Horizontal flight without wings

brenan

Just curious as I saw a movie of one on TV
I'm trying to understand how a cruise missile can fly?

I can understand action v reaction in a rocket pushing against the mass of the earth
I can understand winged flight and air pressure holding the craft up
I can inderstand a hovercraft using air pressure or a helicopter generating upthrust
but I dont see how you can make an object fly long distances without the upthrust of an engine or wings.

My guess is they have to be permanently in a controlled fall ? Would that make sense?
If so what is the thrust pushing against?
And if so how would you calculate how to make such an object fly?

cjl

Cruise missiles do have wings. Some of them have very small wings, but they all fly by pretty much the same set of principles that aircraft use.

brenan

Ah - I didnt realise.
The one I saw was flying very low and not very fast and the "wings" were so small
I would have described them more as fins than wings.
If its normal aircraft flight how are they carrying what must be quite a load
(propellant, engine, explosive etc) with such small "wings"
I'm confused because it just doesn't look right to me.

cjl

A large part of it is because they don't need to take off or land the way an aircraft does. Therefore, they can use smaller wings (which means that they need to fly very fast to stay in the air). A normal aircraft needs to be able to fly at 150-200mph or so when landing, while even a slow cruise missile never needs to fly any slower than 400-500 mph. This means they can get away with a fraction the wing area (specifically, you could fly on a quarter the wing area if you're going twice as fast, since dynamic pressure scales with v², though admittedly once you approach supersonic speed it gets a little more complicated). Which cruise missile in particular are you referring to, out of curiosity?

berkeman

There is also the concept of "lifting body", although cruise missles do not use this in their design:

http://en.wikipedia.org/wiki/Lifting_body

.

brenan

I'm not sure of the the name of the one I saw but a similar flight would be
the footage of the one shown on the TV news during the attack on Iraq.
That one was not much higher than the buildings or so the camera angle made it appear.
It was one of those "double take" moments when you see something that
just doesnt look right and you go "huh?" internally.

brenan

Thanks for the link - One thing I note here is that all the lifting bodies listed
seem to be large area wing shapes rather than the more tubular shape of
the cruise missile. That story about the one winged landing of an F-15 is
pretty amazing though. I bet that pilot would be dead today if he was using fly by wire.

brenan

Ok I've found some full design specs for the tomahawk cruise so I'll have a read
through those and see if I can understand this better.
Essentially it is a 3000lb tube with fins that is accelerated to 550mph to a max height
of 1000 ft.
Right off I can see it's initial acceleration seems similar to a handgun bullet this will
then be maintained by its rocket engine. I'm still unsure about using the term
"flight" but I'll read on.

etudiant

The Tomahawk cruises at about 600 mph thanks to a small built in jet engine.
The rocket booster is just to kick it out of its capsule to the speed and altitude needed before the jet engine takes over.
There are real wings, folded back under the belly for launch and deployed after the booster cuts out.
Faster missiles such as the SRAM by contrast did not have such wings, just some fins.
Of course, they did not have 500-1000 mi range either, only about a tenth as much.
For lifting bodies, check out the NASA archives, you will see a plethora of shapes.
The specifics depend on how fast you are going and how far you plan to go.
Lastly, NASA actually demonstrated that fly by wire was an essential component for operating an aircraft with some of the surfaces shot away. A computer can keep an unstable airframe flying better than any human can. The catch of course is that the control system has to be functional, which may be iffy if a wing has been lost.

brenan

Sure - PROVIDING the situation was previously considered possible and a routine to handle
such unexpected events was 100% reliably written. This was just one the planes designers couldn't believe even after it had happened. So what chance the pilot?

This is the same reason so called "intelligent" cars will always be unaceptable. I'd go so far
as to say the entire concept of personalised transport will be out of date before anyone with brains would allow large numbers of them to be widely used.

Computers can do lots of things - but only if someone has forseen them AND been able to guarantee a software algorithm will be able to cope with all variations.
So far the biggest software companies in the world can't write a simple GUI without it crashing every time there's an "r" in the month. Look at Android. Its the most out of date botched together piece of inneficient rubbish and everyone thinks it's wonderful.
Computers can do things its people that are the problem.

I have had a scan of the NASA files and "lifting bodies" seems to be a bit of a stretched concept to me.

etudiant

Modern fighters are designed to be unstable and needed ongoing computer intervention to stay pointy end forward. The pilot controls essentially add a parameter indicating what the computer should do next.
The NASA/USAF effort mentioned seeks to allow an aircraft to remain controllable even after loosing one or more aerodynamic surfaces. It was very much inspired by examples such as the one you cite and seeks to maintain control by using the remaining surfaces available and engine thrust vectoring to allow the pilot to keep flying even after massive damage.
Re the NASA lifting bodies such as these: http://www.nasa.gov/centers/dryden/h...ing/index.html
why do you see this as a 'stretched concept'?

cjl

It flies using a jet engine, actually - the rocket is just to get it up to speed. It also has wings that fold out after launch. 550 mph really isn't that fast - it's comparable to the cruising speed of a jet airliner, so the aerodynamics of a tomahawk are based on pretty much the exact same principles as airliners. It is very definitely flying - staying in the air by thrust alone is very inefficient, and the designers wanted to maximize the missile's range.

brenan

Yes - sorry I used the word rocket when I should have said jet.

etudient : My point is that a computer will do nothing if an unexpected issue arises.
unexpected = no-subroutine = no action or possibly worse - the wrong action.
The reason fighters are designed to be unstable is for manouverability not safety.
And given the ability of modern tech. a dubious requirement in the first place some might argue.

etudiant

The programming is indeed pretty limited, but in some ways, that is its strength.
The software inputs are from the aerodynamic sensors on the airframe, plus the position sensors on the control surfaces and throttles. The programming tries to keep the aerodymanic inputs within a certain range, using the tools available. If some are MIA, the program will use the rest. So there is considerable built in robustness to damage.
Obviously that can also cause real problems if the sensors are malfunctioning for some reason, but it works pretty wel in real life.

brenan

Oh sure under 99.999% of circumstances I've no doubt the concept is really great.
My only concern would be to ensure the pilot has instant overide capability at all times.
Something probably impossible with fighters but I'd say essential for airliners and the like.
(and I say this from personal knowledge of the number of fighters pulled out of the sea
off the Blackpool coast...)

godscountry

yes it is a weird site to see,as cjl stated,because of the speed[400-600 mph] you don't need a large wing with a large surface area to keep it flying.

brenan

Hmmm yes it seems there is sufficient lift for the design to work that way - it just doesn't
look right on TV !