POH/Aerodyamics question

[flyingcheesehead]

Why is there no published power-on stall speed?

I'll leave it at that for now, and leave my speculation out of it.

 

[Greg Bockelman]

Because there are too many variables?

 

[nosehair]

I'm guessing because there is no FAA requirement to establish and publish it. The only reason a power-off stall speed is published is FAA mandate and to be able to calculate an approach to landing speed.

 

[EdFred]

Why is there no published power-on stall speed?

Are you sure about that? Isn't a stall speed is stall speed regardless of whether you have power in or not. It's an AoA thing, not an RPM thing.

 

[flyingcheesehead]

Are you sure about that? Isn't a stall speed is stall speed regardless of whether you have power in or not. It's an AoA thing, not an RPM thing.

It is an AoA thing, but it is NOT a speed thing. In our 182, power off stall = Vx. You get a lot of lift from the prop wash apparently! And if the engine fails, ya better nose over NOW.

 

[flyingcheesehead]

I'm guessing because there is no FAA requirement to establish and publish it.

Okay, so then... Why doesn't the FAA have such a requirement?

 

[tonycondon]

It is an AoA thing, but it is NOT a speed thing. In our 182, power off stall = Vx. You get a lot of lift from the prop wash apparently! And if the engine fails, ya better nose over NOW.

not just lift due to prop wash, but also the vertical component of thrust.

Okay, so then... Why doesn't the FAA have such a requirement?

too many variables? prop/engine combination, engine age/condition

 

[flyingcheesehead]

too many variables?

But... Aren't there quite a few variables anyway (power off)? And doesn't the existence of a "stall speed" promote the idea among students that stalls are based on speed and not angle of attack?

 

[tonycondon]

But... Aren't there quite a few variables anyway (power off)? And doesn't the existence of a "stall speed" promote the idea among students that stalls are based on speed and not angle of attack?

power off would eliminate engine/prop and engine age/condition variables. perhaps you would still have issues with wing contamination or something like that.

"stall speed" can promote that idea if you dont qualify it with a flight condition disclaimer

 

[EdFred]

Vx = Vs1?

That doesn't sound right.

23.65
(a)(4) A climb speed not less than the greater of 1.1 VMC and 1.2 VS1 for multiengine airplanes and not less than 1.2 VS1 for single—engine airplanes.

and 23.69
(a)(4) A climb speed not less than 1.3 VS1.

 

[jesse]

But... Aren't there quite a few variables anyway (power off)? And doesn't the existence of a "stall speed" promote the idea among students that stalls are based on speed and not angle of attack?

Require AoA indicators in every airplane and get rid of Vs0. Problem solved. Until that happens students are going to want some type of number. Hell--I like to look at Vs0 for an airplane before I fly it. Not because I expect the airplane to only stall at this airspeed and exactly at this airspeed everytime. I do it because it gives me an idea with regards to the flight characteristics of the airplane.

My preferred technique for teaching myself how to fly an airplane is to take off and climb to a safe altitude away from the airport. Do some stalls followed by slow flight as slow as I possibly can get it combined with turns. After I do this the level of control that I have over the airplane during the landing phase is way higher. Sometimes I will go ahead and do a simulated landing in the air by putting it in the landing configuration at the airspeed that I have deemed appropriate based on the above flying. I'll fly it down to whatever the predetermined altitude is and pull back to flare. At that point I can see how long I float above this altitude before I finally stall. The secret to this is that you need to do it first in an airplane you're familiar with at a landing speed you're familiar with. You'll get an idea of what it should look and feel like in other airplanes since it's obviously not the same without the runway a foot below you combined with the increased lift due to ground effect.

 

[nosehair]

I will go ahead and do a simulated landing in the air by putting it in the landing configuration at the airspeed that I have deemed appropriate based on the above flying. I'll fly it down to whatever the predetermined altitude is and pull back to flare. At that point I can see how long I float above this altitude before I finally stall.

Good. That is a good description of the way we are all supposed to teach power off stalls. That is the main purpose of stall practice - to be in control during the landing flare, which is a stall onto the runway.

AoA indicators would be so much better, but, you know, they're way too expensive, or so I've been told.

Airplane manufacturers don't make POH's for student pilots. They are written to pilot knowledge level. We are supposed to know that this published speed is in a specific condition: Gross weight, standard day, power off, and at an AoA rate increase of 1 degree per second.

Power on could be full power-some power - a little power- a lotta power, etc. Even if you go with full power, that changes with altitude, engine condition, angle of engine thrust, etc. Think of a helicopter. That has vertical thrust enough to not stall at all. Same with a big engine, you will get way, way below power off stall speed, because you don't reach stall AoA at the same speed as when power off.

Asking why the FAA dosn't mandate power on stall speeds is like asking why our government doesn't provide that our children get good medical care and good education.

The government isn't in the business of looking out for our welfare. They are in the business of looking out for their own welfare.

 

[Steve]

http://www.hciaviation.com/prod-aoa-a.shtml

Oh, you chose not to go with experimental category?...sorry...my bad

Require AoA indicators in every airplane ...

 

[tonycondon]

http://www.hciaviation.com/prod-aoa-a.shtml

Oh, you chose not to go with experimental category?...sorry...my bad

heh heh, nice steve. the Dynon 10A also has provisions for an AoA indicator. Very handy.

 

[flyingcheesehead]

Vx = Vs1?

That doesn't sound right.

Vs1=63 mph
Vx=63 mph
Vy=90 mph

I'll show you the POH if you want.

23.65
(a)(4) A climb speed not less than the greater of 1.1 VMC and 1.2 VS1 for multiengine airplanes and not less than 1.2 VS1 for single—engine airplanes.

Not applicable. You missed a very important part of the reg:

§ 23.65 Climb: All engines operating.

(a) Each normal, utility, and acrobatic category reciprocating engine-powered airplane of 6,000 pounds or less maximum weight must have a steady climb gradient at sea level of at least 8.3 percent for landplanes or 6.7 percet for seaplanes and amphibians with—

(4) A climb speed not less than the greater of 1.1 VMC and 1.2 VS1 for multiengine airplanes and not less than 1.2 VS1 for single—engine airplanes.

They have to show a climb gradient with that speed. So, the 182 must have a climb gradient of 8.3 percent or more, at a speed faster than Vx. (I can vouch for that, the climb gradient at Vx is pretty insane.)

and 23.69
(a)(4) A climb speed not less than 1.3 VS1.

Again, you're missing an important part of the reg:

§ 23.69 Enroute climb/descent.

(a) All engines operating. The steady gradient and rate of climb must be determined at each weight, altitude, and ambient temperature within the operational limits established by the applicant with—

(4) A climb speed not less than 1.3 VS1.

Similar to above. They're not saying that "climb speed" must be above 1.3VS1, they're saying that they need to demonstrate certain climbing abilities above that speed.

Were it as you stated, they also do not define "climb speed." If the limits were as you posted, I'm sure they could simply use Vy which is 1.43Vs1.

 

[EdFred]

Well, I also saw the "outstanding" Vx performance in that aircraft at a certain fly-in last summer.

 

[CJones]

heh heh, nice steve. the Dynon 10A also has provisions for an AoA indicator. Very handy.

Yep.. There are a lot of AoA indicators flying in RV's today. #1 reason being that there is no 'stall warning horn' built into the RV's. We've considered adding the AoA to 7DL, but haven't gotten around to it yet.

 

[jesse]

Well, I also saw the "outstanding" Vx performance in that aircraft at a certain fly-in last summer.

Classic

 

[jesse]

Vs1=63 mph
Vx=63 mph
Vy=90 mph

I'll show you the POH if you want.

The bonus is I'm assuming that you have flaps out when you are climbing at Vx. What is Vs0?

 

[Greg Bockelman]

#1 reason being that there is no 'stall warning horn' built into the RV's.

How much warning do you get out of the airframe in a Stall, Chris? In other words, does it buffet much before the break? Does it give you any warning?

 

[jesse]

How much warning do you get out of the airframe in a Stall, Chris? In other words, does it buffet much before the break? Does it give you any warning?

Apparently not enough. Ask Tony.

 

[flyingcheesehead]

Well, I also saw the "outstanding" Vx performance in that aircraft at a certain fly-in last summer.

That wasn't Vx. That was Vx minus 15!

 

[flyingcheesehead]

The bonus is I'm assuming that you have flaps out when you are climbing at Vx. What is Vs0?

Actually, I lied.

Vs1 is 68 mph. Vs0 is 63 mph.

 

[gismo]

Are you sure about that? Isn't a stall speed is stall speed regardless of whether you have power in or not. It's an AoA thing, not an RPM thing.

In a single the main difference comes from the vertical component of the thrust vector providing some of the "lift". In a twin the difference is far more significant because idling engines and windmilling props really mess up the airflow over a substantial portion of the wing. That increases the load on the remainder of the wing and that requires more IAS at the critical AOA. Conversely running high power blows a lot of air across the top and bottom of the middle part of the wings and this generates more lift at the same AOA and IAS or the same lift at the critical AOA and less IAS. In my airplane the difference in stall speed between both engines at idle and both at full power is close to 10 KIAS. In the Bonanza I used to have (same wings) the difference was 2-3 KCAS

 

[CJones]

How much warning do you get out of the airframe in a Stall, Chris? In other words, does it buffet much before the break? Does it give you any warning?

Nope.. Not much buffet at all. Still pretty responsive at severely slow speeds and a pretty clean break to recovery. I'll show you at Gaston's.

 

[Greg Bockelman]

I'll show you at Gaston's.

Or sooner.

 

[PHXAvi8tor]

I gotta demonstrate power-on stalls this afternoon, so I'll check the following:

If my memory is correct, you'll feel the buffet and be hearing the stall warning horn at the bottom of the white arc on the airspeed indicator -- the same place as in a power-off stall.

Reason? Angle of attack, primarily. You just happen to have a lot of thrust going, and with a high pitch attitude (simulated takeoff configuration in training), things look different. But, the wing's still at the same old critical angle of attack.

In any case, I'll make a note this afternoon to try to remember to check that and report back.

 

[Bill]

If my memory is correct, you'll feel the buffet and be hearing the stall warning horn at the bottom of the white arc on the airspeed indicator -- the same place as in a power-off stall.

If you're in takeoff configuration (partial flaps), would the stall still be at the bottom of white arc, or somewhere between bottom of the green and bottom of the white?

 

[jesse]

I gotta demonstrate power-on stalls this afternoon, so I'll check the following:

If my memory is correct, you'll feel the buffet and be hearing the stall warning horn at the bottom of the white arc on the airspeed indicator -- the same place as in a power-off stall.

Reason? Angle of attack, primarily. You just happen to have a lot of thrust going, and with a high pitch attitude (simulated takeoff configuration in training), things look different. But, the wing's still at the same old critical angle of attack.

In any case, I'll make a note this afternoon to try to remember to check that and report back.

Let the airspeed decay some into the lower range. Give it power and get aggressive with it. If you load the wings up by pulling you'll be able to get it to stall at a higher speed due to the loading. Of course you can also get it to stall at a lower speed if you are more gentle and don't load it up since you'll have thrust on your side. This will vary airplane to airplane. The more thrust and the lighter the airplane the more the rules start to change. I'm willing to bet Chip can stall his Extra 300L at a MUCH lower airspeed power on than off.

 

[poadeleted20]

In most cases, the power-on stall when performed per the PTS occurs at an IAS well below the bottom of the white arc (power-off landing configuration max gross weight 1g stalling IAS, aka Vs0).

 

[nosehair]

If my memory is correct, you'll feel the buffet and be hearing the stall warning horn at the bottom of the white arc on the airspeed indicator -- the same place as in a power-off stall.

No. It won't be. Check it out. The airspeed color markings are for power off stall. The green arc should stop at the published power off no flaps staal speed, and the white arc stops at the published power off full flaps speed. The power on will get a bit lower before stall.
Full stall that is - don't know where the horn and buffet may start.

 

[Henning]

Are you sure about that? Isn't a stall speed is stall speed regardless of whether you have power in or not. It's an AoA thing, not an RPM thing.

Correct, but most small planes have no way to determine the AOA simply since it is dependant on relative wind so you have to calculate your pitch angle with your rate of decent/climb. Due to varying levels of power caused by condition of the engine and altitude, the pitch angle and the speed of stall will vary from situation to situation so publishing either is pointless, you just feel for stall.

 

[Henning]

But... Aren't there quite a few variables anyway (power off)? And doesn't the existence of a "stall speed" promote the idea among students that stalls are based on speed and not angle of attack?

For power off, your big variable is accounted for by conditioning the number "at max gross". Altitude is compensated automatically by using IAS. We use airspeed to teach it because most planes a student trains in aren't equiped with an AOA instrument, so airspeed is the most consistent thing to use, and it is also an indicator of kinetic energy carried.

 

[cwyckham]

There are two factors that I can see that would decrease the stall speed for a power on stall. First, because some of the thrust is directed downwards, the amount of lift required from the wings is reduced. The effect is identical to reducing the weight of the aircraft. Since less lift is required, the airspeed at the critical angle of attack is reduced.

Second, the wing is broken into two bits, the part with propwash and the part without. The part with prop wash has a high velocity and a lower angle of attack. It will contribute more lift than you would expect based purely on the area (ie. if 1/4 of the wing area is influenced by the propwash, it might be producing 1/3 of the total lift). This again reduces the amount of lift effective lift required of the portion of wing which isn't influenced by the propwash, reducing the airspeed at which the critical angle of attack is reached still further.

Chris