6 ms·

It is not necessary to empirically determine a specific airplane's stall speed in order to operate it safely. It's not required in the US, we just use the numbe

It is not necessary to empirically determine a specific airplane's stall speed in order to operate it safely. It's not required in the US, we just use the number the manufacturer publishes. It's normal for airplanes of the same model to fly differently: I fly a little fleet of six Citabrias, and their stall characteristics are radically different. You'd expect more uniformity from a modern aluminum airplane, but still: nobody should be flying an airplane like this so close to the edge the exact stall speed needs to be known numerically within one knot. The 40lbs of gas I burn flying for an hour decreases the stall speed by more than 1mph on those Citabrias I fly. EDIT: I was mistaken, this isn't a requirement in Europe either.

hinkley 3y ago
Decreases the stall speed? How does that work?
- ketralnis 3y ago
  Things that can influence stall speed include weight, power, center of gravity, flaps/landing gear configuration, and more. Why? Well, stall speed isn't a real thing. There isn't a speed at which you stall, that's not how it works. It's a convenient short-hand that we use for the more complicated reality. The physical reality is that stalls happen at a particular angle of attack (AOA) into the apparent wind. That is, the angle of your wings relative to the airflow. Up to the critical angle a higher AOA means more lift to counteract gravity. As you slow down you generate less lift because there's less airflow over the wings. So as you slow down, in order to generate a similar amount of lift you have to increase your AOA. If you keep slowing down and adjusting your AOA to compensate, you'll reach a speed that's low enough and therefore AOA high enough that adding more AOA no longer adds more lift (the air no longer flows smoothly over the wing). That's the stall speed, the speed at which more AOA no longer generates more lift. But it's the AOA that's the problem, not the airspeed. In addition to lower speeds needing more AOA, you also need a higher AOA if you weigh more. A wrong but illustrative way to think about it might be that you need the engine's thrust pointed more towards the ground the more you weigh. That means that as you burn fuel (lose weight) the AOA that will stall you doesn't change, but the excess AOA available due to your weight-change does so in effect the air speed at which you would be near the critical AOA to stay airborn does change. Stall speed is still a useful concept especially while landing but it's misleading outside of landing and when anything else is remotely unusual like weight or modifications to the plane. For this reason the FAA has been trying to get AOA indicators installed in planes and to train pilots to look at those instead of thinking about stall speeds https://www.faa.gov/sites/faa.gov/files/2022-01/Angle%20of%20Attack%20Awareness.pdf https://www.faa.gov/sites/faa.gov/files/2022-01/Angle%20of%2...
- deleted 3y ago
  [deleted]
- t0mas88 3y ago
  The published speed is at maximum takeoff weight, with the most unfavourable center of gravity (usually most forward) and idle power If the conditions are better (not at max weight, rear center of gravity, engine power adding more airflow over the wings) you can fly below the published stall speed number.
  - hinkley 3y ago
    Yeah I was thinking backward. Lower stall speed is better, not worse behavior.
- matheusmoreira 3y ago
  This is a good resource: https://ciechanow.ski/airfoil/ https://ciechanow.ski/airfoil/ Useful to think of the airplane as standing still while the engine accelerates the air around it. To fly, you need the air to move over the wings quickly and in the right direction. You can sort of trade how fast you need the air to go for how ideally the air is flowing over the wings. If you angle the wings just right against the air flow, and/or you bend them out of shape just right with flaps, you can slow down a lot while relying on the air itself to carry your plane. If you're flying against the airflow, you need to go faster. This is usually done during take off and landing. The pilot lowers the flaps when approaching to land and flares the aircraft before touchdown, all to make the air flow efficiently into the wings, thereby allowing the aircraft to slow down without falling straight down like a stone. That's why weather is so important for flights. Pilots need to be ready to call TOGA and go maximum thrust at a moment's notice just in case some crosswind or heat wave or something screws up the direction of the air flow just as they're about to land. Many an admiral cloudberg article has been written due to that sort of thing. You angle the plane just right, slow it down just to the edge of stalling, then some phenomenon happens and increases your stall speed past your current speed...
- sokoloff 3y ago
  The Robertson STOL mod droops the ailerons with flaps, changing the effective angle of incidence of the wing. A friend had a Robinson-equipped 182 and we could quite comfortably operate in/out of Marlboro Airport (1650' paved with trees at one end and a fence at the other).
  - vdqtp3 3y ago
    That's not TERRIBLY impressive, considering a stock 182P has a 50' obstacle take off distance of 1350' of at max gross on an ISA day
macintux 3y ago
I'll admit to complete and total ignorance, but: > ...we just use the number the manufacturer publishes. From the article it sounds like this plane was radically modified, to the point where the manufacturer's stall speed would be irrelevant. Why wouldn't you want to confirm for yourself where the speed is after so many changes?
- hluska 3y ago
  The plane had a cargo pack and a Robertson STOL. Cargo packs are essentially for bush planes and as an example, the 1975 Skywagon’s owners manual even had one diagrammed. Robertson STOL’s are extremely common in Northern Canada to the point that even as a passenger I know about them. It’s nothing too radical. Edit - Here’s a copy of the 1975 owners manual: https://www.seaplanescenics.com/documents/1975-cessna-185f-poh-1.pdf https://www.seaplanescenics.com/documents/1975-cessna-185f-p...
- jcalvinowens 3y ago
  My understanding is that in the US, part of the modification would be updating the plane's official operating limitations, and there could be a new stall number. Still a number from the manufacturer, not a number empirically determined by testing that specific airplane. For special one-off modifications, I don't know: I've always been told that's almost impossible with certified airplanes. One Citabria I've flown had vortex generators installed on the leading edge of the wings, but the club sold it over a year ago and I don't remember if it listed a modified stall speed. I do remember it said "not airworthy if more than N of the VGs are broken off", I think it was three?
- dghlsakjg 3y ago
  Stall speed depends on so many factors that it can change significantly in a single flight. Weight, altitude, density altitude, angle of attack etc. are all going to have an effect. In other words, sure, you might want to confirm it, but you should also give yourself some margin since you don’t ever really know what the stall speed is until you stall.
  - WalterBright 3y ago
t0mas88 3y ago
European pilot here :) it's also not a European thing. It works the same here as it does in the US, you use the published number in the flight manual. But this plane had significant modifications done. And if you do things that significantly change performance, you'll need to get updated performance data. Or the provider of the supplemental type certificate that allows the modification has to provide an updated flight manual with that data.
WalterBright 3y ago
> nobody should be flying an airplane like this so close to the edge the exact stall speed needs to be known numerically within one knot. An experienced pilot can feel the stall coming on with a bit of a "burble" in the stick. My dad (fighter pilot) told me that knowing exactly where the stall point is is life and death. When you're in a dogfight, the winner often is the one that can turn inside the other. Turning as tight as you can requires getting exactly on that edge of the burble. It's the same thing as in automobile and motorcycle racing. How close you can get to a slide without sliding is the difference between victory and ignominy.
- peteradio 3y ago
  Imagine dogfighting in a Cessna! I imagine the arms to be pilot wielded colt 45s and first officer to be a jug of whiskey.
  - logical_proof 3y ago
    Love this description lol
  - buildsjets 3y ago
    One of the last aircraft shot down in the European theater of WWII was a Piper Cub that shot down a Fieseler Storch with a .45 pistol. They landed next to the crashed Storch, captured the pilot, and treated his injuries. Then handed him over to the Russians, who I am sure also treated him in accordance with the provisions of the Geneva convention. https://theaviationgeekclub.com/that-time-a-usaaf-piper-l-4h-grasshopper-crew-shot-down-a-german-fieseler-fi-156-storch-by-means-of-their-colt-45-pistols/ https://theaviationgeekclub.com/that-time-a-usaaf-piper-l-4h...
    - wizzwizz4 3y ago
      Perhaps in accordance with the spirit, but the Geneva convention was in 1949, so those provisions didn't exist yet.
      - 3y ago
sokoloff 3y ago
> nobody should be flying an airplane like this so close to the edge the exact stall speed needs to be known numerically within one knot Citabria's are often flown in aerobatics (Citabria backwards is airbatic and for a while, they were the only aerobatic aircraft being commercially manufactured in the US) and a lot of aerobatic maneuvers involve stalling the wing.
- SR2Z 3y ago
  Yes, but anyone doing aerobatics so close to the ground that they can't recover from a stall is understood to be doing that at their own risk. Anyone doing the above with unfavorable wind has a death wish.
- jcalvinowens 3y ago
  You don't really look at the airspeed indicator for that, you feel the stall in the reversible controls. That feel is incredibly precise.
taneq 3y ago
> It's normal for airplanes of the same model to fly differently In my experience any equipment this large is effectively handmade, with all the variability implied by this. I'd hope aircraft are made to tighter tolerances than the stuff I work with but a couple of millimeters is often enough to have a noticeable effect on things like actuator travel. > nobody should be flying an airplane like this so close to the edge the exact stall speed needs to be known numerically within one knot. This is exactly my takeaway as well. Rather than trying to determine the characteristics of the machine to the n'th degree, assume a realistic degree of variation and design for it accordingly.
rob74 3y ago
I'm not a pilot or an aviation expert, but I think you're arguing against a strawman here. The investigation report said nothing about determining the precise stall speed, it just recommended "that pilots be informed about the stall behaviour of the Robertson short take-off and landing kit on the Cessna 185 aircraft, both by listing the issue in the aircraft flight manual supplement and through education through the aviation authority". The fact that the stall speeds were not listed on the test flight report was just further proof that the pilot didn't actually test the stall behaviour of the aircraft. What the investigation report also didn't mention is that pilots should actually practice stall recovery and not just think "if I believe strongly enough that my plane is unstallable, I can ignore stall recovery". But that probably goes without saying...
fransje26 3y ago
> It is not necessary to empirically determine a specific airplane's stall speed in order to operate it safely. It's not required in the US, we just use the number the manufacturer publishes. It is not necessary as in "there is no legal obligation", true. But if you want to live, it is absolutely necessary. The numbers provided by the manufacturer will tell you absolutely nothing about the stalling characteristics of the airplane as soon as you start modifying it. The non-linearity of the phenomena involved in stalling also mean that "intuition" and "small changes" will be of absolutely not help to determine by how much you changed the characteristics of your airplane.

hinkley 3y ago

Decreases the stall speed? How does that work?

ketralnis 3y ago
Things that can influence stall speed include weight, power, center of gravity, flaps/landing gear configuration, and more. Why? Well, stall speed isn't a real thing. There isn't a speed at which you stall, that's not how it works. It's a convenient short-hand that we use for the more complicated reality. The physical reality is that stalls happen at a particular angle of attack (AOA) into the apparent wind. That is, the angle of your wings relative to the airflow. Up to the critical angle a higher AOA means more lift to counteract gravity. As you slow down you generate less lift because there's less airflow over the wings. So as you slow down, in order to generate a similar amount of lift you have to increase your AOA. If you keep slowing down and adjusting your AOA to compensate, you'll reach a speed that's low enough and therefore AOA high enough that adding more AOA no longer adds more lift (the air no longer flows smoothly over the wing). That's the stall speed, the speed at which more AOA no longer generates more lift. But it's the AOA that's the problem, not the airspeed. In addition to lower speeds needing more AOA, you also need a higher AOA if you weigh more. A wrong but illustrative way to think about it might be that you need the engine's thrust pointed more towards the ground the more you weigh. That means that as you burn fuel (lose weight) the AOA that will stall you doesn't change, but the excess AOA available due to your weight-change does so in effect the air speed at which you would be near the critical AOA to stay airborn does change. Stall speed is still a useful concept especially while landing but it's misleading outside of landing and when anything else is remotely unusual like weight or modifications to the plane. For this reason the FAA has been trying to get AOA indicators installed in planes and to train pilots to look at those instead of thinking about stall speeds https://www.faa.gov/sites/faa.gov/files/2022-01/Angle%20of%20Attack%20Awareness.pdf https://www.faa.gov/sites/faa.gov/files/2022-01/Angle%20of%2...
deleted 3y ago
[deleted]
t0mas88 3y ago
The published speed is at maximum takeoff weight, with the most unfavourable center of gravity (usually most forward) and idle power If the conditions are better (not at max weight, rear center of gravity, engine power adding more airflow over the wings) you can fly below the published stall speed number.
- hinkley 3y ago
  Yeah I was thinking backward. Lower stall speed is better, not worse behavior.
matheusmoreira 3y ago
This is a good resource: https://ciechanow.ski/airfoil/ https://ciechanow.ski/airfoil/ Useful to think of the airplane as standing still while the engine accelerates the air around it. To fly, you need the air to move over the wings quickly and in the right direction. You can sort of trade how fast you need the air to go for how ideally the air is flowing over the wings. If you angle the wings just right against the air flow, and/or you bend them out of shape just right with flaps, you can slow down a lot while relying on the air itself to carry your plane. If you're flying against the airflow, you need to go faster. This is usually done during take off and landing. The pilot lowers the flaps when approaching to land and flares the aircraft before touchdown, all to make the air flow efficiently into the wings, thereby allowing the aircraft to slow down without falling straight down like a stone. That's why weather is so important for flights. Pilots need to be ready to call TOGA and go maximum thrust at a moment's notice just in case some crosswind or heat wave or something screws up the direction of the air flow just as they're about to land. Many an admiral cloudberg article has been written due to that sort of thing. You angle the plane just right, slow it down just to the edge of stalling, then some phenomenon happens and increases your stall speed past your current speed...
sokoloff 3y ago
The Robertson STOL mod droops the ailerons with flaps, changing the effective angle of incidence of the wing. A friend had a Robinson-equipped 182 and we could quite comfortably operate in/out of Marlboro Airport (1650' paved with trees at one end and a fence at the other).
- vdqtp3 3y ago
  That's not TERRIBLY impressive, considering a stock 182P has a 50' obstacle take off distance of 1350' of at max gross on an ISA day

macintux 3y ago

I'll admit to complete and total ignorance, but: > ...we just use the number the manufacturer publishes. From the article it sounds like this plane was radically modified, to the point where the manufacturer's stall speed would be irrelevant. Why wouldn't you want to confirm for yourself where the speed is after so many changes?

hluska 3y ago
The plane had a cargo pack and a Robertson STOL. Cargo packs are essentially for bush planes and as an example, the 1975 Skywagon’s owners manual even had one diagrammed. Robertson STOL’s are extremely common in Northern Canada to the point that even as a passenger I know about them. It’s nothing too radical. Edit - Here’s a copy of the 1975 owners manual: https://www.seaplanescenics.com/documents/1975-cessna-185f-poh-1.pdf https://www.seaplanescenics.com/documents/1975-cessna-185f-p...
jcalvinowens 3y ago
My understanding is that in the US, part of the modification would be updating the plane's official operating limitations, and there could be a new stall number. Still a number from the manufacturer, not a number empirically determined by testing that specific airplane. For special one-off modifications, I don't know: I've always been told that's almost impossible with certified airplanes. One Citabria I've flown had vortex generators installed on the leading edge of the wings, but the club sold it over a year ago and I don't remember if it listed a modified stall speed. I do remember it said "not airworthy if more than N of the VGs are broken off", I think it was three?
dghlsakjg 3y ago
Stall speed depends on so many factors that it can change significantly in a single flight. Weight, altitude, density altitude, angle of attack etc. are all going to have an effect. In other words, sure, you might want to confirm it, but you should also give yourself some margin since you don’t ever really know what the stall speed is until you stall.
- WalterBright 3y ago

t0mas88 3y ago

European pilot here :) it's also not a European thing. It works the same here as it does in the US, you use the published number in the flight manual. But this plane had significant modifications done. And if you do things that significantly change performance, you'll need to get updated performance data. Or the provider of the supplemental type certificate that allows the modification has to provide an updated flight manual with that data.

WalterBright 3y ago

> nobody should be flying an airplane like this so close to the edge the exact stall speed needs to be known numerically within one knot. An experienced pilot can feel the stall coming on with a bit of a "burble" in the stick. My dad (fighter pilot) told me that knowing exactly where the stall point is is life and death. When you're in a dogfight, the winner often is the one that can turn inside the other. Turning as tight as you can requires getting exactly on that edge of the burble. It's the same thing as in automobile and motorcycle racing. How close you can get to a slide without sliding is the difference between victory and ignominy.

peteradio 3y ago
Imagine dogfighting in a Cessna! I imagine the arms to be pilot wielded colt 45s and first officer to be a jug of whiskey.
- logical_proof 3y ago
  Love this description lol
- buildsjets 3y ago
  One of the last aircraft shot down in the European theater of WWII was a Piper Cub that shot down a Fieseler Storch with a .45 pistol. They landed next to the crashed Storch, captured the pilot, and treated his injuries. Then handed him over to the Russians, who I am sure also treated him in accordance with the provisions of the Geneva convention. https://theaviationgeekclub.com/that-time-a-usaaf-piper-l-4h-grasshopper-crew-shot-down-a-german-fieseler-fi-156-storch-by-means-of-their-colt-45-pistols/ https://theaviationgeekclub.com/that-time-a-usaaf-piper-l-4h...
  - wizzwizz4 3y ago
    Perhaps in accordance with the spirit, but the Geneva convention was in 1949, so those provisions didn't exist yet.
    - 3y ago

sokoloff 3y ago

> nobody should be flying an airplane like this so close to the edge the exact stall speed needs to be known numerically within one knot Citabria's are often flown in aerobatics (Citabria backwards is airbatic and for a while, they were the only aerobatic aircraft being commercially manufactured in the US) and a lot of aerobatic maneuvers involve stalling the wing.

SR2Z 3y ago
Yes, but anyone doing aerobatics so close to the ground that they can't recover from a stall is understood to be doing that at their own risk. Anyone doing the above with unfavorable wind has a death wish.
jcalvinowens 3y ago
You don't really look at the airspeed indicator for that, you feel the stall in the reversible controls. That feel is incredibly precise.

taneq 3y ago

> It's normal for airplanes of the same model to fly differently In my experience any equipment this large is effectively handmade, with all the variability implied by this. I'd hope aircraft are made to tighter tolerances than the stuff I work with but a couple of millimeters is often enough to have a noticeable effect on things like actuator travel. > nobody should be flying an airplane like this so close to the edge the exact stall speed needs to be known numerically within one knot. This is exactly my takeaway as well. Rather than trying to determine the characteristics of the machine to the n'th degree, assume a realistic degree of variation and design for it accordingly.

rob74 3y ago

I'm not a pilot or an aviation expert, but I think you're arguing against a strawman here. The investigation report said nothing about determining the precise stall speed, it just recommended "that pilots be informed about the stall behaviour of the Robertson short take-off and landing kit on the Cessna 185 aircraft, both by listing the issue in the aircraft flight manual supplement and through education through the aviation authority". The fact that the stall speeds were not listed on the test flight report was just further proof that the pilot didn't actually test the stall behaviour of the aircraft. What the investigation report also didn't mention is that pilots should actually practice stall recovery and not just think "if I believe strongly enough that my plane is unstallable, I can ignore stall recovery". But that probably goes without saying...

fransje26 3y ago

> It is not necessary to empirically determine a specific airplane's stall speed in order to operate it safely. It's not required in the US, we just use the number the manufacturer publishes. It is not necessary as in "there is no legal obligation", true. But if you want to live, it is absolutely necessary. The numbers provided by the manufacturer will tell you absolutely nothing about the stalling characteristics of the airplane as soon as you start modifying it. The non-linearity of the phenomena involved in stalling also mean that "intuition" and "small changes" will be of absolutely not help to determine by how much you changed the characteristics of your airplane.