How does lift generate on lifting bodys?

[Elthy]

I tried to read a bit abut lifting bodys, but sadly there isnt much information about them on the internet. Im interested in the rules they follow because i want to design some myself, but when looking at different lifting body designs i wasnt able to find the features that define them. E.g. here:

http://www.nasa.gov/centers/armstrong/news/FactSheets/FS-011-DFRC.html

The picture over the article shows a completly flat bottom of the plane, while e.g. the M2-F1 is the complete opposite, it looks almost like a boat. This troubles me greatly, why does it stay in the air? It cant be compression, since the bottom seems to be round on purpose. Its not pressure difference due to different airspeeds on top/bottom, since the flat top would inverse the rule i know from normal airplanes, it should create negative lift...

[K^2]

That is a somewhat oversimplified view of lift. The reason lift is generated on any body is due to circulation. The precise statement is in Kutta-Joukowski Theorem. Circulation is generated in the air stream by combination of body geometry and angle of attack. A brick will generate lift given proper angle of attack. The main reason for all the curves on the wing is actually to reduce drag and increase the critical angle of attack, which allows for much better flight characteristic. Camber on a wing can also allow for positive lift with zero angle of attack, which may be convenient, but isn't strictly necessary. Many jet fighters have symmetrical wings and generate lift purely due to angle of attack. A body can also have a negative lift due to camber, and still have net positive lift due to high angle of attack. M2-F1 appears to be such a case. All of the pictures have it flying with significant nose-up attitude, which is probably necessary for it to generate lift. IIRC, such aircraft tend to have pretty lousy flight characteristics at high speeds, but allow for very low speed landings/takeoffs, making them of some interest to navy.

[Nibb31]

Note that lifting bodies do not really "fly". They are designed to "glide" down in a controlled manner. The slope of the "glide", and the rate of descent, typically ranges from "scary" to "pants-ruining". The only thing that saves your day in a lifting body is the ability to flare just before touchdown.

[prophet_01]

Lifting bodies create a lot of their lift by a typically high angle of attack (watch a video of a landing shuttle, the angle of attack is pretty impressive). To get a better idea of how that works look at capsules that reenter the atmosphere. Apollo for example also created lift by not flying perfectly straight through the airstream, but angled. The heat shield was jsed to create lift. It's typically used to stay longer in the high parts of the atmosphere which helps to reduce reentry temperatures as you reduce your velocity slower and over a longer periode of time.

Lifting bodies also create some lift like regular wings do. Compare the shape of a wing (or basically the concept drawings that pop up if you google 'lift') to a MiG-105 for example.

(From doing that kind of stuff in KSP with FAR) a lifting body usually tries to roll and fly upside down, because your center of lift is very low and often even below your center of gravity. In order to counter the resulting roll movement you can use big fins and/or the boat like belly to keep it stable. The space shuttle had a huge fin and also slightly upwards angled wings.

[Elthy]

Thanks for those great answers!

[K^2]

Note that lifting bodies do not really "fly". They are designed to "glide" down in a controlled manner.

That's what flying ever is. Applying thrust is equivalent to changing the angle of gravity, at which point, you just happen to be "gliding down" at a constant altitude.

[pincushionman]

Note that lifting bodies do not really "fly". They are designed to "glide" down in a controlled manner. The slope of the "glide", and the rate of descent, typically ranges from "scary" to "pants-ruining". The only thing that saves your day in a lifting body is the ability to flare just before touchdown.

Keep in mind, you can fly a barn door if you put a big enough engine on it.

[PB666]

Keep in mind, you can fly a barn door if you put a big enough engine on it.

Down here we call those tornados and hurricanes. Given that most aircraft do not fly over the speed of sound..

Some points.

Flight control. Simple example, you have a 747 at FL400 and you want to turn in a circle, you are roughly talking about 25km in a 30 bank turn. Why, 747 in level flight has one of the best glide characteristics do to its wing shape, it can almost approach the speed of sound, but it can't turn worth much at 400.

momentum, it GS is near 500 kts. IAS is in the upper 200s kts. It can slow down however at flight configuration it would stall, and fall.

So just before landing, say 20 km aircraft are expected to make 30 bank turns changing about 30 degrees.

So as the 747 descends it begine to lower flaps, this makes the wing much less able to glide at high speed but much more capable of flying at lower speed. Flaps increase the differential distance air has to travel over the top of the wing. This increases lift, but in a turn it also increases the change of angle per volume of air traveled through, because turnig is steered from the tail and produce from the wings.

As a result the plane can turn full circle over a few kilometers.

Landing. Most planes could survive a landing withou flaring, but not at flight level speed, 170kts and traveling 1000 feet per 3 nauticle miles they are traveling downward at 17 feet per second. Contact the insurance company afterward. If the craft was traveling at flight level configuration, that would be 265kts, thats 26kts -verticle and you have airframe collapse on the runway as well as a wreckage that flies off the runway. Flaps are responsible for flight control at low speed, they allow the AC a nose level on approach an a tiny flare puts the front gear just a little bit off the runway so that a moment after wheels down the thrust diverters can be deployed.

So to correct these. The shape of the wing is adjusted during the flight to make it more or less like a plane for the purpose of flight control. If you need lift at low speed bernoulli is your friend, if you need efficiency at high speed low profile into the angle of attack is your friend. if you want to turn fast lots of control surfaces are going to need to be deployed.

[wizzlebippi]

As a general guideline, the round fuselage of many civilian aircraft produces as much lift at the section of wing it obscures. The catch is the fuselage has a much lower aspect ratio than the wing, making it produce more drag per degree AoA. A lifting body takes this to the extreme.

[K^2]

Simple example, you have a 747 at FL400 and you want to turn in a circle, you are roughly talking about 25km in a 30 bank turn.

Any aircraft traveling at 500kts doing a 30° turn, which is max bank set by FAA for commercial flights, will make a circle nearly 25km in diameter. This has nothing to do with flight characteristics of an aircraft, though. You can't make a tighter turn at that speed without increasing the bank angle.

[PB666]

Any aircraft traveling at 500kts doing a 30° turn, which is max bank set by FAA for commercial flights, will make a circle nearly 25km in diameter. This has nothing to do with flight characteristics of an aircraft, though. You can't make a tighter turn at that speed without increasing the bank angle.

True because of what level flight near coffin corner requires. At that alt the IAS is near the minimum in trim flight to support the craft, the engines cannot produce much more thrust, they are often close to the N1 limit. Increasing flap would certainly allow a slower craft but dramatically increase drag, which a slower craft has less engine output, which means stall is more likely. The point though is that at FL400 and 500kts Flight control is heavily restricted by a performance application. If there was a runway at 39,000 feet the craft could not land because a precision turn would be impossible, it could not make a proceedure descent 6nm out. and it would hit the runway at 52 fps. dare say the wheels would prolly explode if the a/c flared in time.

There are craft that could do this, not commercial, but have alot more wing and thus fly much more slowly

I should also make the point in that increasing the crossectional curvature of the wing does not increase lift indefinitely, on many commercial craft the last one or two flap settings do little more than increase drag and make it somewhat easiler to stop a heavy load. The V-land is set pretty much by weight and assumes that the optimal flap is engaged.

[LN400]

Someone mentioned that given enough speed, a brick can fly. Case in point:

Speed and AoA can keep pretty much anything flying.

[K^2]

The point though is that at FL400 and 500kts Flight control is heavily restricted by a performance application. If there was a runway at 39,000 feet the craft could not land because a precision turn would be impossible, it could not make a proceedure descent 6nm out. and it would hit the runway at 52 fps. dare say the wheels would prolly explode if the a/c flared in time.

That's just Reynolds number doing its thing, though. If we design an aircraft to fly at 250kts clean at sea level, it will need to fly at 500kts clean at 36k feet, because dynamic pressure needs to stay the same. Consequently, former is the typical early approach speed, while later is minimal cruise. Again, not a whole lot you can do here in terms of aerodynamics. If you wanted to design an aircraft for slow maneuvering at FL400, you absolutely could. It'd be extra slow and clumsy at sea level, however, which isn't what you want with a commercial aircraft.

[PB666]

That's just Reynolds number doing its thing, though. If we design an aircraft to fly at 250kts clean at sea level, it will need to fly at 500kts clean at 36k feet, because dynamic pressure needs to stay the same. Consequently, former is the typical early approach speed, while later is minimal cruise. Again, not a whole lot you can do here in terms of aerodynamics. If you wanted to design an aircraft for slow maneuvering at FL400, you absolutely could. It'd be extra slow and clumsy at sea level, however, which isn't what you want with a commercial aircraft.

Yeah, but turbofan AC are not really designed to run clean well at 250kts MSL. The heavies tend to barrel up to the <FL100 speed limit until they supercede 10k and speed up past 290 kts IAS. The engines are burning hot air at that altitude, they lean out alot as they climb, they are producing less thrust as they climb past 180 but burning much less fuel. The worst example was the concorde, Not a turbofan but it is barely off the runway at 200 kts with afterburners and is suffering to get past fl100 at 250 kts, but get a little more air under the wings and cool it down a bit and its acceleration all but doubles. That poor ac was an accident waiting to happen it really needed a 4nm runway for abort takeoff at v rot.

Of course there are not too many good reasons to make a tight bank turn at FL600 at mach 2.0

[K^2]

We are still talking about things that are built that way because that's the conditions under which they are being used. None of these are limitations of a particular lifting surface configuration or engine type. A fixed prop will also only work well at certain air speeds. But these can be anything from an ultralight trike speeds to transonic high-altitude transports and bombers. And you can build a biplane that'll handle a reentry from orbit if you were set on it. So what I'm really missing is how any of this factors into discussion of lifting bodies. Especially when we clearly see lifting bodies used both for hypersonic flight and for very slow takeoffs/landings. These are very differently looking lifting bodies, but you kind of expect that.

[WestAir]

Elthy,

(...) Im interested in the rules they follow because i want to design some myself (...)

That's awesome! I assume you mean you want to design one to build as an RC aircraft? I'm obviously jumping to conclusions based on your vague statement, but if that's what you mean then best of luck to you. I've personally always intended to design and build my own little RC plane but due to time constraints (or laziness?) I've never found a good time to stop and give it a try.

(...) but when looking at different lifting body designs i wasnt able to find the features that define them. (...)

Picture a flying wing; A lifting body is the exact inverse of that. Interestingly both are equally challenging to design and build.

(...) why does it stay in the air? (...) since the flat top would inverse the rule i know from normal airplanes, it should create negative lift...

That's a pretty expansive question. The cross section of an airfoil has everything to due with its purpose. You'll find most aircraft have wings cambered or curved on the top, and the rear wings (horizontal stabilizers) cambered on the bottom. The reason is because aircraft are generally designed to have the center of lift behind the center of mass. This creates a torque or pull that would generally send an aircraft into a nose-down spin - the negative lift from the stabilizers at the rear help cancel that out. (Though in my personal experience some companies design planes so that help is needed in that department. ~3 weeks ago a buddy of mine had to maintenance ferry an airplane from Tuscon AZ to Mesa AZ, and since the aircraft was empty of fuel bags and people it was so tail heavy they had to round up about 900 lbs of junk to throw in the front galley before they could legally fly. I guess they designed the AC to be nose heavy only when loaded)

Back to your design, I personally would build a lifting body RC as a sort of very thin-width flying wing because it appears a lot easier to put flight controls on a wing than a fuselage, especially if you are in fact designing a small RC. It's also easier to see what orientation the aircraft is in from the ground when it's shaped more like an airfoil and less like a cylinder. I'd be extremely interested to see what you design. I find the most interesting (and difficult) part of designing any non traditional aircraft is the placement (or lack thereof) of stabilizers and flight controls. In a lifting body you'll want a way to stop the plane from rolling around and you'll certainly want a way to stop it from becoming a propeller driven missile. To this I'm wondering if you shouldn't look into a design more like a stubby Space Shuttle and less like the Fat Man atom bomb with engines.

(...) Landing. Most planes could survive a landing withou (sic) flaring, but not at flight level speed, 170kts and traveling 1000 feet per 3 nauticle miles they are traveling downward at 17 feet per second. Contact the insurance company afterward. (...)

I'm not sure what this has to do with lifting bodies, but to be perfectly fair, this statement is only accurate under the assumption you're trying to perform a "standard" descent. My guess is that if your runway is long enough you could make a shallow approach and landing to a runway even if that runway were at 36,000 feet. A better pilot could keep it at 50 fpm until the mains touch. Depending on where you are in your cruise and how much you weigh, chances are you're already well pitched for a flare - ground effect from the runway will do the rest. I don't imagine such a landing would use flaps, and I don't imagine the gear would survive the roll speeds at the air pressures and temperatures involved, but as K^2 said, if it can't be done by the aircraft you're trying to use, someone could design one that could.

Luckily for us the OP is in the designing stage.

Edited November 12, 2015 by WestAir

[Elthy]

I dont plan to build an RC plane, its more about designing an reusable hydrogen upper stage. Not to be build, just as a feasibility study. The idea is:

A reusable heavy launch vehicle, that can get 100t to orbit. To do this i thought about an kerosene or methane lower stage with a flight profile of the Falcon 9R, it would be similar in performance to the Saturn V first stage. For the upper stage hydrogen would be ideal, not only because of the high efficency, but also because of the low density. This would result in a very large upper stage with extremly low ballistic coefficient, which would result in lower reentry temperatures (Skylon is supposed to work that way, according to wikipedia it would only heat to 800°C) so a lightweight, easy to maintain heatshield would be enough. This second stage would be about as large as an space shuttle external tank, with engines similar to 4 SSMEs. A dry weight of 100T should be possible.

The question would be how to build the tanks so you get the best compromise of good tank weight and flight characteristics on reentry...

[PB666]

yes but from a level flight to dro 2000feet using the 3nm per 1000 feet, the standard final approach, the craft would have so much momentum at 500 kts that by the time it reached the OM it would have past the threshold before it began a safe descent vector. You could anticipate the outermarker by a mile, and the same for the threshold, but that is not a precision landing.

[WestAir]

Very true. My only question is why a crew would attempt a precision landing when already unstable due to speed. At that point, if you HAVE to approach at vref+100, why not just perform a shallow approach?

Edited November 13, 2015 by WestAir