Thrust distribution
 

At our morning coffee we opened the age old discussion . 1: What is the lift distribution between the top & bottom surfaces of a wing . On a prop or turbine what is the thrust dist: between the suck at the front and the blow at the back .

RE: Thrust distribution
 

Thanks to Multiflyer& BMathews .May I pose these basic FAQ 's once again, somebody out there maybe can help . When helping newcomers or chatting with my buddies I often find myself unable to properly answer what at first seems some simple questions. What makes the wing lift ? Quickly followed by ;why does the top surface provide more lift than the bottom (one engineer friend says I have it backwards )and what is the ratio ? This can quickly go to props .Which side provides most thrust & why, & so on into turbines with reversers .
Once this happens we go thru a very unconvincing outbursts of Newton , Bernoulli ,Coanda & under camber . Is it possible simple answers are not to be had and I'm being silly ?

RE: Thrust distribution
 

There was a couple of rather lengthy threads a while back about wing lift (and prop lift since props are just wings that are spun up rather than pulled through the air).

Try a search on "wing lift" within the Aerodynamics forum only and you should find a bunch of old threads that deal with this. The juicy ones are those with reply numbers over 40.

Basically for a wing to produce lift it must fly at a positive angle of attack, have camber or have a little bit of both. Once the wing is developing lift you will see a higher than ambient pressure on the lower surface and a lower than ambient pressure on the upper surface. Linked to this pressure you will see the air undergo a change of direction as the wing passes by that produces a downward motion of the air. If you analyse all this you will find that the upper to lower pressure difference over the wing accounts for the lift being produced. Similarly if you study the air being deflected downwards and do a mass to velocity comparison you will find that the energy added to that air to move it down also accounts for the lift upwards. Which of these is the actual lift is like asking which came first, the chicken or the egg. And that was the basis for quite a bit of colorful posting in past threads. In the end the two effects are simultanious and linked.

So you see, one surface does not supply more or less than the other. It's the pressure difference between them added together. Alternately you can show them that the air is defected downwards so the wing is just acting in a classic Newtonian action and reaction mode. But as said, both effects occur together and change in step with changes in angle of attack, speed or camber (flaps). They are in lockstep and one effect is totally dependent on the other. In fact I think it's fair to say that they are different aspects of the same thing or that they are actually both part of one integrated effect that "we" just find it easier to look at as separate effects.

....at least that's my story and I'm sticking to it... :D

RE: Thrust distribution
 

Bruce is right. Momentum and pressure distribution are simply two ways of looking at the same thing.

The simplest explanation is that a wing deflects air downward producing lift upward, and a propeller throws air rearward producing forward thrust. The “how” involves study of pressure changes and can get complicated pretty quickly.

While both explanations are basically correct, the momentum exchange isn’t particularly useful to engineers who are designing stuff, so they nearly always delve deeply into flow and pressure factors. On the other hand the deflection explanation is pretty graphic and helps pilots (both model and full scale) to visualize effects of their control.

RE: Thrust distribution
 

I’ll add a quick explanation of Bernoulli’s principle since it is so often misrepresented. All he did was restate conservation of mechanical energy in terms of fluid motion.

When you stretch a rubber band, your work of stretching it is captured as potential energy within the elastic. When you let it fling off your finger, that potential energy is used to put the band into motion. The potential is transferred into kinetic energy.

Bernoulli is simply saying for fluid flow, that when pressure is high velocity is low and vice versa. It’s very obvious if you think about it. If the flow is speeding up then the pressure must be dropping. Why else would an air molecule accelerate. The pressure difference is pushing it. And likewise, the motion of a fluid as it flows and swirls around an object like a wing is creating pressure differences all around that object. You can measure the pressure anywhere along the surface by simply poking a small hole and attaching a pressure gage. When you add up the total effect you get lift, drag, and moment (twist).

Airfoil surfaces (wings) are just shapes chosen because they generate higher overall pressure differences (lift) with lower amounts of waste (drag and twist).

Hope this helps demystify fluid dynamics.

Multiflyer

RE: Thrust distribution
 

I prefer the roller coaster analogy when explaining Bernoulli's laws of fluid motion. A roller coaster has kinetic energy and potential energy and the sum of the two is constant. A roller coaster's potential energy is it's altitude, a fluid's potential energy is it's head.
When a fluid flows, the sum of it's potential energy and kinetic energy is also constant.

Potential energy for the roller coaster = mass X altitude X gravity

Potential energy in fluids = density X head X gravity = pressure

RE: Thrust distribution
 

Libby's third theorem on flight:
"things fly, when it is easier to fly than to remain at rest."
This is based on the observation that nature always takes the path of least resistance.
If you want to figure out where air flows when a wing moves thru it , look for the easiest , obvious paths .

RE: Thrust distribution
 

I was going to babble on about the Coanda effect but found this to be better. That theory about molecules dividing and meeting at the trailing edge does not cut it for me any longer....years of misinformation!!


http://jef.raskincenter.org/publishe...da_effect.html

when u have time, go thru this thread. many of the points have been discussed:

http://www.rcuniverse.com/forum/m_26...CRaskin/tm.htm

RE: Thrust distribution
 

yep - Libby looks at the direction the frisbee by is spinning and runs to intercept it -in the correct direction.
She has no concept of numbers but can learn from the obvious.

RE: Thrust distribution
 

Regarding the turbine portion of the questin I do not beleive thrust and suck are equal do to the expansion of gas due to the heat of combustion of fuel but I can not say for sure what the ratio is.

RE: Thrust distribution
 

A spectator came up to me while I was flying my Cub model. I carefully explained to him how Cubs could not fly inverted because of their flat-bottom airfoils as I was flying the pattern inverted the whole while, contradicting everything I was saying. He caught on to the joke BTW.:D

RE: Thrust distribution
 

Just proves that anything will fly in anyway possible, given the right circumstances. Even the space shuttle or a brick, take your pick

MattK