Twin Wasp

That's the big part, you can get yourself in a 25 knot window which might be ok if it's smooth but you never know.

Then there's the V1 cut, I've heard for CV-580s and HW-500s, you pull the good engine back. I've seen a low speed abort after an engine failure on a 747, I can imagine what a 70 series Eight would be like.

Cubdriver

There are many factors to consider just as with your questions with how many engines and engine mounting. First of all, I have to give the usual disclaimer here that you cannot answer broad technical questions like yours adequately on the internet, it just isn't the best venue for it. I recommend you run to your local aeronautical engineering school for a couple of years of aircraft design. We can't show graphs here, it takes too long to type all this, complex relationships are impossible to model, and so on. That's what books and classrooms are for. Nevertheless, I'll mention a few things to be a good sport. It's fascinating stuff after all.

1. Mach number goes up with altitude. A wing designed to stay under the critical mach number at FL350 is going to be at another location on the drag curve at higher altitudes. The wing has to be used at the altitude it was designed for, and this also has important consequences on low speed performance. It can be done, but as always tradeoffs will dictate the best situation. The best aspect ratio for a wing that cruises at FL350 (7-9) is much lower than one designed for FL600 (12-14). Such a wing would have trouble using standard length runways. There is no question of using a low-altitude wing at altitudes above its design regime. Same for engines, there are problems with mixing them up. You can't drop in a high-thrust engine and simply go higher, an engine has to be designed for high altitude operations. For example, compressors will stall at higher altitudes, as will wings. If getting out of the weather is the goal, it can be accomplished at FL350 and there is little need to go higher for transport purposes.

2. For subsonic aircraft using high bypass ratio (5) engines, thrust goes down strongly with altitude, and less so with increasing mach number. Going faster at higher altitudes does not offset losses due to lower density and air temperature. Thrust specific fuel consumption also enjoys little change with altitude, so you are not saving much jet-A. You can get more bang for your buck (thrust) out of a given engine design at lower altitudes, all other things being equal.

3. Too low is arbitrary. In fact, most airlines specify the cruise altitude they want when the order the aircraft they intend to use. They are mostly interested in getting out of the weather and saving money. This is easily accomplished at FL350.

Quite to the contrary, it would have inadequate landing and takeoff performance. "Low speed" is a technical term for the aerodynamics that are not affected by compressibility. This usually occurs below Mach 0.3. Higher speed wings must have sweep, multi-section tapering, washout, varied thickness, supercritical airfoils, and many other aerodynamic features in order to reduce drag and push drag divergence up to the high subsonic range (Mach= 0.82+). All of these techniques to a one are at the cost of slow speed lift. This is why these wings must have complicated high-lift devices such as slats and 2- or 3- section flaps to push the lift curve up enough for landing and takeoff.

Aspect ratio is not as strongly tied to speed as it is to efficiency. The Boeing B-52 has a high aspect ratio but it is very fast. A glider has a high aspect ratio wing, but is slow. High aspect ratios provide efficiency because they mimic the effects of 2-dimensional aerodynamics by separating wingtip vortices from one another. This leads to lower drag. However, due to the onset of critical mach number at higher speeds, either wing must be swept to delay critical mach number as much as possible.

This is true and it is due to higher altitudes having lower air density- the wing must fly at a higher angle of attack to provide the same lift. Induced drag is a byproduct or co-attribute of lift.

Composites offer many gains in strength, weight, formability, drag reduction, longevity, part count reduction, efficiency, etc. but are not being used necessarily for altitude gain. While they offer some strength gains that may permit the use of higher aspect ratio wings with slightly higher wing loading, which improves the ride and enhances cruise efficiency, the material does not do very much for slow-speed flight ie., landing and take-off. High-lift devices are still required for that. The term "low speed" is also an aerodynamics term to signify speeds that are affected by compressibility, above M=0.3.

With questions like yours I would seriously consider taking some aerospace engineering courses. Start with low speed aerodynamics, flight dynamics, and flight performance. When you get to about 25 posts you will have personal messaging ability at APC (PM), and you can ask me for more advice if you like.

Twin Wasp

You could, and then someone would just 'up' the engine. It's always a trade off. The 727 started with dash one engines, 14,000 pounds of thrust. It ended up with -219, over 21.000 pounds of thrust. There are a couple STCs to tweak the wing or add winglets but not a 33 percent change.

It's a lot easier to lift 80,000 pounds than 800,000.

tuna hp

Cubdriver,

Thank you very much for the knowledgeable responses. I am fascinated by flight but I don't think that aviation engineering is in my cards. I'm at university now but my graduate studies are going to be in law. I still very well could get in the business: aviation law and tort related to aircraft operation is a large industry.

If you don't mind, I have a couple questions about your responses.

So engines are designed for their mission, including the altitude of operation. That makes sense. An turbofan designed for FL500 cruise should be less fuel efficient in the lower part of the climb than one designed for cruise at FL350. I understand that some technologies are already in operation that should decrease this difference (variable geometry/ angle of attack turbine blades is one that I can think of) but it still makes sense that an engine should be designed around its mission including altitude. Given that you're going to design the engine and wing to fly at whatever your cruise altitude is, most of the technical information that I've been able to find on the subject seems to conflict with what you're saying.

What I have read is that for turbine engined airplanes, the higher you fly the better, only limited by structural considerations of flying that high (wing, also possibly pressurization). The idea is that thrust decreases with altitude because the air is less dense and has less oxygen. However, decreasing air density also reduces parasitic drag, to the extent that you can "always" design an airplane to fly more efficiently at X speed at a higher rather than lower altitude. Maybe the maximization of altitude has to do with diminishing marginal returns of added weight in structure to added high altitude lift. Is there something I am missing that determines that optimal cruise altitude of airplanes, other than structural considerations and practical consideration of operating a large airliner in and out of existing airports? I mean, I realize that a 777 size plane with aspect ratio 10 wings would have outrageously long wingspan that would prevent it from operating out of a lot of places making it impractical.

A good way for me to rephrase my question is: "is flying at some higher altitude "A" at some higher speed "V" aerodynamically equivalent to flying at some lower altitude "a" at some lower speed "v"?

Is the onset of transonic drag also relative to altitude/ air density?

Thats where I get the idea that an airplane fully designed to cruise at M.80 at FL500 should have better takeoff, climb, and approach performance, than one designed to fly M.80 at FL350. I understand that it could still use high lift devices to a benefit at the lowest speeds, but generally shouldn't the higher aspect ratio, lower swept wing outperform the lower aspect ratio wing during the relatively low and slow takeoff, climb, and approach phases?

You say that aspect ratio relates to efficiency more than speed, but isn't efficiency relative to drag, and isn't drag a function of both speed and altitude?

You example of the glider and the B52 seems to me to support what I'm thinking rather than counter it. They have high aspect ratio wings. The low-flying glider goes slower. The high flying B52 (50,000' max altitude) goes faster. In each case, the high aspect ratio wing is efficient. To me this says that B52 should have better climb and approach performance than a plane that is optimized to cruise at the same speed, but at a lower altitude. In other words, for the same reason that the high aspect ratio wing works for the glider, it should work for the B52.

An example I know of because of my interest in the question of 2 vs 3 engines are the Dassault 3 engine business jets (Falcon 50, 900, and 7X). Dassaults are known for their extremely low runway requirements and extremely efficient cruise performance, typically far ahead of their competitors in both respects. They are also known for their relatively high aspect ratio, low sweep wings. The latest model (7X) has a 70,000lb MTOW with which it can take off in 5,500' (with standard assumptions). It can then cruse efficiently very fast, in the M.85 to M.87 range, depending on load, towards its FL490 flight level. The aspect ratio is 9.7 and the inboard portions of the wing are swept 34*, outboard 30*.

Now the third engine should help with runway requirement, but from what people are saying it shouldn't necessarily help with cruise efficiency or climb performance. The plane uses ~30% less fuel than its competitors while flying just as fast, and its time to climb to initial cruise altitude from MTOW of FL410 is also competitive with its higher powered competitors, I believe it to be about 18 minutes. An explanation for the Falcon's performance could be that its wing, suitable for fast cruise at FL490, also provides good lift-to-drag ratios at the lower and slower portions of the flight. And also Falcons have very low approach speeds, 104 knots on the 7X, which I would think would only be possible if the wing was conducive to it.

Cubdriver

I am suddenly very busy and will try to get back to this soon... assuming no one else does first. Got to make a buck sometimes .

YAKflyer

Hey winglet.....

You forgot one important fact. For many, like me, the 727 was the very best jet they ever flew. I spent over 20 years with her in one seat of another because I just couldn't tear myself away. Finally I decided I needed to make some retirement money and spent 10 years on 767s & 777s, but the 72 has my heart.

III Corps

I got to fly one of the -200s with that config and a -100 with a -7 in the center and -217s on the pods. In both cases it was like adding a 4th engine to the Mighty Tri-motor and it was easy to see how one could ride it right out of the top of the envelop and well above what the wing wanted to do.

III Corps

That video was up for a while. Don't know if it still is but that was sad and ugly.

galaxy flyer

Four thousand hours in the C-5 taught me that military transports are designed for "roll-on, roll-off" and that is why they have high mounted wings--ours (US) and theirs (the Russkies). Military cargo is usually drive on, drive off; we need to have a low fuselage allowing easy unloading in austere airports.

Off prepared surfaces are almost never a deciding factor, there is just too much concrete around the world. We had no lack of it in Somalia, Iraq or old Zaire.

GF

flyboycpa

You might post this question over on the Corporate or Fractional side of the website as there are a lot of pilots flying the Falcon 50/900/7X (all "modern" tri-jets) that probably have recent twinjet currency, as well. Those would be a good comparison.

The trijets you are referring to (B-727s mostly) are fast, but old straight-jet technology. The more modern DC-10 and MD-11 you excluded from your sample because of the layout of the engines, so that's why I think you could compare efficiencies better using the Falcon series, as they are all still being manufactured, and as modern as they come, and all the engines in a little pile at the back of the airplane.

It should be pretty easy to find some comparisons, even with the twinjet Falcon 2000 (which, I think, has the roughly the same size fuselage as the 900 but with one less engine).