2 vs 3 blade props

Shortening the prop reduces its tip speed. Reducing RPM would also reduce tip speed but would also reduce horsepower, which would be foolish. RPM and torque are the producers of HP:

(Torque [in ft.-lbs.] x 6.28) x RPM, and the result divided by 33,000 = HP.

So you see that you can increase HP by increasing RPM or by increasing torque, or both. Increasing torque usually means more cubic inches. Increasing RPM usually means a lower-pitched prop. Of the two, increasing torque is better because more of the HP goes into useful thrust, instead of increasing drag as higher RPM does.

Dan

One of the biggest problems facing full scale Reno racers is not how to get more HP, but how to use it. There are a fixed number of gear ratios for these engines and most of them are for somewhat less than 3000 hp! Many of the top racers are getting 3500-4000 hp now and struggle to not overspeed. Anyone seen the prop on Rare Bear? Looks like it came fresh off the titanic! Tranferring all that power into thrust is the main area of reseearch. Several teams were experimenting with composite blades a few years back.

Get 10 books and you will get 10 different explainations as to how planes fly. Some use Bernoulli's exclusively while others use Newtonian physics and others use combinations.

Rest assured that if you measured the air pressure in front of and behind the prop they would be different.

Another good place to look would be centrifugal compressors. Suction and head pressures are different even with no flow.

"Paul McIntosh" wrote irrelevant drivel in message news:...

What is the matter with you Paul. Can you not read? I did not say a word about pressure. Not one. All I addressed was density. Pressure and density are two different topics. Is that too hard for you to understand? Maybe you should go back to high school physics?

Why so angry? Pressure and density go hand in hand. If you compress a gas, the density goes up. Would you be willing to bet that the air inside a compressors tank at 100psig is the same density as outside air?

You are right. I stand corrected. If the density decreased, there would be no increase in velocity as pressure dropped. The density will essentially stay constant and decrease only with altitude. It's pressure and *temperature* that decrease with increased velocity. The incompressible fluid theory helps to explain the turbulence ahead of a rotating prop. Any movement of an airfoil through a medium is going to disturb the fluid in all directions, not just in the direction of flow behind the prop.

Dan

I believe I very carefully said the density could not change at subsonic velocitys in an unconfined space. Now you are talking about in a confined space. It simply is not the same at all.

And before you go spout nonsense about the atmosphere being more dense at lower levels then at high altitudes let me point out that the atmosphere is in a confined space. It is bounded by the physical presence of the earth and the non physical presence of the fact that the earth exerts a force of one g.

Pressure and density DO NOT go hand in hand under all physical processes. They do in some special circumstances that have nothing to do with how a prop generates thrust under most circumstances.

As long as you brought up the incompressible fluid water we may as well dismiss your argument. Water is a fine example where pressure and density do not go hand in hand. If I measure the density of water in the top inch of a 1000 foot deep lake I find it to be 1 g/cc. If I measure the density at a depth of 1000 feet the density is still 1 g/cc. The above ignoring temperature effects. Lets just assume the lake is constant temp from top to bottom.

Now, if I measure the pressure at a depth of one foot I get a vastly lower pressure then I get from the same measurement at 1000 feet.

I would hope this is simple enough for you to understand Paul. I do not know how to make it any simpler.

Cavitation is the result of a prop being configured and spun at an rpm such that the water is being forced to try to flow at faster then the speed of sound in water. If the prop is designed so that the flows are all subsonic then cavitation can not occur. Properly designed props do not force the water to flow at supersonic rates as the cavitation can destroy the prop in short order. Cavitation has nothing to do with my statements about the behavior of air in an unconfined space at subsonic velocitys.

If you can not understand these simple concepts you have no hope of ever understanding why a prop generates thrust. Worse, by continually and very forcefully representing yourself as an expert you mislead all the many very fine non technical readers of this group and give them false beliefs. It simply is not fair to do this to these fine people.

I suggest when you understand why a prop absorbs power in proportion to the diameter to the fourth power times the pitch you will understand what makes a prop work. Until then do not claim to be an expert.

By the way, Bernoulli is a direct and straightforward consequence of Newton's laws. So when you spout stuff about Bernoulli and Newton's laws and imply they are in some sort of conflict you are making it obvious that you do not know anything about first year undergrad physics much less what causes a prop to work.

Very nicely put Dan. Thank you.

What a condescending turd you are!

But an accurate one, foir all that ;-)

Liquids and solids are incompressable - hence no change in density with pessure. Measue the weight of a gallon of gaseous oxygen, then a gallon of liquid oxygen and tell me what you have.

David

Good day to you Pe:

If I implied anything to the effect that I did not believe in Bernoulli's equations or the ideal gas law I am sorry. Not sure what it was that I said that caused you to come to such a conclusion thou.

I would not call treating lift by considering air incompressible the largest mistake ever. In fact I would not call it a mistake at all. There are two reasons. First, if you can not understand lift with an incompressible fluid you sure are never going to get through all the non linear equations and high powered computer calculations required for a compressible treatment. Second it is a very, very good approximation for low speed stuff such as we do with our models.

Compressibility was not simply dismissed pre computer either. For instance you might want to look at the classic areo E book written by the German, Richard Von Mises. The book was originally published in German if you are more comfortable in that language. The book is "The Theory of Flight." In the english language version Mises translated when he came to the US to teach at Harvard, published in 1959, he treats the correction for compressibilty at small mach numbers starting on page 275. For instance at 300 feet/sec the correction is

But the subject was props. Well, an 11 inch prop spun at 13000 rpm has a tip speed of 425 mph. At this speed the compressibility correction amounts to about 20%. Getting important but still far from the dominating factor. And recall that this is the tip correction. As you move in on the diameter of the prop the correction drops fast. And yes, I know most of the thrust comes from very near the tip. But not all of it.

Both of the above corrections are also a function of alpha. As Mises clearly points out in a later part of his book at high alpha the correction is greater. So for instance the physics of what is happening to the air around the spinning prop when it is reved up and sitting restrained on the ground can be quite different from what happens in the air.

I used this book as a reference not because I think it is the best book on the market. I used it as a rather old reference to show that consideration of compressibility was not some new revelation that happened in the recent past.

A big problem with even talking about compressiblity to the non tech people is that they immediatly jump to the very logical conclusion that air must pile up behind a prop or under a wing creating a high pressure region which pushes the prop forward or the wing up. This all makes perfectly good every day common sense. And from what I have seen you write I am pretty sure you know is not the case at all. If you do actual pressure measurements on an airfoil you find out that this is not the physical facts. For instance under many conditions the leading third of the bottom of an airfoil will actually show a reduced pressure relative to the surrounding still atmosphere exactly where common sense says it should show an excess pressure.

I doubt very much if there is a mm of difference between your thinking and mine Pe. Have a great day.

A nice day to you as well. I did not answer to you directly, but more to Dan. Yes I think we argue along the same principles, which certainly is not the skipping stone theory. I also consider Bernoulli equal to Newton's third law when considering thrust or lift calculations. Yet, I want to elaborate on the subject a bit more, without expanding the framework too much, or enter a disagreement discussion. By the very token of the pressure gradient in the flow perpendicular to any foil in a moving liquid, it means that if the liquid is a gas and not a liquid, the specific density at any point in this gradient is in inverse relationship to this pressure. This is compensated for by the presentation of "flow lines" which have an increased density, inverse proportional to the reduced pressure. This is the direct result of the incompressibility theory for gasses in fluid dynamics. The temperature drop and associated icing problems on airfoils further illustrates the adiabatic expansion of the thin layer near to the foil, and creates a distance to the earlier in this thread mentioned cavitation problems in liquid, which does not follow the gas laws I mentioned. This cavitation is caused by the local pressure dropping below the vapour pressure at the specific temperature, and returning to above that pressure further downstream, where the damaging implosions occur. Some dissolved gas that is freed by the low pressures may even take a while before it is absorbed by the fluid again, which causes the nice spiral trails.

I would love to read the German publication you mentioned, if I had it available. German is my second language, English my third.

Perhaps for all who follow this thread, There is good reading on aerodynamic subjects to be found under this knowledgebase link:

Nasa also has a wonderful foil Java applet available: see

ROFLMAO re your skipping stone theory. What a great line!

No problem at all with the rest of the stuff you said.

| I also consider Bernoulli equal to Newton's third law when | considering thrust or lift calculations.

Well, a plane with flat wings (like a $2 balsa wood plane you'll find at Wal-Mart) still creates lift, so obviously Bernoulli's principal is not *essential* to flight.

Still, using Bernoulli's principal is an efficient way of deflecting air downwards and therefore keeping your plane up in the air, so it's certainly important.

ccur." --Quayle

The flat plate also follows Bernoulli' s law, though most are not aware of the fact. A circulating airflow is set up due to the lower upper pressure near the leading edge. This makes the boundary layer on the board underside want to flow towards that low pressure. This flow, which rotates around the flat foil, directs the incoming stream up- and over the flat board, thus increasing velocity and decreasing pressure, mainly near the leading edge. An illustration of this fact: hold a floor board horizontally against your breast and shove it forwrd. When you let go, the board starts to drop, increasing AoA. Then the leading edge will tip up, and the board starts to rotate. The tip-up is started by the flow up, and over the boards leading edge. Once rotating, it will keep on rotating and still generate lift like a Flettner rotor, as the board glides down to the ground. There are a few other principles involved though that keep the rotation going.

If you examine the streamlines on a flat plate wing, you will see why Bernouilli works for flat plates as well.