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Bank Limits  
User currently offlineIFACN From Italy, joined Nov 2005, 153 posts, RR: 0
Posted (6 years 7 months 1 week 3 days 12 hours ago) and read 3606 times:

Hi everyone!

I was wondering about some questions I can't figure out by myself. Older threads gave me some more details, but I still haven't found what I'm looking for...  Wink

I'm flying my C172, equipped with a 150hp engine (reading the POH it's rated for +4g flaps up) and doing turns at 30, 45 and 60 degrees of bank angle; a simple computation shows that the load factor is 1.2g, 1.4g, 2.0g; greater load factors needs more AOA to increas lift so I apply a greater back pressure as steeper is the turn; over 45 I need to add power, at 60 the throttle is full in.
At this point, if I try to turn at 75 degrees the load factor is almost 4 but I don't have any power to keep altitude.

Q.1) Am I correct in assuming that a 60-deg bank turn is the steeper I can do?

Q.2) In such scenario, I can't figure out what happens if I exceed 60. Do I enter a spiral dive (nose down+full power+back pressure)?

Q.3) Apply the same scenario to a liner; Airbuses are limited to 67deg. bank. This means about +3g load factor. Is this a structural limit or is a limit imposed by control software specifications (please allow me to ask something stupid: maybe exceeding 67 degrees exceeds attitude sensors/flight control actuators specs?); this kind of limit is found also on other liners (a B737 for example?).

Q.4) Am I correct in assuming that an airplane (speed in the green arc) stalls before reaching max load factors (for example, I put it into a dive, reach the green arc limit and then apply back pressure as hard as I can).

Of course I prefer your theoretical replies and discussions before grabbing a C172 at our airfield and find my answers by practicing...

Kind regards,
A.

8 replies: All unread, jump to last
 
User currently offlineTbanger From Australia, joined Jul 2004, 266 posts, RR: 0
Reply 1, posted (6 years 7 months 1 week 3 days 11 hours ago) and read 3583 times:

This has been a while for me but I think I can shed some light.

Firstly stall has nothing to do with speed. It is a direct result of the angle of attack on an aerofoil exceeding the amount to generate left....hence the aerofoil stalls.(airfoil for most on this forum). It becomes speed related because in level flight when your speed becomes less than what is required to keep the angle of attack less than the given stall angle of attack it stalls. Speed is also the only real indicator available to the pilot in a small unsofisticated aircraft to see when a stall in iminent. On larger aircraft there is usually an AoA indicator. Also note that "Attitude has nothing to do with Angle of Attack".

Quoting IFACN (Thread starter):
Q.1) Am I correct in assuming that a 60-deg bank turn is the steeper I can do?

It's probably about the steepest "sustained" turn you can make without descending. At speed you can probably go to 75 deg as you mentioned but even at full power the speed will bleed off and your nose will tend to drop as you slow.

Quoting IFACN (Thread starter):
Q.2) In such scenario, I can't figure out what happens if I exceed 60. Do I enter a spiral dive (nose down+full power+back pressure)?

Becasue a C172 is a fairly "balanced" aircraft as you slow down or exceed 60 degrees you will need to use opposite rudder to keep your nose at a level that will sustain a given altitude. If you don't use opposite rudder, your nose will tend to drop hence putting you into a "spiral dive". This is where students get into trouble as a spiral dive will lead to a spin if not dealt with correctly. (Release back pressure, level the wings, gently pull out of the dive).

Quoting IFACN (Thread starter):
Q.3) Apply the same scenario to a liner; Airbuses are limited to 67deg. bank. This means about +3g load factor. Is this a structural limit or is a limit imposed by control software specifications (please allow me to ask something stupid: maybe exceeding 67 degrees exceeds attitude sensors/flight control actuators specs?); this kind of limit is found also on other liners (a B737 for example?).

I don't think this is a stupid question. YES these aircraft use attitude censors linked to flight control computers which "prevent" the pilot from exceeding these limits. I am pretty sure it isn't g force related as the computers would simply "unload" the wing. As I mentioned above attitude is different to AoA.

Quoting IFACN (Thread starter):
Q.4) Am I correct in assuming that an airplane (speed in the green arc) stalls before reaching max load factors (for example, I put it into a dive, reach the green arc limit and then apply back pressure as hard as I can).

Not 100% sure that the aircraft will stall "prior" to meeting load limits. For example: an aircraft that has a VmO of 120kts say and a max load of 4g gets put into a 30 degree nose dive. The speed is increased to 120knts and you pull back as hard as you can on the elevator. If the g limit exceeds 4g you will over stress the aircraft but you may not necessarily stall the aircraft. On the other hand, you may only pull 3g and exceed the stall AoA at a speed of say 90kts hence the aircraft continues to descend.

Quoting IFACN (Thread starter):
Of course I prefer your theoretical replies and discussions before grabbing a C172 at our airfield and find my answers by practicing...

I did many hours in a C152 Aerobat with an RAAF pilot who had spent 2 years with the RAAF Roulettes needless to say he was flying a desk for the RAN when I flew with him, but that guy really knew how to put theory into practical situations.

Off topic here but when I was doing unusual attitude recoveries, he would get me to close my eyes while he spent the next 5 minutes doing full on aerobatics so that I became extremely dissoriented prior to saying "open eyes, handing over". I must say it was the best fun leading up to my PPL...but all that is now just a distant memory. (...something to do with a wife that thinks mortgages and kids are more important than spending money on flying).  duck   flamed 


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 2, posted (6 years 7 months 1 week 2 days 19 hours ago) and read 3482 times:



Quoting IFACN (Thread starter):
Q.1) Am I correct in assuming that a 60-deg bank turn is the steeper I can do?

Legally, anything above 60 degrees will be considered aerobatics. Unless you are operating the aircraft in the utility category (check the POH) and have the appropriate training, you cannot legally perform steeper turns (as always, you regulations may vary).

There is a bank angle where the induced drag resulting from generating the lift required to achieve the load factor associated with the bank angle in level flight will mean the power required for maintaining your airspeed equals the power available. At this point, you cannot bank any steeper without either slowing down or descending.

There is also a bank angle at which you will exceed the load factor limit of the aircraft. For a C172P, this limit is 3.8G in the normal category and 4.4G in the utility category.

Quoting IFACN (Thread starter):
Q.2) In such scenario, I can't figure out what happens if I exceed 60. Do I enter a spiral dive (nose down+full power+back pressure)?

You enter a descending steep turn. In my mind, when you talk about a spiral dive you are talking about a flight condition which is more out of control, although strictly speaking they are both the same. Airspeed rapidly approaching Vne and so on, which will not be the case if you are simply pulling hard at 75 degrees of bank and lowering the nose in order not to lose airspeed.

Quoting IFACN (Thread starter):
Q.4) Am I correct in assuming that an airplane (speed in the green arc) stalls before reaching max load factors (for example, I put it into a dive, reach the green arc limit and then apply back pressure as hard as I can).

No. Below Va, the aircraft will stall before reaching the load factor limit. Above Va, it will not.



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineIFACN From Italy, joined Nov 2005, 153 posts, RR: 0
Reply 3, posted (6 years 7 months 1 week 2 days 16 hours ago) and read 3421 times:

Many thanks for your replies, now I got it clearer (expecially Q.4).

I didn't know that exceeding 60deg banks goes into aerobatics. At flight school my instructor never told me it, anyway the steepest turn we did with the 172M was 60.
Just after steep turns he demonstrated and trained me about spiral dives, (the recovery procedure I got was Power-Push-Roll - first put engine to idle, because increasing airspeed when diving will accelerate the propeller and engine near or over max RPM, then release back pressure, level.
I agree that spiral dives are a big trouble for students, because the students must be quick in detecting the situation (increasing IAS, increasing ROD and pulling the yoke means worsening the situation).

Regards,
A.


User currently offlineTbanger From Australia, joined Jul 2004, 266 posts, RR: 0
Reply 4, posted (6 years 7 months 1 week 2 days 2 hours ago) and read 3339 times:



Quoting IFACN (Reply 3):
(the recovery procedure I got was Power-Push-Roll - first put engine to idle, because increasing airspeed when diving will accelerate the propeller and engine near or over max RPM, then release back pressure, level.

Yeah, I'd go with your instructor not me for recovery procedure. Like I said it's been a while.


User currently offlineBarney Captain From United States of America, joined Nov 2001, 965 posts, RR: 13
Reply 5, posted (6 years 7 months 1 week 1 day 23 hours ago) and read 3321 times:



Quoting Tbanger (Reply 1):
I don't think this is a stupid question. YES these aircraft use attitude censors linked to flight control computers which "prevent" the pilot from exceeding these limits. I am pretty sure it isn't g force related as the computers would simply "unload" the wing. As I mentioned above attitude is different to AoA.

No such monster on the 737. The only thing that even comes close is on the NG's beyond 35 degrees of bank , you will get an aural "Bank Angle" alert.



...from the Banana Republic....
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2556 posts, RR: 24
Reply 6, posted (6 years 7 months 1 week 1 day 20 hours ago) and read 3289 times:



Quoting Barney Captain (Reply 5):
No such monster on the 737. The only thing that even comes close is on the NG's beyond 35 degrees of bank , you will get an aural "Bank Angle" alert.

Not just NGs. Apart from Airbus and MD which have their own system of aural alerts, any aircraft will have this BANK ANGLE alert if it has a recent GPWS/EGPWS, so it can be heard on a 737 Classic, for example. The alert bank angle is a function of radio altitude, at very low heights the alert can be triggered by just 11 degrees bank.

It is only an aural alert, not a bank angle limiter, of course.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineIFACN From Italy, joined Nov 2005, 153 posts, RR: 0
Reply 7, posted (6 years 7 months 1 week 1 day 18 hours ago) and read 3267 times:

Thanks again for the clarifications.

Speaking with my instructor, we decided to review these issues in a briefing and then do a bit of practice in an aerobatic plane (being limited to 2500ft QNH by Milan TMA, or 3000ft QNH in the aerobatic working area, we don't have enough altitude to practice safely in a C172).

Quoting Jetlagged (Reply 6):
any aircraft will have this BANK ANGLE alert if it has a recent GPWS/EGPWS, so it can be heard on a 737 Classic, for example. The alert bank angle is a function of radio altitude, at very low heights the alert can be triggered by just 11 degrees bank.

As "very low heights" you mean something like minima or DH or some other kind of altitude threshold (say, 5000ft)?

Regards,
A.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2556 posts, RR: 24
Reply 8, posted (6 years 7 months 1 week 1 day 14 hours ago) and read 3216 times:



Quoting IFACN (Reply 7):
As "very low heights" you mean something like minima or DH or some other kind of altitude threshold (say, 5000ft)?

I can't remember the shape of the rad alt function exactly. The bank limit is about 40 degrees most of the time but it ramps down quickly to around 11 degrees when rad alt is very low (if pressed I would guess 100 ft or so). I remember it was low enough to make it difficult to verify in the simulator I was working on.

The limit is part of EPGWS/GPWS Mode 6, the same mode which provides altitude callouts. It can be disabled by pin programming so not all airline configurations will have the BANK ANGLE callout.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
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