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C152's maneuvering speed

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C152's maneuvering speed

Old 25th Dec 2013, 20:07
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C152's maneuvering speed

Hi all,

I'm a flight student, doing PPL right now. There's one question with a precise answer, that yet leaves me confused.

Cessna 152's maneuvering speed is a range of 93 to 104 knots. Depending on weight, so the POH says. That is, 93 when i weigh 1350 pounds in total and 104 at MTOW of 1670 pounds.

I keep trying to penetrate the physics behind this dependence, but get to conclude, it has to be inverse. Can somebody explain me why it isn't?

When i make an abrupt maneuver, in what effect does it differ from a smooth input? An increased load on the airframe, right? The strenght of the wings, the struts, the fuselage etc is limited, so a too big force of a jerk might brake them. Rip off wings, for example.

What does force consist of? It's acceleration times mass. Now if i understand aerodynamics right, the same deflection of a control surface at higher speed causes a faster change of situation (pitch, roll, whatever). Is that right?

So the higher the speed, the less we would want the mass to be, for the resulting load to stay the same. If we introduce a range of maximum maneuvering speeds, we would allow the pilots to make maneuvers at higher speed if the weight is well below maximum. Right now what i see in the POH is the opposite of it: you may make turns at 104 KT with MTOW, but only 93 KT if you're very light.

Where am i wrong?

Thank you
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Old 25th Dec 2013, 22:07
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This should explain: http://www.safepilots.org/documents/...ring_Speed.pdf
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Old 25th Dec 2013, 22:10
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Heya Creaze,

If I am right, it is as follows:

Vertical acceleration is induced by a Force. Given Newton's formula, we have F=m.a , or a=F/m

So if you would apply the same upward force (when pulling on the yoke) to the light and heavy aircraft, the upward acceleration of the lighter aircraft will be higher.

That means a heavier aircraft has a higher Va than a light aircraft
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Old 25th Dec 2013, 23:59
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overthinking things can actually make a pilot neglect other things, like airspeed, or not hitting a mountain.

first off, just believe that the heavier a plane, the higher the maneuvering speed.

think of stalling a plane as blowing a safety valve on a boiler.

if the plane doesn't weigh too much, it is farther from the stall, so you have to be closer to the stall to protect it, so the lower speed brings you closer to the stall, and the protection for the structure.

accelerating a plane (like in a steep turn) makes the plane''heavier'' and then it is closer to the stall.

heavier means closer to the stall

lighter means farther from the stall

so, a higher speed at heavy weight is the same '''distance'' to the stall
as a slower speed at a light weight.

or you can sit in the plane, flying around thinking about newton and run into another plane instead of looking around for other traffic


now I'm sure someone will belittle me in some way for trying to over simplify and that's fine.

still remember the student who worked for nasa as an aerodynamic engineer who got lost on a back course apch.

yup..don't think too much
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Old 26th Dec 2013, 00:51
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Rayfil, it doesen't make sense.

Faster acceleration of a lighter plane results in the same force being applied to structure. It's the force (acceleration times mass), that loads the structure, not merely accerelation. Am i right so far?

So this logic would only make sense in setting a fixed Va speed. Yet, they made it a range.

All above assumes that my yoke inputs, like you say, directly apply acceleration to a 1600 pound body. I don't think that's true, i see it as changing of spatial situation (in plain words, turning the airfoil for higher angle of attack), which in turn directs a bigger part of horizontal speed into increasing lift, which in turn produces that vertical acceleration, that loads our structure.

Otherwise how did the old 100-tonne planes work, where yokes were directly connected via cords to control surfaces, with no hydraulic enforcement?

Flare Pilot, so you're saying, it's all about staying clear of stalling speeds? The POH says, don't enter turns faster than this. Why would they limit me with 93 knots in a light airplane? What happens if i go 99 knots? (The Vno is 111 KIAS, the Vne is 149 KIAS)

Besides, even at 93 knots i'm some 50 knots away from stalling speed. The cruising speed at 2300 RPM would be less than 93. Looks like quite a safe margin to me.

As for overthinking, i'm totally with you. Got a lot sad stories to tell as well. But right now i'm not in the plane, i'm on the couch, school is closed for holiday. Think i can afford me some curiosity, ain i? And besides, i think precise understanding of the chemistry behind what's happening in the plane adds up to my confidence and flying ahead of the airplane. Ain it?
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Old 26th Dec 2013, 01:00
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no creaze...it is not about staying clear of the stall speeds

the stall is the protection that keeps the structure intact

the stall is the safety valve


VA is all about the stall protecting the structures.

simple question...would you rather stall and be able to recover from the stall or would you rather not stall and have the plane fall apart.??????????

a lighter plane stalls at a slower speed (more complicated, but lets leave it at that for now)...if you were going faster and imposed a higher load on the structure the plane would break apart rather than stall.
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Old 26th Dec 2013, 01:18
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I'd rather stall and recover. I've been trained well to do that. I feel very confident 20 knots above VSO in that airplane, let alone 50 knots above.

My question was, why the relationship between weight and VA is direct? What is there about a heavier plane that makes it withstand higher structure loads?

Or you're saying, since a lighter plane's stalling speed is lower, the engineers thought, let's have them pilots stay further away from structural damage on turns?
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Old 26th Dec 2013, 01:24
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what is it about a heavier plane that makes it withstand higher loads?

because it is closer to the stall at the heavier weight than the lighter weight

so

the safety valve(the stall) works sooner at heavier weight

(again, simplification)
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Old 26th Dec 2013, 01:34
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By closer to the stall, you mean, like 40 knots above it as opposed to 50 knots above it? Just to make sure i get you right; cause a heavy c-152 in a 30 degree bank would stall maybe at 50 knots.

And by the way, what exactly do you mean with safety valve? Am i supposed to stall just before my wings get ripped off?
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Old 26th Dec 2013, 04:51
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Remember, the Manoeuvering Stall Speed is proportional to the square root of the G, times the 1G clean stall speed.

eg at 4G, your new stall speed is doubled (2 being the square root of 4G)

and at 9G, your new stall speed is tripled (3 being the square root of 9G).

So at 6G (if that is a Cessna Aerobat limit), the Vman = 2.449 x Vbs
(2.449 being the square root of 6G, and Vbs being Basic Stall Speed).

If your Basic Stall Speed is higher, due to heavier Mass, then your manoeuvre speed also increases.

If you are flying slower than your Man Speed, you cannot pull 6G before you stall.
If you are flying faster than your Man Speed, you can overstress the airframe.

PS. Keep in mind the Rolling G-limits, and the effects of rolling with aileron PLUS pulling G. You can easily overstress just the one wing....
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Old 26th Dec 2013, 10:06
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Flarepilot: "overthinking things can actually make a pilot neglect other things, like airspeed, or not hitting a mountain."

He was sitting at his computer when he asked the question, not flying round mountains. It's a perfectly good question (which you were kind enough to answer); the sort that students should ask and the rest of us benefit from by revising.

Different people have different learning styles. "Monkey see, monkey do" might work for some, but more intelligent students are far more likely to accept and retain information if it is based on understanding.

It is rather counter-intuitive that the same plane can apparently be flown faster safely when it is more heavily loaded.
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Old 26th Dec 2013, 10:19
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Hi Creaze, As well as the 'breaking the wings off' there is also the 'throwing the engine out of its mounting' to think about if you exceed the max allowable G forces.


At speeds below Va it is impossible to achieve that amount of G force.


With a heavier plane that G Force Max occurs at a higher speed.


You need to study the V-n diagram to understand what is possible at what airspeed. (page 299 in '4 The Airplane - Technical')
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Old 26th Dec 2013, 11:49
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Good idea also if you 'keep the numbers in your head' as I do just to remember the lowesr turbulence penetration speed and stick to that rather than trying to do O level maths when you're getting bounced around in some Pennine wave.

Edit: Just noticed your location, add mountain range and high school maths to suit.
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Old 26th Dec 2013, 12:28
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creaze

what do I mean by safety valve? I mean a safety valve as in a boiler like on a ship/steamship.

if a boiler will blow up at 1000psi, a valve that allows dumping of pressure that opens at 900 psi will prevent the boiler from exploding.

so too VA. it is a safety valve that causes the plane to be unloaded (by stalling) before it breaks apart.

yes, you STALL before the wings reach failure and fall off. by stalling the load is reduced and the wings DON'T FALL OFF ( no comment on airbus 300s) ;-)

IT is counterintuitive as jacko says. Flying faster when heavier doesn't make sense does it...but it is the way it works.


as to jacko's comment about learning styles, yes intelligent students do seem to learn better when they understand. and if I was employed by creaze as his CFI, I would spend hours explaining it to him. And I would charge him current rates I've seen incalifornia are between 80 and 90 dollars an hour.

I am reminded constantly about the Sergeant in the Army who taught his men how to fix/unfix bayonets on their rifles. Place rifle stock between legs, place bayonet on barrel lug without touching barrel.

troops went on maneuver and were told to fix bayonets...smart guys said: its dumb to put "GUN" (soliders laugh here) between legs to put bayonet on...why not just hold gun barrel by hand and fix bayonets.


men went on to the maneuver, including firing over 100 rounds of live ammo.

command to unfix/remove bayonets.

SMART GUYS burned hands

DUMB guys did what the sgt told them to do, and didn't burn hands

what happened.

In the army, or anyplace, there are reasons to learn things by rote...the rifle barrels were hot from shooting and the smart guys hadn't thought of that and placed their hands on the barrel to remove bayonet.

dumb guys didn't think it through too much and did what they were told


IN FLYING, sometimes its better to know something and do it the right way without over thinking it in critical situations.
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Old 26th Dec 2013, 13:34
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Thanks everybody, specially Runaway Gun.

Flare Pilot i might be starting to see your point. So in a light plane, if i go 97 knots instead of 93 knots, this would allow me to pull destructive Gs, and the airplane would not stall, the safety valve would not work for that light airplane. Right?

Whereas, in a heavy plane, the stall valve works, and i can be allowed to go up to 104 knots and be sure i'll stall before i can do damage to airframe. Is that how it is?

Starts making sense to me now. Only, i thought, with increasing weight, all flight characteristics would walk along with each other, and as stalling speeds rise, the Gs, that produce harmful load, would decline. Well, maybe i'll figure that sometime later when i get to study theory books.

As for learning styles, i'm rather on Flare Pilot's side. Just keep in mind that since my curiosity is a factor as well, i'm better off knowing why, by the time i'm inside the plane, rather than guessing and burning hands after all. That's how it is with us, smart guys ;o)
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Old 26th Dec 2013, 13:53
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i'm better off knowing why, by the time i'm inside the plane
What you actually do in the plane is straightforward: if it's bumpy enough to be uncomfortable, slow down a bit.
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Old 26th Dec 2013, 14:05
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.....O-o-o-o-oh, wa-a-a-ait a second..... i see now!

So it's not the speed that loads the airframe, it's the acceleration! Regardless of speed, i might pull that yoke hard enough to rip off wing or tear off engine. So the engineers don't want me to enter turns at a too fast speed, to make sure i stall before i rip off my wings.

And since a lighter airplane is harder to stall -- that is, stalls at a lower speed -- so they limit me with 93 knots. So that whatever i do in a turn, how ever steep i turn, at that low speed i reach stall condition before i damage airframe.

Allright, i see now what you've been trying to tell me Flare Pilot. What a graceful way to compose a safety belt out of the laws of aerodynamics!

Thank you all again!
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Old 26th Dec 2013, 19:19
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remember too that tubulence is a form of acceleration or increased g loading and that is part of the safety valve, you might hit turbulence so bad,it increases the ''weight'' enough to break the plane


but th eplane will stall before it breaks if you are at the right speed ( I could put a big IF here but I won't).


now, quit thinking about it too much, look out the window, and it will suddenly come to you.

readthe following books

stick and rudder

modern airmanship

and if you really want to be informed, ''aerodynamics for naval aviators"

just follow the rules for now, the ''ahha'' will come soon enough
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Old 27th Dec 2013, 02:27
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I have to reread this thread carefully, because I put this same question to one of my FI's just a week or 2 ago, and I did not get it. I kinda shuffled it away with a mental note to self to research a little more when I have time. So I will sit down on this thread and go through it on the weekend. Be prepared for more questions from me! Maybe when you got it sorted creaze, you can explain it to me. It will also help you understand it better!

I agree with flare pilot when stated "overthinking things can actually make a pilot neglect other things, like airspeed, or not hitting a mountain." For us new pilots, we dont need in depth understanding of the physics to an engineers level, just a dumbed down version of it that explains how the plane flies is enough. This is why I like stick and rudder so much. As experience grows, the level of the physics can (and should) increase. In saying that, I personally get as much enjoyment from understanding the physics as I do flying the plane!
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Old 27th Dec 2013, 03:40
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just remember that Va is a safety valve that allows the structure of the plane to be preserved by unloading the structure.

the structure can and is unloaded by stalling before the structure fails.


some smart guy with a slide rule (remember those) has figured out the speeds and published them.

the speeds depend on weight

so , know your weight (plane's weight) and observe the speed limits when the air is not smooth or if you plan to make anything but very smooth, gentle maneuvers with the plane.


Now, you would think that if the plane weighs MORE the speed should be less to keep safe....but because the plane weighs MORE it stalls at a higher speed and there is a margin between the Va and stall that is adequate

why? because STALLING is the safety valve and preserves the structure.

but if you are flying too fast and the plane is too light, you won't stall in time to preserve the structure. why? because the plane won't stall at higher speeds because it is too light.


now...play around with it, think about it, but on the ground where you can't be hurt.

loading comes in forms like .high g maneuvers (steep turns), turbulence,

you can also unload a plane (to a degree) by pushing forward on the yoke (do it too much and you might end up in the negative g range and that's not good either...stick and rudder talks about unloading a plane in a turn to avoid a stall)

and tomake things even more difficult, jets use something called rough air penetration speed and DP Davies talks about this and its selection in his book, "handling the big jets". but unless youare going to fly a jet, don't bother.

I know of one accident many years ago with a brand new private pilot, flying at night, in a complex/high performance single, running into a thunderstorm on autopilot. He was flying above Va and overloaded the structure and the plane broke apart in flight. He overcontrolled the plane and bam.

He was in over his head...all 4 aboard were killed.

avoid problems, esp for your first 200 hours. don't fall for traps that others have paid for with their lives.

oh, and look at the airspeed indicator and the different ''arcs'' on small planes...you know, stuff like the red line, yellow arc ,green arc, white arc andknow what they mean.
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