Vertical stablizer
 

G'day Have a question about sweepback and its effect on increasing the stalling angle of attack of the surface.. I understand how it moves the cp rearwards and increases the moment arm but i can't get my head around how a swept back fin stalls at a greater angle of attack. So if somebody could please shed some light on this would be tops!

Same reasons why they sweep the wing [many excellent explanations for this phenomena abound] remember the tail exerts a downward force but this has not much to do with your question--- basically, it delays the onset of compresibility effects on the tail that hinder its effectiveness.

the Cp shift that aides Mach tuck may be just one term the other is loss of downward balance force by fiddling with these two parameters.

designers try to impart as much positve natural longitudinal stability before turning to artificial stabilty augmentation like pitch trim conpensators etc


PA

oops I gave the answer for horizontal stabilizer]

but in terms of directional stabilty this time:O it increases the amount of directional stabilty because ---again ---it delays the onset of directional instabilty cause by compressibilty effects the same way as above and samwe way as wing

lets call this mistake I made above a happy accident:}

PA will read more carefully :ugh:

PA

I remember reading many years ago about the design of one particular type that the fin was designed with sweepback purely for aesthetic reasons. I've always believed that a swept fin did not do much but make the design look 'cool'!

Correct. Somewhere back in the early 60's if memory serves, and it probably doesn't, the Cessna 150 went from a uhhmm, vertical, vertical sabilizer to a swept, vertical stabilizer. The C150 pilots, mostly CFI's, said the deterioration in handling was definitely noticeable. The design change was made so it'd look more like a jet- even tho the high wing and struts remained.
Bob.

I think it would be a very bad day if your fin got anywhere near 'stalling angle' at high subsonic speeds:)

As far as I recall the reasons for sweepback are:-

More fin area for same vertical extent
Moment arm longer = more directional stability (c of p further aft as you say)
Looks cool - I mean, compare the appearance of the OLD 150 with the new 152 man!

Regarding stalling, ALL sweepbacks increase stalling angle as far as I recall. I'd have to go into the attic to dig out my text books to remember why!

I think the B747-400 fin is a 'looker' but for me nothing will ever beat the VC10.

For the VC10, the swept fin does have a purpose. On a T-tail aeroplane, it allows the horizontal tailplane to be mounted far further back. As the engines will be at the back, you need all the moment arm you can get, so it serves a very useful function. I think it fair to say for 707-planform aircraft, the swept fin is not greatly useful. The Comet was quite happy with a stubby, vertical fin.

Suseptability to spanwise airflow....some designs (Reynolds Number) more than others.

Oh, but it is, on aircraft with greater wing sweep (than the Comet, for example).
Two reasons.
Better yaw suppression and...if it was not swept, it would need to be either taller or broader, and in turn, the aircraft would have more difficulty meeting the certification criteria for positive spiral stability.

Many trade-offs in design.
Have a look at DP Davies book, 'Handling the Big Jets' as these issues (and a lot more) are explained in good detail.
Davies, more than any other one single individual, had a profound effect on certification for jet transport airplanes.

DP Davies was my God! We are not worthy! A very experienced test pilot and wonderful book he wrote too! Reprinted repeatedly. Along with First Light by Geoffrey Wellum, the only 2 books of aviation totally worth reading. Maybe that American author too flying back in DC3 days, but I was never really sure how much of that one was fiction.

Couldn't remember the name- 'twas Ernest K Gann's 'Fate is the Hunter'. Entertaining, but I felt I was reading a lot of embellishment.

'Stretch the truth' might perhaps be the best term.
As regards, Davies, I expect that if you were to bring up his name with many of the newby pilots...they would either never have heard of him...or...WTF did he know?:rolleyes:

3 books influenced my (working) life. 'Fly For your Life' by Robert Stanford Tuck, 'Handling the Big Jets' of course and the 3rd I'm too embarrassed to mention here!!

Even today during interviews I ask whether prospective employees have read any of these books. If they want to work within my organisation, a negative response is not good.

I still pull out my treasured 38 year old copy of Davies' 'Handling the Big Jets'. It contains stuff vital to know and understand, and describes techniques I follow religiously today. Stuff that is not even in Flight Crew Training Manuals. I have pored more over that book than I ever did of the Bible when I long ago thought there might possibly be something in this religious malarky before I realised it was all just paganist claptrap and blokes with white beards did not live in the sky!
I cannot conceive of a serious jet pilot not having thoroughly read and digested that book.

I'm pretty sure 411's 'spanwise flow' is a red herring in terms of stalling angle of sweptback wings - we really need JF here to correct us all, but I seem to recall that it was vortex generation at the L Edge, parallel to the LE, that re-energised the boundary layer and 're-attached' the airstream, thus permitting greater unstalled angles of attack, along with a widened buffet boundary (a lot of swept-wing fighters spend a lot of their time in manoeuvre in buffet and the BAC Lightning speed on approach could be controlled with a higher AoA producing more 'stall' buffet:eek:) plus, of course, an overall reduced CL/alpha slope.

Many variables at work, especially with early designs that needed wing fences and vortex generators for proper airflow control.
Boeing, at one time, produced a very nice discussion about all this, and might well be worth finding a copy....that is, if one were truly interested.
Or, one might travel to Arizona, and speak directly to a retired Boeing design engineer, who lives about a quarter mile from my present home...some enlightening listening...presuming of couse, if that desire to actually learn was still on the agenda....:rolleyes:

Yes - why don't you? Please tell us what he says about higher stalling angles with sweepback.

Already did, see reply #9.
Now, if you are asking about some exotic military design, have no idea about those, I only fly (and am interested in) civil types.

- actually that looks a bit like your ideas? - nope - actually TALKING about sweepback, pure and simple.

Was your 'line' on spanwise flow this Boeing man's explanation for higher stalling angles with sweepback or your attempt at explaining tip stall?? What is your understanding of vortex flow?

Yup.
Military aircraft are (or, can be) totally different.
I repeat, my only interest is in civil airliner designs.
I leave the military designs to others.

Talk to your optician - sheesh! How are you doing with vortex flow?