Ask The Pilot

[RSD]

Like everything in aviation, there are always competing priorities when an aircraft is being designed. Twin tails have some structural advantages, as well as being lower for the same cross sectional area (a plus on aircraft carriers). But, the reason that sticks in my head, from Pilots’ Course aerodynamics, relates to the coefficient of lift across mach numbers. As an aircraft increases speed, the CL remains about the same up until about mach .9. Then it decreases rapidly. Once the aircraft gets past mach 1, the CL increases again, but only to about 60% of its original value. The upshot of that is that the tail is less effective, and so the aircraft could be directionally unstable at higher mach numbers. Some SR71s took that to an extreme, by having a third fin, that folded down at high speeds.



Basically because they want the space where it is normally for something else. Generally engines.

Cheers JB747! If this thing ever gets anywhere near mach 1 then either -
a) we are design geniuses (unlikely), or
b) we have really screwed up and the nose is pointing straight done (lets hope not!)

 

[jb747]

Cheers JB747! If this thing ever gets anywhere near mach 1 then either -
a) we are design geniuses (unlikely), or
b) we have really screwed up and the nose is pointing straight done (lets hope not!)

Which thing are we talking about? There are lots of other reasons for twin tails, but mach 1 plus is by far the sexiest answer.

 

[trooper]

Have to ask an engineer. I do vaguely recall discussion about such system in the aircrafts’ early days, but it isn’t something that has relevance to the coughpit, so it wasn’t in our manuals.



It doesn’t really....as long as any flex is allowed for in the design of the ailerons.

Ailerons are a somewhat poor flight control. Because the system is split across the left and right wings, one will always go up, and the other down. The down going aileron increases the chord of the wing, which has the effect of making that part of the wing stall at a lower angle of attack than the rest of the wing. The side that’s trying to increase the lift also makes more drag, which means that the ailerons create differential drag across the wing, which tries to yaw the aircraft, but in the opposite direction to what is desired.

Boeing doesn’t have ailerons? On what aircraft? On the 747, they had inner and outboard ailerons. The outboards were locked out above about 200 knots. I had a set fail to unlock once, and the inners plus spoilers gave plenty of control.

The tendency of ailerons to cause earlier stall, and adverse yaw, is one reason that aspiring pilots are taught to never use aileron in a stall recovery. The alternative flight control is the spoiler. Airliners use them as well for roll control. In some cases when large amounts of input are needed, and at other times depending upon configuration. As spoilers work by dumping a small amount of lift, their behaviour is always predictable. The yaw is in the correct direction, and they can be used at any angle of attack...even well into the stall.



They differ on just about everything. They both have good ideas, and bad. It took me a while, but I did come to appreciate the AB way of doing things though.

I'd LOVE to see a 747 with WING WARPING!!!! LOL

(Sorry to quote an older post )

 

[RSD]

Which thing are we talking about? There are lots of other reasons for twin tails, but mach 1 plus is by far the sexiest answer.

A part homebuilt that we are playing around with - using a fuselage and wings that we picked up cheap and then making some major changes to the rear end to increase its sexiness

 

[sydks]

And why would an aircraft designer move a horizontal stabiliser to the top of the tail instead of the more normal location?

Basically because they want the space where it is normally for something else. Generally engines.

It appeared as though Concorde didn't have a horizontal stabiliser. Assume this was due to her unique wing?

Is she the only passenger jet to date without one?

 

[jb747]

It appeared as though Concorde didn't have a horizontal stabiliser. Assume this was due to her unique wing?

Is she the only passenger jet to date without one?

Tail-less delta. Quite a common military configuration, but generally not on very large aircraft. Vulcan and Space Shuttle used a similar configuration. Delta winged aircraft with tails are rarities, but that was the configuration of my beloved A4, and the contemporary Mig 21.

Deltas are very good high speed wings, but they do have some interesting characteristics at high angles of attack.

 

[straitman]

Tail-less delta. Quite a common military configuration, but generally not on very large aircraft. Vulcan and Soace Shuttle used a similar configuration. Delta winged aircraft with tails are rarities, but that was the configuration of my beloved A4, and the contemporary Mig 21.

Deltas are very good high speed wings, but they do have some interesting characteristics at high angles of attack.

This comes down to the differences between subsonic and supersonic aerodynamics and there are quite a lot of them.
For example the Mirage flew well at high speeds but more like a brick with an engine attached at lower speeds. It had a landing speed around 180 kts with quite a bit of power required.

 

[Quickstatus]

Doesn’t Delta wings need high angle of attack at low speed to maintain lift, but this comes at increased drag - which fits in with what @straitman observes?

 

[jb747]

Doesn’t Delta wings need high angle of attack at low speed to maintain lift, but this comes at increased drag - which fits in with what @straitman observes?

In many ways, a delta is simply a normal swept wing, given an extreme angle of sweep.

Sweep angle on airliners is in the mid thirty degrees area. As you increase the sweep, you delay compressibility effects (transonic), and so allow for faster cruise speeds. At the same time though, you decrease the amount of lift gained for any given angle of attack change, and increase the structural issues. If you aren't going any faster than .9 mach or so, there is no point in having a wing designed for mach 1.2.

Delta configurations basically start at about 45º of sweep, and move up from there. The Mirage was, if I recall correctly, 60º, and the A4 was 45º. They have a lot of wing area compared to their span (which means more space for fuel, and a better wing loading). Structurally, they are exceptionally strong.

Downside really just goes with the sweep. Again, you need more pitch change for a given AoA change. Their stalling characteristics are gentle...in fact they may not have a classic stall, but rather keep letting you get more and more AoA, whilst giving decreasing amounts of lift, and more drag. That's not necessarily a bad thing in a fighter, but undesirable in a civilian aircraft.

Landing speeds are not necessarily all that high, just because they are delta. There's a huge difference between tailed, and tail-less use. In an aircraft with a conventional tail (or a canard for that matter), the wing can have the usual high lift devices (slats, flaps), which will allow lower speed operation. But, in a tail-less configuration, the use of such devices is restricted, as the trailing edge of the wing will carry the control surfaces for roll and pitch...and any use equating to flaps would give a pitch input. So, whilst the Mirage, with no high lift devices at all, had an approach speed of around 180 knots, the A4, with slats and flaps, was back around 125 knots.

Another issue that will affect landing speeds is geometry. You don't want the nose so high that the pilot cannot see the runway.....and you also need to avoid tail strike. Both issues give a practical maximum to the pitch angle for the approach and landing....and for a given pitch angle, there will be a corresponding AoA/speed.

There is a bit of discussion about sweep angle here: What is the optimal wing sweep angle for a passenger airliner?

 

[Quickstatus]

This reminds me of the SST Concorde
20deg for TO and 12 deg for approach and TO speed over 200kt. Control surfaces at rear for pitch and roll. No high lift devices such as flaps and the movable nose was required for the high pitch requirements during TO and approach not just because of the pointy nose.

Differential airflow over an airfoil contributes to lift, but I see a lot of pictures where there is a lot of turbulence on the upper side. I don’t understand why this contributes to lift. I’m certain that’s why golf balls have dimples but I don’t understand...

 

[NM]

So, whilst the Mirage, with no high lift devices at all, had an approach speed of around 180 knots, the A4, with slats and flaps, was back around 125 knots.

I guess that is one reason why the Mirage was not used on carriers.

Another issue that will affect landing speeds is geometry. You don't want the nose so high that the pilot cannot see the runway....

Especially when its a steel runway that is both moving and very short.

Would the F4 Phantom be considered a delta with with tail?

I guess this is why aircraft like the F14 Tomcat and F111 had variable sweep wings - best of both worlds (sub-sonic and super-sonic performance characteristics). The F14 was obviously designed with carrier ops as an objective. While the F111B did perform some carrier tests (USS Coral See in 1968), I believe the F14 killed off the opportunity for the F111B production for the USN.

Did the A4's delta wing make it especially trick for carrier ops due to high AOA and/or approach speed? Or were these mitigated by the slats and flaps you mentioned previously.

 

[jb747]

I guess that is one reason why the Mirage was not used on carriers.

Especially when its a steel runway that is both moving and very short.

Carriers are all about minimum energy at the deck. The difference between 180 and 150 knots (for the same weight) is getting close to 50% extra energy. A tailless delta planform just wouldn't work. On the other hand, Rafale seems to work perfectly well, but it is a canard configuration.

Would the F4 Phantom be considered a delta with with tail?

I'm not sure that the F4 would be considered anything but a hodge podge. I don't know if there's an arbitrary cutoff, that makes one wing an X whilst another is a Y, but most of the aircraft that we recognise as delta winged have zero sweep on the trailing edge. So, the F4 is more of a very thick swept wing design. The anhedral on some surfaces, with dihedral on others, makes it unusual. I think you need to jump to the Harrier to see so many varied angles.

I guess this is why aircraft like the F14 Tomcat and F111 had variable sweep wings - best of both worlds (sub-sonic and super-sonic performance characteristics). The F14 was obviously designed with carrier ops as an objective. While the F111B did perform some carrier tests (USS Coral See in 1968), I believe the F14 killed off the opportunity for the F111B production for the USN.

Robert McNamara was the US defence secretary for most of the '60s. His background was business management. He decided that having the USN, Marines, and USAF developing aircraft for their own needs was wasteful, and forced a consolidation of purchases across the military. With some of these he was quite successful, as both the F4 and A7 lent themselves to shore basing. But, conversely, taking the F-111 and trying to make it a carrier aircraft was a disaster. It was a bomber, notwithstanding the F nomenclature, and although the F111B actually made it to sea trials, it would have been a horrid choice. The engines and weapons system were kept, and a new airframe was wrapped around them....which gave us the F14.

Did the A4's delta wing make it especially trick for carrier ops due to high AOA and/or approach speed? Or were these mitigated by the slats and flaps you mentioned previously.

The A4 was designed using some principles that seem to have been forgotten today. Keep it light. Small. Build in simplicity, not complexity. It is an extremely agile aircraft. Its approach speed was high for some of the small carriers it was used on, but for the USN with its large ships, it was probably the easiest of the jets to operate at sea.

 

[Franky]

We're going back to the 60s from memory here JB, though the F111 entered service known as that, during its development phase it was named TFX (Tactical/Fighter/Reconnaissance), whilst the British RAF version was known as TSR2 (Tactical/Strike/Reconnaissance). I believe that once Australia opted for the TFX, the British model was cancelled due to lack of potential sales. Once the swing wing issues were sorted on the F111, I understand that the RAAF pilots had great respect for the aircraft, which had a fairly long period in service.

 

[ozfflyer]

have flown into Mt Hotham airport a few times in winter, including commercial Qantas flights which are no more.

Have also flown into Queenstown NZ dozens of times in winter including once we did a go around seemingly only a few metres off the ground & sat in aircraft waiting for cloud/fog to lift.

What are the biggest problems pilots face getting into such airports?

 

[jb747]

We're going back to the 60s from memory here JB, though the F111 entered service known as that, during its development phase it was named TFX (Tactical/Fighter/Reconnaissance), whilst the British RAF version was known as TSR2 (Tactical/Strike/Reconnaissance). I believe that once Australia opted for the TFX, the British model was cancelled due to lack of potential sales. Once the swing wing issues were sorted on the F111, I understand that the RAAF pilots had great respect for the aircraft, which had a fairly long period in service.

Most aircraft have varying names during their development phase. The TSR2 was a totally different aircraft, which was a very different take on things compared to the F-111. Whilst the F-111 matured into a decent bomber/recon/electrical warfare aircraft, I doubt that the TSR had anything like the same potential. Nevertheless the Brits were doing very interesting things in aviation in the 60s.

But, the F-111 was never any sort of fighter, and that's more or less how it got its Australian name...the Pig...'cos it turned like one. Very fast though.

 

[tgh]

A few years back a folk singer friend wrote a song about the pig that actually got a bit of traction.
We sang it everywhere because folks loved it..

The F-111

Words: Lyell Sayer Tune: Traditional (Johnny Lad) Now, Mr Robert Menzies was walking down the street, And thinking of our airfor...

ozfolksongaday.blogspot.com

 

[jb747]

have flown into Mt Hotham airport a few times in winter, including commercial Qantas flights which are no more.

Have also flown into Queenstown NZ dozens of times in winter including once we did a go around seemingly only a few metres off the ground & sat in aircraft waiting for cloud/fog to lift.

What are the biggest problems pilots face getting into such airports?

Low level go arounds are not a big deal.

Aircraft and nav aids are both getting smarter, so there are automated arrivals for places like Queenstown now. But, still no ILS, nor GLS, so fog will still defeat you. It's surrounded by large hills, so you aren't going to get a nice straight finals of any duration. Those same hills will probably provide quite a bit of turbulence in the right (or wrong) conditions.

I expect that Hotham had limited facilities across the board...as well as being up in the clouds/mountains. Not the sort of place I want to fly to, in any seat.

Approach aids are very limited at any airfield in Oz that isn't a major one. GPS helps, but is not a cure all. ILS is rare, even at RPT airfields (like Mildura).

 

[Franky]

Most aircraft have varying names during their development phase. The TSR2 was a totally different aircraft, which was a very different take on things compared to the F-111. Whilst the F-111 matured into a decent bomber/recon/electrical warfare aircraft, I doubt that the TSR had anything like the same potential. Nevertheless the Brits were doing very interesting things in aviation in the 60s.

But, the F-111 was never any sort of fighter, and that's more or less how it got its Australian name...the Pig...'cos it turned like one. Very fast though.

And loud - during one of my postings to NAS Nowra ('75 I think) we had a joint exercise with the RAAF and had a couple of F111s there for the period. Not sure if they were just showing off, but when they planted the foot on take-off, they made every building on the base shudder. Scary things.

 

[ChrisGibbs]

Was on Friday afternoons 10 May QF432 1230 service MEL-SYD. Our departure was delayed by about 30 minutes due to the late arrival of the inbound service from Brisbane. We pushed back at around 1300 and the airport was then closed due to a storm over the airport with all arrivals and departures being halted. We eventually got going around 1400 off RWY16 with an unusual left hand turn after departure and out over the city.

While waiting to depart I was tracking the QF682 service from ADL-MEL. From the looks of things it was over a 3hr sector for them with what looked to be a couple of missed approaches onto RWY16. When planning an ADL-MEL sector would you typically take on that much fuel for that much holding?

 

[tuppaware]

I see old de Crespigny has released a new book.
You going to grace our book shelves soon JB747 ?