Cessna 172R stall speed wrong?

[Nick - A2A]

Hi primeGT - thanks for the post.

Yes, I think we need to make sure we're all on the same page, so to speak. Certainly the tests I've done have been specifically related to VS0, so MTOW and full flaps. Basically, I've been trying to maintain level flight with power at the slowest speed possible. Not sure what (if anything) the results show, but it's good to have an excuse to practice handling at these low airspeeds.

Cheers,
Nick

[taildraggin68]

The 172 and big brother 182 are two different animals in handling....although they both share an improved handling with weight, that weight in the 182 offsets the heaviness of the nose and smooths out the handling...on the 172, with it's nose to the sky, I wanna fly disposition, can get a bit unnerving loaded to mtow with the CoG back....she will handle smooth, but settle into a descent quickly at low speeds. Trimming to maintain level flight as you slow produced an interesting AoA with one stall break noted a wing drop and a tail drop at the same time.....recoverable with no spin and minimal altitude loss but now your in a position of low airspeed, trimmed nose high, with a rear CoG and a large opportunity for error. I personally would not let the plane get below 55 knots indicated at any point other than touch down just for a safety margin, but it is fun to practice slow flight and it never hurts to know how long you can go once the stall horn starts blaring

[Oracle427]

PrimeGT,

What I can tell you from flying a couple of 172S and a 172N, is that you can slow fly very close to the bottom of the white arc in the published landing config.

When flying smoothly in a steady descent at idle power in a landing config, the aircraft will stall very close to the bottom of the white arc when the yoke is pulled gently back until the aircraft stalls.

Both of these flights were at around 2400lbs.

Low to the ground, I'm not staring very closely a the ASI, but if I'm in hilly terrain with anemic climb performance at altitude, you bet I'm going to increase my scan of the ASI when making any config changes to make sure I am stable and not getting too close to the stall. I haven't found the ASI to be inaccurate at all. The ASI is reading what the airplane "feels", and I have found that 172's will stall pretty reliably at the published V speed for a given configuration when flown smoothly. The differences are usually only a couple of knots due to weight differences and rigging.

You do these types of manuevers so much in PPL training that you need to be very comfortable flying at the edge of the stall down to the bottom of the white arc.

Nick,

When a wing just begins to drop, you need to pick it up with the opposite rudder. It may take a little punch of the rudder to do it, but the wing will stay up. Think of balancing a broomstick on the palm of your hand. Your feet need to dance around like that hand to keep the aircraft straight and level using the rudder. You will be able to keep it up in the air for a little longer once you get the hang of that. Also note that slow flight as I was taught is demonstrating maintainance of aircraft heading and altitude and making 180 turns at a stable airspeed with the stall horn going the entire time. Once the manuever is complete, you need to leave slow flight without losing or gaining altitude while cleaning up the aircraft and resume cruise.

All these basic manuevers are what I like to practice especially so I can familiarize myself with unfamiliar aircraft.

[Nick - A2A]

Thanks Oracle, yeah - I can actually disable the aileron axis in FSX and do a pretty good job of keeping the wings level in slow flight with just the rudder using that technique. (The problem with a 'twisty' joystick is that it's rather hard to avoid inadvertent aileron input while twisting the stick.)

Anyway, even with 'auto-rudder' enabled (which still seems to be the easiest way to mitigate most of the wing drop with my hardware) I just can't maintain level flight at the bottom of that white arc. The aircraft settles into a descent beforehand as Taildraggin' put it, and I just don't have enough power to stop it, even with the throttle wide open. Maybe I should try this with the 'S' prop instead - I think that's what you mentioned you were using?

Note: I seem to recall that from one of Great Ozzie's earlier remarks, VS0 strictly references the stall speed at idle throttle. My understanding (correct me if I'm wrong) is that, if anything, level flight at an even slower airspeed than VSO should be possible with power? (Presumably due to the prop wash providing added lift as primeGT mentioned.) Something like this perhaps...

[youtube]http://www.youtube.com/watch?v=yPG05Dz8P6c[/youtube]

Guess it's a C172S in the video and presume the white arc ends at 40 kias on those C172S models you fly Oracle?

Cheers,
Nick

P.S. Of course, the other way to make level flight possible at VS0 is just to move the realism sliders all the way to the left...

[Caldemeyn]

There really is a difference between 172S and R's airspeed indicators, i must say that i didn't see until now.

[Oracle427]

Yes, Vso is with idle power. In a nose high attitude with power, the lift is pointing somwhat rearward and adding to drag as induced drag and thrust is pointing upward adding somewhat to lift. Weight still acts straight down because gravity hasn't changed its rule.

You can get the needle below the white arc that ends are 40KIAS in those aircraft with a lot of power and the nose pitched up quite high.

I've never flown a 172 with the cruise prop modeled here, so I can't comment on the performance.

I understand that a twisty stick will pose a great challenge to avoid introducing aileron input. It's the reason I don't use one and invested in rudder pedals.

I wasn't able to maintain level flight at the bottom of the white arc in the A2A 172 with the S prop. I'm not sure that it is possible, though I didn't spend a lot of time trying that last night. I just tried to confirm where the stall occurred. I'll try some extra slow, slow flight at lunch.

[Caldemeyn]

Looking at the video and comparing the readings in the A2A one, i think that they are quite the same, taking into account the difference in the way the scales are drawn. During the slow flight they stop somewhere the same.

[AKar]

A few loose comments - I attempt to finish my P3D reinstallation tomorrow so I can join the party.

Re: airspeed indicator errors at extreme low airspeeds. It is true that there is a big discrepancy in many airplanes. But if stall occurs at given IAS, then this error is not of concern, because by definition your indicator displays indicated airspeed! No matter how bad your pitot system is, it will show you IAS. If stall airspeed is given in IAS, the airspeed indicator errors are accounted for.

The 'perfect' airspeed that is proportional to the dynamic pressure would be equivalent airspeed, or EAS, which is very much the same that calibrated airspeed, or CAS, is at low Mach numbers. If you've got a perfect pitot-based airspeed indicator, then its IAS would be equal to CAS. This would be the case with ctrl+z airspeed because no system error is introduced to it. But as in real life we don't have perfect instruments, Cessna publishes both CAS and IAS figures for stall airspeed: IAS is what you'd expect to see in the cockpit, while CAS is what your airspeed indicator would show if it was perfect, like ctrl+z speedometer is. Note there is a big, big difference in between them.

Regarding the difference in between the stall speeds of R anf S models: at the same weight, I'd expect them to stall at the same airspeed. The difference in figures comes from the added weight. If you load your 172R to 2550 lbs you'd need to fly by the S model's figures in regards to stall performance.

-Esa

[Nick - A2A]

Looking at the video and comparing the readings in the A2A one, i think that they are quite the same, taking into account the difference in the way the scales are drawn. During the slow flight they stop somewhere the same.

Yes, I'm still somewhat mystified as to why the marked VSO is so much higher on the 'S'. The gross weight difference alone doesn't seem to be enough to account for it. One suggestion (see here) was that the C172S has a more accurate pitot static system. Or could it just be that white arc was reduced for the newer model for legal/liability reasons?

I wasn't able to maintain level flight at the bottom of the white arc in the A2A 172 with the S prop. I'm not sure that it is possible, though I didn't spend a lot of time trying that last night. I just tried to confirm where the stall occurred. I'll try some extra slow, slow flight at lunch.

Thanks, sounds pretty similar to my experience. Interested to hear how you get on!

Nick

[taildraggin68]

Altitude 4200 and around 2200 rpm in the video, will try in a bit, but I do believe that is very doable in the 172.....power on, you can hang it there, power off, you would be descending before you got that low of an airspeed.

Good discussion either way

[Nick - A2A]

Regarding the difference in between the stall speeds of R anf S models: at the same weight, I'd expect them to stall at the same airspeed. The difference in figures comes from the added weight. If you load your 172R to 2550 lbs you'd need to fly by the S model's figures in regards to stall performance.

Thanks Esa - hadn't seen this when I made my previous post. I guess it's possible the actual stall speed of the two models isn't that far apart (as the vid seems to suggest) but that for some reason the published VS0 number for the 'S' is just a bit more conservative.

will try in a bit, but I do believe that is very doable in the 172

Cheers Scott - no cheatin' now and dragging those realism sliders to the left...

[AKar]

Regarding the difference in between the stall speeds of R anf S models: at the same weight, I'd expect them to stall at the same airspeed. The difference in figures comes from the added weight. If you load your 172R to 2550 lbs you'd need to fly by the S model's figures in regards to stall performance.

Thanks Esa - hadn't seen this when I made my previous post. I guess it's possible the actual stall speed of the two models isn't that far apart (as the vid seems to suggest) but that for some reason the published VS0 number for the 'S' is just a bit more conservative.

Well, I quickly calculated that it should make perhaps a one knot difference, so that 100 lbs increase is almost negligible in this respect. There is something else going on if the figures are much different. Got to check out some pdf:s it seems...

-Esa

[Oracle427]

I can't hold it at the bottom of the arc. Anywhere above it is fine, but as soon as that needle gets within a hair's width of the bottom of the arc the airplane will drop. The stall break also coincides with the stall warning horn making the "aggravated" sound.

[Nick - A2A]

Thanks Oracle - that sounds like exactly what I was experiencing. Going by your earlier remarks, you wouldn't actually need to be that close to VS0 to demonstrate slow flight during a checkride anyway; just below stall horn activation speed (VSW)?

Anyway, I think for those that tend to fly with the 180hp 'S' prop fitted, it would actually be closer to reality to make a slight edit to the gauge graphics so that the bottom of the white arc is at 40 kias. Presumably the 'R' models with the 'S' prop share the same VS0 as the C172S itself? Photos like this one seem to suggest so anyway...

Perhaps a tendency for unreliable ASI readings below 40 kias is another possible reason the published VS0/white arc was upped for the 'S' model?

Hopefully Esa can dig something out...

Cheers,
Nick

[Oracle427]

NickM,

Correct, all that is needed is to slow down from cruise and reconfigure the aircraft and enter slow flight with the stall horn continuously sounding without gaining or loosing altitude or changing heading. Then you perform the turns and go back to cruise without gaining or loosing altitude. There are tolerances for the maneuvers.

The FAA PPL testing standards can be found here (check page 125 of the PDF, 103 of the document):
https://www.faa.gov/training_testing/te ... 81-14B.pdf

Note this objective below. It doesn't mean that you need to get right up to Vso. Having the stall horn sounding steadily should easily meet this criteria unless the aircraft is rigged unusually.

3. Establishes and maintains an airspeed at which any further increase in angle of attack, increase in load factor, or reduction in power, would result in an immediate stall.