Aero 101 - ask your burning questions here

[Achilles]

I'm a long time reader, very little poster and I was just reviewing some material for the ASTB and I had a random question. I know that for supersonic or transonic flight an aircraft uses a stabilator to control pitch. I semi-understand that this is due to a loss of controls in the elevators in high speed flight due to the shockwave moving to the trailing edge of the elevators (if this is too dumbed down I apologize, not an engineer). My question is why isn't the vertical stabilizer required to move as a whole similiar to the horizontal stabilizer on a stabilator? Is the vertical stabilizer not affected by the shockwave like the horizontal stabilizer? Is this because the horizontal stabilizer produces a certain amount of lift while the vertical doesn't produce any? Sorry if the answer to this requires an engineering degree but I was just curious when I was reading instead of writing a paper. Also if you can keep the answer as easy a read as possible for the uniniated, just because I can ask the question it doesn't mean I can understand the answer.

 

[HeyJoe]

....just because I can ask the question it doesn't mean I can understand the answer.

Now, that's funny and a keeper for future use!

I'll let one of the aero wizards confuse you with the answer you seek.

 

[ftrooper]

I'll give this a shot, but I'm definitely no engineer. It looks like you may have a couple things confused. An Aircraft with all moving horizontal stabs (the flying tail of X-1/ F-86 Fame and all supersonic aircraft since) doesn't have elevators, it has stabilators. Said named stabs will also provide roll control at transonic-supersonic speeds, as shockwaves will have bad effects on ailerons at those speeds(wing warp, reversal of controls, etc). The vertical stab is affected at high speeds by the same forces, however, yaw control is not used to the extent of a pitch authority at transonic or supersonic speeds, and so an all flying vertical stab is unnecessarily heavy and complex. Think of it this way, at 600+ knots, you can still pull the max G authorized in most fighters. You will NOT step on the rudder and yaw it as hard as you can, as you will depart controlled flight. There is no purpose in designing an aircraft to perform that manuever, and making the rudder more effective would only exacerbate the high speed departure. Clear as mud?

 

[ftrooper]

Also, I think the RA-5 Vigilante had an all moving vertical stab, but I don't know why or if that affected it's flying qualities. From what I've read it was a big, fast, and very tricky S.O.B

 

[Schnugg]

Have done many high speed FCFs in the F-14A where I've had a mach lever fail to hold an engine at speed during a mach run throttle back.

Engine stalls and bangs like someone hitting the side of the jet with a sledge hammer...jet yaws ever so slightly, but yaw SAS keeps it pointing into the wind quite nicely. So it works with a fixed tail and moveable rudder surface. Plus, 3000 psi of hydraulics helps.

Never had an engine cough like that with an F-14B/D on a mach run. Did have one eat it's high pressure turbine on an FCF.

 

[MIDNJAC]

Also, I think the RA-5 Vigilante had an all moving vertical stab, but I don't know why or if that affected it's flying qualities. From what I've read it was a big, fast, and very tricky S.O.B

So did the X-15, although thats an entirely different proposition

 

[Achilles]

I'll give this a shot, but I'm definitely no engineer. It looks like you may have a couple things confused. An Aircraft with all moving horizontal stabs (the flying tail of X-1/ F-86 Fame and all supersonic aircraft since) doesn't have elevators, it has stabilators. Said named stabs will also provide roll control at transonic-supersonic speeds, as shockwaves will have bad effects on ailerons at those speeds(wing warp, reversal of controls, etc). The vertical stab is affected at high speeds by the same forces, however, yaw control is not used to the extent of a pitch authority at transonic or supersonic speeds, and so an all flying vertical stab is unnecessarily heavy and complex. Think of it this way, at 600+ knots, you can still pull the max G authorized in most fighters. You will NOT step on the rudder and yaw it as hard as you can, as you will depart controlled flight. There is no purpose in designing an aircraft to perform that manuever, and making the rudder more effective would only exacerbate the high speed departure. Clear as mud?

I think I have a decent grasp on it. Basically at higher speeds not as much yaw control is needed as pitch control so while a stabilator is needed for pitch control no additional modifications are needed for the rudder. Does using the rudder at that speed present a problem of producing adverse yaw or does the computer help/prevent this? From my understanding adverse yaw comes from too much rudder usage or not enough rudder usage during a bank, if that's completely wrong please correct me.

Schnugg - What is an FCF? Forgive the ignorance as I can't search
acronyms. Is SAS the computer responsible for the fly by wire controls?

Thanks for all the answers and help.

 

[MasterBates]

FCF- Functional Check Flight. A post-maintenance check where thing can and do go wrong with much greater frequency than normal flights.

SAS- Stability Augmentation System. Can't speak to Fly By Wire, but in the 60 it helped damp out the instability of the helo, while being able to "fly thru" it. Gyros, computers, and actuators. Can be analog or digital. The Sh60B had one analog SAS (SAS1) and one digital SAS (SAS2).

SAS 3 is an organic SAS activated when copilots start to suffer from cranial rectal inversion on approach or over the deck.

 

[Achilles]

FCF- Functional Check Flight. A post-maintenance check where thing can and do go wrong with much greater frequency than normal flights.

SAS- Stability Augmentation System. Can't speak to Fly By Wire, but in the 60 it helped damp out the instability of the helo, while being able to "fly thru" it. Gyros, computers, and actuators. Can be analog or digital. The Sh60B had one analog SAS (SAS1) and one digital SAS (SAS2).

SAS 3 is an organic SAS activated when copilots start to suffer from cranial rectal inversion on approach or over the deck.

Methinks I confused fly-by-wire with stability control. I thought fly-by-wire was an electronic computer which gradually adjusted small flight variations for stability while also "translating" pilot control inputs into changes on the flight control surfaces? Sorry if I'm playing 20 questions here but I'm bored at work and was curious.

 

[insanebikerboy]

Isn't yaw in a turbojet/fan not as much of an issue since there isn't p-factor associated with the fuselage? Phrased another way, I thought thrust in a jet was almost entirely down the longitudinal axis and very little across the lateral axis.

Caveat: obviously if an engine is lost, yaw becomes an issue, but I can't imagine a jet going supersonic if on one engine when designed for two.

 

[Catmando]

Isn't yaw in a turbojet/fan not as much of an issue since there isn't p-factor associated with the fuselage? Phrased another way, I thought thrust in a jet was almost entirely down the longitudinal axis and very little across the lateral axis.

The yaw comes not only from the asymmetric thrust of the remaining engine in burner, but more importantly at higher supersonic speeds, the tremendous and instantaneous drag on the other engine as the variable inlet slams full open and supersonic air and shockwave hits the fan.

As Schnugg correctly said, it's "like someone hitting the side of the jet with a sledge hammer." It's a simultaneous big bang, yaw, and deceleration that I never got used to, although it was not uncommon during FCF's so I could expect it happening.

It was even a bigger bang in the F-4 because the variable inlet ramps and engine were closer to the cockpit than the F-14. Also quite interestingly, there was much greater yaw in the F-4 than the F-14 when it happened, even though the F-14 engines are further apart. This was probably due to a better SAS in the F-14. (Indeed, the FAA considers an F-4 centerline thrust but the F-14 asymmetric thrust)

Regardless, the yaw in each while it got your attention, was not all that severe and certainly controllable. A moveable vertical stabilizer was hardly needed. A little rudder did just fine. I think most aircraft are inherently extremely stable in yaw, and obviously far more so than in pitch or roll.

 

[insanebikerboy]

Are you talking about losing an engine? I was referring to normal flight with both engines operating normally, that there would be (I think) very little yaw, hence the lack of need for a rudder similar to a stabilator.

Of course, it has to be big enough to handle the yaw from a loss of an engine, but can a dual engined aircraft still maintain supersonic speed in the event of one engine loss?

 

[Catmando]

... but can a dual engined aircraft still maintain supersonic speed in the event of one engine loss?

Yes, but why? The restart envelope is far below the "number".

As for rudders.... we used them every day, especially for ACM when they were in near constant use, and full-throw.

 

[insanebikerboy]

^^^ Ok, that's what I was getting at/asking, thanks.

 

[Death Rattler]

One item that hasn`t been discussed here is the fact that rudder throw is restricted at higher mach numbers, either by a bellows, computers or pfm.