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Vso

  • Andrewnielsen@me.com
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Andrewnielsen@me.com created the topic: Vso

You said that Vso was with full flaps. Vso is with flaps in the landing configuration. You need to fix that.
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Stuart Tait replied the topic: Vso

So you don't land with full flaps?
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  • Andrewnielsen@me.com
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Andrewnielsen@me.com replied the topic: Vso

No. You definitely don't land with full flaps on all aircraft. Standard landing configuration in a Foxbat is one stage of flaps. Short field landings are with two stages of flap.

I have never flown one of the old Cessna 172's that could deploy 40 degrees of flaps. From what I have read, they could not do a go around with full flaps, it was dangerous and they limited the flap travel. I imagine it would be much safer to fly one of those C172's by not deploying 40 degrees of flaps for standard landings. I am not sure what the POH says.

The second issue is that it is best not to improve on official definitions even if you think that what you are saying is exactly the same, or better. IMHO, if an examiner is wanting a first-stage, wrote response, the student is best off giving them exactly what they expect to get. If the examiner wants to find out the examinee's understanding, then the student can give more information. Your readers have noting to gain from the improved definition and only things to lose.

Turns out 172N standard landing configuration seems to be 40 degrees, based on 1978 POH.
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bobtait replied the topic: Vso

You have made a valid point when it comes to the definition of Vso. I will include the phrase 'landing configuration' in the print of the text. In every case I've ever heard of, that is full flap and landing gear down. It is hard to imagine a situation where full flap would ever be used except for landing.

By the way there are many of us old instructors who have flown many thousands of hours in the 'old' Cessna 172s and 150s who are still alive. That includes thousands of practice go-arounds with pre-solo students.

Also, concerning Va. Every definition I can find lists it as a structural limitation. It is imposed to indicate the maximum IAS at which full nose-up elevator can be applied without exceeding the maximum permitted load factor that the airframe can tolerate. That's a structural limitation.

Thanks for your valuable contribution to this forum.
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Andrewnielsen@me.com replied the topic: Vso

Thank you for being kind to a noob.

What you say about Va is true: it is a structural, airframe limitation. I agree with you wholeheartedly. (You did write the book, after all).

To be specific, Va does not decrease with decreasing weight to protect the *main spar* of the wing. That is because, at a given airspeed and AoA, the bending force on the main spar will be the same. The force on the main spar of the wing does not increase if the airplane gets lighter. The force on, say, the engine mounts will get higher.

(To go overboard, maybe if the airplane is lighter, and you pull full aft as fast as you can, the airplane will have less angular momentum and will pitch up faster. But I don't think that that is how they decided that Va should decrease with decreased weight.

If you could explain why the bending force on the main spar would increase with the aircraft being lighter, I would really like to hear.)
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  • John.Heddles
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John.Heddles replied the topic: Vso

There are lots of strange quasi-definitions around regarding aspects of certification. Usually, this is a well-intentioned aim to make things a little easier for the new pilots to understand and put into the wider picture of the flying learning process.

Generally, the better approach is to go back to basics if you want to argue the toss on any points.

The definition for Vs0 (and that is zero, not the letter O - notwithstanding that the documents contain plenty of typos and sometimes you will find the number and sometimes the letter) has a few considerations associated with it. The following is a superseded definition but probably more useful for the discussion.

rgl.faa.gov/Regulatory_and_Guidance_Libr...06CAE12?OpenDocument

As a sideline note, the reference to 61 kts is the subsequently metricated number relating to 70 mph, which was the original limitation. This number goes back to the very, very early days of aircraft regulation and was a number which the early certification engineers plucked out of the air over coffee as being a suitable balance between what was achievable by Industry and a figure which offered some reasonable probability for post-mishap survival. This snippet of information I got from an ancient engineer who was a junior chap in the office at the time.

If you want to know what the FAA thinks their regulations mean, you read the relevant ACs. For flight testing associated with certification, that would be here ..

www.faa.gov/documentLibrary/media/Adviso...ircular/AC_23-8C.pdf

Not a document you might read over one cup of coffee but, certainly, a document which you should run your eyes over to get a feel for what goes into the POH.

Engineering specialist in aircraft performance and weight control.
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Andrewnielsen@me.com replied the topic: Vso

Page 15 says that "landing configuration" is gear down and full flaps! Ouch!

Since you're an engineer, what is your take on the Va question? I reckon that Va is not primarily there to protect the main spar of the wing (or horizontal stabiliser), because if the plane is lighter with a particular AoA and airspeed, the force on the main spar will not be greater, even though the airplane will accelerate faster.
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Andrewnielsen@me.com replied the topic: Vso

It seems there are layers of definitions. Is there a standard Australian reference for wrote definitions that CASA use, and examiners are expected to use?
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  • John.Heddles
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John.Heddles replied the topic: Vso

what is your take on the Va question

Again, you are trapping yourself within the "simplified" pilot training story arena rather than getting into the Design Standards where the story is given in more detail .. but, even then, you need to look at individual regulator determinations from time to time to see what a particular rule might mean for a particular certification. Things are not always as simple as you might think that they ought to be.

I'll give some thought to the question, figure out just how I see it to be appropriate to answer, and come back in due course ...

Just going back to an earlier comment you made the airplane will have less angular momentum perhaps you meant inertia, rather than momentum ?

Is there a standard Australian reference

'Fraid not. Especially post-Yates Report changes to the Oz regs, you have to go back to the relevant NAA rules, in general, the FARs for US-originating Types.

Keep in mind that, while Bob may not have the answer to every single, last question (and, I'm sure, he would be the first to acknowledge that) he has been around the traps for a long time and has a good story on the great majority of things relevant to the training arena .. folks in the training pipeline would do well to heed his counsel.

Engineering specialist in aircraft performance and weight control.
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  • Andrewnielsen@me.com
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Andrewnielsen@me.com replied the topic: Vso

I meant inertia, rather than momentum.

I spend 99.9% of my time trying to memorise Bob's books and 0.1% of my time trying to pick holes in them. By attempting to pick holes in them, and getting egg on my face, I have learnt valuable stuff about definitions.

Here's an interesting thing, to me anyway. The POH of Foxbat (A22LS-POH-04, page 7) says that the Vso is 32 kts. Page 20 says to do a short field landing approach at 49 kts, with full flaps. That makes the approach speed 1.5 Vso, which is how it should be, apparently (1.3 for "heavy" aircraft like C172's). From that I conclude that Vso in the foxbat is the stall speed with full flaps. And that if I am doing a standard approach at full weight, the aircraft will stall before the bottom of the white arc!

(The Foxbat is not certified in the standard way, of course). Maybe everyone who flies knows the bottom of the Foxbat white arc is full flaps. I didn't. That really does make the Bob Tait definition *actually* better than what I thought the standard definition was!
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