on center line, or if it was way off, go around.

Am I correct that the slower you are going and the steeper you turn the higher the stall speed (plane stalls more easily) ?

The slowness doesn't change the stall speed, it just gets you closer to it. The bank angle doesn't tell you stall speed either, you could be in a descending spiral at a high bank angle and low AOA. Or you could be mushing at high AOA descending rapidly and on the verge of a stall.

Someone tell the difference in stall speed in the above scenarios when descending vs. holding altitude. I think if you are turning steep and descending the stall speed is lower (safer) than if you were holding altitude. Is that correct?

Which has a greater angle of attack? Steep altitude holding turns or steep descending turns?

It depends on how much you're pulling and how fast you're going and AOA(relative wind) regardless of whether you're holding altitude or not, I give up, Concentrate on nailing your speeds and staying coordinated

disclaimer, i am a student pilot, i don't know squat about squat

 

[RotorDude]

As long as the student doesn't get so hung up on the 30 degree bank limitation and either decides to, or unconsciously, cheats with bottom rudder.

Absolutely!
The one takeaway for a student is that coordinated turns may be a matter of passenger comfort up high, but near the ground they could save your life. What typically kills down low is a spin, and you can't spin with a centered ball.

 

[Witmo]

Your graph assumes level flight, and for that the load factor is increased (assuming coordinated turns) as depicted.
And I fully agree that for student or new pilots (or even tired/distracted veteran pilots) keeping the bank angles to no more than around 30 degrees makes sense. But I assume some military (and military wannabe) pilots will tell you that in the "break" they routinely fly 60 degrees (2g) banks, and somehow still manage to live another day.

The "break" is a level 2 g turn to bleed off enough airspeed to lower the gear. Military pilots don't pull g' s in the final turn where they're fully configured and descending to final. Add to this the fact that they have an AoA indexer which references where they should be as far as their weight.warrants concerning speed to fly plus they've probably calculated a back-up final approach speed based on their weight which should agree with the speed they see when the indexer is telling them they have the AoA nailed.

 

[dtuuri]

True, but the neat thing is that the same stall speed reduction also happens for a climb (the cosine function is symmetrical about zero). Which means that in a steep climb (at a steady speed) the stall speed is reduced, potentially all the way to zero when aiming straight up, as I noted above.

Yep. That time I tried a hammerhead at 8000' I was going straight up with a zero stall speed. It stalled anyway. Now I know why they call 'em "whip stalls".

dtuuri

 

[RotorDude]

The hammerhead "stall", which would happen if you are late on the rudder, is not a real stall. It's simply running out of inertia and falling backwards. I have only done hammerheads in dual, and my instructor (who was a champion aerobatics pilot) always had me punch the rudder well before the top of the trajectory. But of course, if you have an airplane going straight up vertically, its stall speed is always zero, but once there is zero airspeed and no inertia, you'll start sliding back on your tail, and that's considered "bad form".

 

[dtuuri]

As long as the student doesn't get so hung up on the 30 degree bank limitation and either decides to, or unconsciously, cheats with bottom rudder.

I haven't read all the posts, so I don't know if anybody said 30° was a limit. I didn't. I've been on the record in past threads here and elsewhere advocating for steeper banks rather than skids and I've said many times "unloading the wing" avoids the stall at the expense of tightening the turn. However, it takes some skill and experience to plan the pattern, making timely small adjustments, so as not to exceed 30°.

dtuuri

 

[dtuuri]

The hammerhead "stall", which would happen if you are late on the rudder, is not a real stall. It's simply running out of inertia and falling backwards. I have only done hammerheads in dual, and my instructor (who was a champion aerobatics pilot) always had me punch the rudder well before the top of the trajectory. But of course, if you have an airplane going straight up vertically, its stall speed is always zero, but once there is zero airspeed and no inertia, you'll start sliding back on your tail, and that's considered "bad form".

Heh, it was bad form alright. I learned a good lesson too, though it took me a few beers afterwards to puzzle through what had happened. FWIW, here, for free, is that lesson: You become used to kicking the rudder at a certain kinesthetic feel in the seat of your pants, so the plane will make a nice pinwheel turn. But that is based on true airspeed as you decelerate vertically. The feel is the same no matter the altitude, but the air density is not. I was real surprised when I kicked the rudder at the same "feeling" and nothing happened. For one long second I knew I was in real trouble, then wham, whip stall! I bet your CFI BTDT.

dtuuri

 

[Everskyward]

I haven't read all the posts, so I don't know if anybody said 30° was a limit. I didn't. I've been on the record in past threads here and elsewhere advocating for steeper banks rather than skids and I've said many times "unloading the wing" avoids the stall at the expense of tightening the turn. However, it takes some skill and experience to plan the pattern, making timely small adjustments, so as not to exceed 30°.

dtuuri

You didn't use the word "limit", but...

 

[RotorDude]

Heh, it was bad form alright. I learned a good lesson too, though it took me a few beers afterwards to puzzle through what had happened. FWIW, here, for free, is that lesson: You become used to kicking the rudder at a certain kinesthetic feel in the seat of your pants, so the plane will make a nice pinwheel turn. But that is based on true airspeed as you decelerate vertically. The feel is the same no matter the altitude, but the air density is not. I was real surprised when I kicked the rudder at the same "feeling" and nothing happened. For one long second I knew I was in real trouble, then wham, whip stall! I bet your CFI BTDT.

dtuuri

Could be. OTOH, some years later, with tons of experience over many years (airshows, competitions, you name it), he (or his partner) messed up doing this same maneuver, and paid the ultimate price.

 

[dtuuri]

You didn't use the word "limit", but...

View attachment 44139

No "buts" ma'am. That quote only proves what I was saying. Of course, I was thinking about pro pilots who fly passengers, not dust crops, fight wars or do airshows for a living, but I'm sure those pros could line up on final without exceeding 30° too.

dtuuri

 

[Everskyward]

No "buts" ma'am. That quote only proves what I was saying. Of course, I was thinking about pro pilots who fly passengers, not dust crops, fight wars or do airshows for a living, but I'm sure those pros could line up on final without exceeding 30° too.

dtuuri

You were clear that you were talking about safety, not comfort.

 

[dtuuri]

Could be. OTOH, some years later, with tons of experience over many years (airshows, competitions, you name it), he (or his partner) messed up doing this same maneuver, and paid the ultimate price.

I'm sorry to hear that. Aerobatics will get you sooner or later if you do them real low. I think crop dusting will too. I had an aerobatic instructor back in the '60s who did air shows. One day he showed me pictures of all the plane wrecks he survived dusting crops--about seven of 'em. Then I went over to a Pawnee to ask the pilot how I could learn to do it. He was climbing out of the cockpit on the other side of the plane. I met him at the tail and saw how disfigured his body was, burn scars and limping. "What can I do for you?" he said. "Uh, nothing. I just wondered if I could look at your plane." Changed my career orientation right then and there.

dtuuri

 

[dtuuri]

You were clear that you were talking about safety, not comfort.

Our good friend Herve sarcastically accused me of saying something I did not really say, "Only banking 30° or less" as a strawman safety argument he could knock over. So, I reminded him that some pilots (pros) that do that are demonstrably safer.

What's going on with the quotes? They aren't showing up. Also, how are you supposed to get back to the original post that was quoted without searching for it?

dtuuri

 

[Everskyward]

Our good friend Herve sarcastically accused me of saying something I did not really say, "Only banking 30° or less" as a strawman safety argument he could knock over. So, I reminded him that some pilots (pros) that do that are demonstrably safer.

dtuuri

Interesting to watch you walk your statements back. I don't care what you believe yourself, only that some students might misinterpret some of the things you post.

Also interesting that you saw yourself in Herve's post since he didn't mention you in it.

 

[dtuuri]

Interesting to watch you walk your statements back. I don't care what you believe yourself, only that some students might misinterpret some of the things you post.

I think you constantly misinterpret my posts, on purpose. When I explain further you accuse me of "walking back". I find it disturbing that the OP, in my opinion, was looking for validation to do something without aerodynamic merit and a bunch of people here, you among them, gave that validation. You could have got the OP killed someday. At last I heard, s/he didn't believe me, probably because of your attempts to marginalize my arguments.

dtuuri

 

[Everskyward]

I think you constantly misinterpret my posts, on purpose. When I explain further you accuse me of "walking back". I find it disturbing that the OP, in my opinion, was looking for validation to do something without aerodynamic merit and a bunch of people here, you among them, gave that validation. You could have got the OP killed someday. At last I heard, s/he didn't believe me, probably because of your attempts to marginalize my arguments.

dtuuri

Your arguments shift with the wind. What was uniform motion again? And how did that relate to flying the pattern?

 

[dtuuri]

Your arguments shift with the wind. What was uniform motion again? And how did that relate to flying the pattern?

First off, I said "uniform flight", not motion. But that's ok, because I meant uniform motion of all flying types. I knew you were trying to catch me, you weren't interested in helping the OP understand. So, I found a little cute video to explain uniform motion. You weren't satisfied. So, I played along. From about 50 years ago I wrote what I could remember about uniform motion as though you were a student and I was the CFI and I was trying to answer the question while standing on a tarmac somewhere. Pretty good, too, I've since found out from my physics book. But, I got pilloried for it. "Uniform motion is straight line velocity!" You all chimed. So, I found another presentation that had uniform circular motion explained with a model airplane. "You don't have the context right!" go the critics. BS. I did fine. Wouldn't change a thing. Well, I won't take your bait again in the spirit of fair play, so maybe that's not true.

dtuuri

 

[2Airtime2]

Just to get everyone on the same page. My original post was basically a made up scenario to ask the question I wanted answered. Sure, I've been blown slightly on base to final but have always been able to turn enough to get back lined up.

I understand the proper procedures to stay on track in the pattern and I understand it's safer to never get in a situation (especially at low altitude) that requires evasive maneuvers. My question stemmed from a possible future "what if" scenario where (hypothetically) a skydiver or rotorcraft (both very prevalent at my field) suddenly got in my path and I need to make evasive maneuvers. If I had known you guys were going to pick my question apart vs just answering what was asked I guess I could have phrased it differently.

So, enough of the safety lectures (that's for my cfi).

-Straight and level flight has a specific stall speed.
-a 30 degree bank turn while holding altitude has a slightly higher stall speed
-a 45 degree bank turn wha has an even higher stall speed
-for a 60 degree bank turn wha, the stall speed is yet higher

-a descending banking turn is safer than a altitude holding turn (of the same bank angle) in that the stall speed is lower

Got it

 

[RotorDude]

Just to get everyone on the same page. My original post was basically a made up scenario to ask the question I wanted answered. Sure, I've been blown slightly on base to final but have always been able to turn enough to get back lined up.

I understand the proper procedures to stay on track in the pattern and I understand it's safer to never get in a situation (especially at low altitude) that requires evasive maneuvers. My question stemmed from a possible future "what if" scenario where (hypothetically) a skydiver or rotorcraft (both very prevalent at my field) suddenly got in my path and I need to make evasive maneuvers. If I had known you guys were going to pick my question apart vs just answering what was asked I guess I could have phrased it differently.

So, enough of the safety lectures (that's for my cfi).

-Straight and level flight has a specific stall speed.
-a 30 degree bank turn while holding altitude has a slightly higher stall speed
-a 45 degree bank turn wha has an even higher stall speed
-for a 60 degree bank turn wha, the stall speed is yet higher

-a descending banking turn is safer than a altitude holding turn (of the same bank angle) in that the stall speed is lower

Got it

I would add just one point to your summary: coordinated turns (centered ball) are critical when you are low and slow.
Slips (extra top rudder) are OK when you need them, skids (extra low rudder) are never OK, and dangerous.

 

[MAKG1]

This is about evasive action? You left the impression it was for routine turns in the pattern.

The answer is different. Low power is not your best option. Study and practice the canyon turn. Not a basic maneuver, but you can get turned around in a few seconds that way.

If a skydiver jumps in front of you, you're not going to have time for a leisurely turn. And clipping an open canopy is very likely to be fatal.

 

[Everskyward]

-a descending banking turn is safer than a altitude holding turn (of the same bank angle) in that the stall speed is lower

This is not necessarily true. Are you starting or increasing your descent as you begin the turn? The pattern is a dynamic place, especially if you are taking about making evasive maneuvers, so it's hard to talk about mathematical models unless you are referring to one instant in time. If you let the nose drop or push it over, lowering the angle of attack during the turn, that would put you further from a stall than if you pull back to hold altitude, given the same angle of bank. Is that what you were asking?

 

[dtuuri]

-a descending banking turn is safer than a altitude holding turn (of the same bank angle) in that the stall speed is lower

Got it

Do you? If you looked at the earlier post I made where I quoted from another forum, even if you triple your angle of descent the difference in stalling speed is less than a single MPH. Of course that assumes you are holding your rate of descent constant the same as you were holding your altitude constant. If you're trying to avoid a stall, all you need remember is to not pull the stick back. If you're trying to avoid a jumper, you need to pull it (or push it) and not far enough to stall. Stall speed is meaningless in your new scenario because you can do an accelerated stall at any speed.

dtuuri