I’m arguing with someone about planes taking off on a treadmill. He’s saying that prop planes will take off because the propellers push air over the wings and give lift to the wings and jet planes won’t.
I understand that both types of planes will take off, but is it true that propellers directly aid in giving wings lift?
If you remember that it’s air going over the wings that generates lift, and you look at a typical single-engine prop plane, you’ll realize that commonly the prop is mounted in such a way that it can’t directly blow air over a wing. Therefore, in that case, it does not.
Sometimes, though, the prop is mounted directly in front of a wing - this is more commonly seen on multi-engine prop planes. However, the prop disc is a relatively small thing in comparison with the wing, so whatever lift is added is pretty minimal.
To a very minor degree, the prop output flowing over the wings increases lift. Unless the props are pusher props located behind the wings, in which case they have negligible effect.
Some jet aircraft also place the engines at least partly ahead of the wings to provide addtional lift via the same mechanism. it’s called “upper surface blowing”.
In either case the effect is small, not nearly enough to lift the airplane off the ground.
P.s. arguing with somebody about planes on treadmills is hopeless.
He’s wrong. Though it might be possible to build a really weird plane that worked this way, essentially every real-world plane requires motion through the air in order to take off.
Most aircraft it doesn’t help, since half the airflow is on top, and half is on bottom.
There is at least one aircraft designed to take advantage of the propellers though…
http://www.custerchannelwing.com/
I have to think that there is something fundamentally wrong with the idea though, since its never caught on.
Why would this not help, since both (normally) yield lift?
[quote=“LSLGuy, post:3, topic:462353”]
To a very minor degree, the prop output flowing over the wings increases lift. Unless the props are pusher props located behind the wings, in which case they have negligible effect.
QUOTE]
How is a pusher prop mounted (directly) behind a wing any different than a puller (?) propeller ahead of a wing?
Doesn’t the air that a pusher pushes essentially have to be pulled from in front of the propeller and thus from over the wing? (there’s something about that sentence reminscent of ‘She sells sea shells…’)
Both propellers and jets have the same principal goal-providing thrust by pushing air backwards. As noted, most propellers probably don’t give much of a boost to lift by the airflow directly from the propeller-
–but on the other hand, the airflow backwards from the propeller is the only thing that makes a propeller plane fly, as it’s providing thrust.
in terms of airflow just caused by the propeller, there might be a difference between a pusher and a puller: try standing in front of and then behind a fan (as I’ve done with both fans and propellors)-
the air pushed back is in a stream, whereas the air “coming in” comes in from all angles, and isn’t a coherent stream (unless, say, confined by a duct— but that’s a different issue).
To provide one swing at the treadmill issue (almost certainly futile)–
You can certainly fly an airplane two feet above a treadmill with no ill consequences.
When you put the wheels down, would it suddenly stop? No, because the airplane is moving, and the wheels aren’t driven.
Could you slow down? Sure–just cut power, and the airplane will slow down.
So if you can do it that way, surely the reverse is also true.
Mythbusters did a show on Planes on a Treadmill. http://mythbusters-wiki.discovery.com/page/Episode+97%3A+Airplane+on+a+Conveyor+Belt The prop is for thrust and the lilft comes from the shape of the wind going over the wings.
I’m quite sure I don’t want to fly in a plop plane. :eek:
I started another thread regarding a plane on a treadmill if anyone is interested:
Air that’s pulled into a propeller isn’t nearly as directed as what’s pushed out. You can see the same principle with a fan in your home; although there’s force behind the fan, it’s not nearly as strong as the force in front, because it’s pulling air in from all directions and then shoving it out in (mostly) one direction.
Induced drag (making it slow), mainly. When you produce lift, you produce drag, and there was no way to reduce the plane’s lift.
Yes, it had a spectacularly short takeoff, but the need for that wasn’t all that great. When landing, it had to use such a high angle of attack that the pilot couldn’t easily see the tiny patch he was trying to land on - and if it was a regular airport, the short roll wasn’t helpful. Basically, the helicopter turned out to do all the things the Channelwing did, but better.
To the OP, no, props don’t generate much lift in a conventional wing-mounted layout. High-velocity airflow over the control surfaces does improve control response, though.
Correct me if I’m wrong, but didn’t the Channelwing have failure mode problems as well? I seem to recall that losing an engine during take-off would be very bad indeed.
I haven’t come across that one, but it makes sense. Even on a conventional twin, an engine failure will induce a roll, but it would be a lot worse on a Channelwing.
Another problem with landing it was that the approach airspeed was so low that control effectiveness was poor. The propwash over the tail kept that from being a problem on takeoff, but landings are at idle power or just above it. Again, not a problem with a helicopter.
Propeller-driven flow can be used to augment lift if you choose to design it that way, but it’s never going to account for much. Jet exhaust can also be used to provide extra lift at slow speeds, by blowing over the wings and that effect can actually be significant. One of the most well-known examples is the YC-14. Of course, jet exhaust can be used for more direct lift, like what the Harrier does…but the YC-14 used wing upper-surface blowing to reduce takeoff distance, without angling the exhaust nozzles down.
I’m pretty sure the C-17 is designed to allow the engines to strongly augment lift in order to reduce takeoff distance. They tried this out on a modified C-130 known as HTTB (hot tub…er High Technology Test Bed) in the mid 80’s when the C-17 was on the drawing board. The boys (OK, there was one girl…smokin’ hot she was too!) in the next department over were working on HTTB flight controls and then the C-17 until management mismanaged itself into having the contract taken away from them. They were intially awarded the contract due to HTTB effort.
The C-17’s engines are mounted underneath the wings, and so the jet exhaust is not used for upper-surface blowing. However, when flaps are deployed (like in this picture), some of the exhaust plume blows through the gap between the wing and flaps, and subsequently over the top of the flap. Some of the exhaust is also deflected downward by the bottom surface of the flap. This actively blown flap does create a lot of additional lift, but not in quite the same way as we’ve been discussing upthread.
I’m not sure that’s entirely correct.
In the mid-70’s, the Air Force launched a design competition for a new cargo plane, something of a replacement for the C-130. aerodave already posted a link to the Wikipedia entry on the Boeing YC-14. McDonnell-Douglas built two prototypes of the YC-15, which used blown flaps. Neither aircraft went into production, but the YC-15 design formed the basis of the C-17.
The HTTB you describe may have been used for further research, but the blown-flap configuration was known to work ten years earlier.
Nah, back then airplanes were cheap relatively speaking, that design was hard & $$$ to build for what it was. It was a great flying plane and could get off quick but so can others more conventional looking and that killed a lot of stuff. (Can you say Edsel?)
YMMV
I have a little experience with controllable-pitch ‘props’, and I can say that absolutely they provide lift.
