RE: Engine right thrust-why?
 

P Factor can be explained as the difference in angle of attack on the descending blade compared to the ascending blade. This happens when the plane is in a high power, and high angle of attack position. Take off is an example. To ilustrate this point, think of the plane moving through the air at an exagerated nose high attitude. Now think of the prop, and how it rotates. The right side of the prop is moving into the relative wind, and the left side of the prop is moving away from the relative wind (this is of course on a engine that rotates clockwise as you view from the cockpit). The right side of the prop generates more lift moving into the wind (or forward thrust in this case). This pulls the nose of the plane to the left requiring input of right rudder. The larger the radius on the prop, the more pronounced this force is because of the moment arm.

Tourque, as stated earlier, is a contributing factor to a tendancy for the plane to roll left. For every action, there is an equal and opposite reaction. The prop is turning clockwise, so the plane tries to turn counterclockwise, so you add right aileron and rudder to fix this.

Gyroscopic Precesion is what you study in physics as angular momentum. Wind a toy car up, and let the wheels go while you are holding it in your hand. That twisting force you feel is angular momentum. It impacts yaw, roll, and pitch to be exact.

OK, the last one mentioned was the prop slipstream twisting around the fuselage, and hitting the vertical stabelizer. There are varying degrees of impact that this force has. It has a lot to do with stabelizer surface area, location, etc. That is a course all it's own, and I don't pretend to understand it.

Thrust offset is a way to deal with all of these as a combination. Full size training aircraft use both thrust offset, and vertical stabelizer offset to deal with this. As you get into faster aircraft, everything tends to get streamlined to lower drag. As a pilot, you are expected to know the forces and deal with them by proper control inputs when needed. That way, you only put drag on the airplane to fit the situation.

Same should apply to modeling. The more in-line you construct everything, the straighter track you will have when you are flying straight and level. Learn to use the rudder when you need it, and leave it alone when you don't.

Or.....build a twin engine with counter rotating props, and you can put all of these worries behind you. Of course you pick up a lot more.

RE: Engine right thrust-why?
 

Can you elaborate upon this, I'm in the mindset of basic physics here, you are displacing the volume of molecules outward, causing them to be less dense within the traveling path of the blade, now within it's absence they equalize out due to the natural air pressure, then do the same yet again on the returning cut into the air, I'd think the longer the interval in between cutting the air and allowing it to reintigrate back into it's orignal configuration the more adventagous. I'm sure after enough velocity of the actual prop assembly moving foward, the volume of air molecules pushing into the field make up for this effect.

RE: Engine right thrust-why?
 

Without seeing the clips I just don't know.

RE: Engine right thrust-why?
 

Up or down thrust to correct pitching while hovering is done to correct a design flaw: the relationship between the thrust line and the center of gravity. If there is too much of a difference, the plane pitches up or down while hovering. The same thing could be accomplished by leaving the thrust angle zero and moving the battery up or down on the airframe to get rid of the pitching.

Adjusting the thrust angle should ideally be done after the above to correct for any small aerodynamic asymmetries in the way the slipstream hits the tail.

RE: Engine right thrust-why?
 

You've mentioned this, so think about what that means. In the air, the prop is moving forward, right. And the air that's being encountered by that prop isn't. It's basically "sitting there waiting for the prop to come along". And then the disturbed "molecules" are being left behind at the speed of the plane, right? No, they're actually being left behind FASTER than the speed of the plane. Think about that "FASTER" deal. It happens on the ground too.

RE: Engine right thrust-why?
 

This interval and reintegrating you write about doesn't occur. The action is continuous.

Before everyone starts arguing about this, might I suggest that all interested parties carefully review the following trustworthy link?

http://www.grc.nasa.gov/WWW/K-12/air...ropeller.html# (click the link to "fundamentals" where it talks about momentum theory)

It might avoid some unneccessary argument.

Detailed propeller theory is so complex that to argue about anything other than generalities requires many specifications.

Momemtum (or actuator disk) theory is a good first-order approximation but to go beyond that in terms of accounting for the many losses, requires specialized training. It's well worth the look-see though.

As far as being able to quantify thrust from a prop, the above theory is about as basic as you can get.

RE: Engine right thrust-why?
 

a propeller does not have to look like a wing -or "airfoil"
a simple flat board angled into the direction of rotation -from a turning shaft is a propeller. (an oar is a type of propeller)
You don't need any formulas to figure out what is going on.
simply put
the angled board -creates low pressure in front and high pressure behind it, as it turns
The efficiency of the prop is an altogether different thing.
Ideally - a "perfect" propeller converts all of the power required to turn it - into thrust. (some miss by a mile)
How well it does that and what shapes it must assume to do it - are an ongoing source of of conjecture and study .
the path the air (or water) takes going into the prop -- then away from the prop-- can make or break any propeller design.
Restated -
The prop INTERACTS with the craft -making any propeller design a task which must include desired results on a specific airframe .
- the best prop for any job can only be found by testing in actual application to the task .

RE: Engine right thrust-why?
 

This is almost entirely true in practice with models, if you limit yourself to available props. The momentum model does not account for all the losses. So an approximate efficiency factor is applied to account for the losses, getting us pretty close to the truth. Accounting for the losses can be done, however, by specially trained people, very accurately with detailed propeller theory. Just because you and I can't do it doesn't mean it can't be done.

How do you know where to start? how do you know not to put a 16" prop on a .40? Either by past experience, trial and error, or by some first approximation calculation, then tweak from there. Sometimes the first aproximation is close enough. after all, how do you know the best prop for your particular racer doesn't have a 10.345" pitch? Does anyone make one of those? The only way you could find out the "perfect" prop for your application is with detailed propeller theory. Then, if one isn't made with exactly the right pitch/diameter/blade width, tip shape, you could make it.

RE: Engine right thrust-why?
 

You have to start with an idea of what should be happening (a theory).
The problem is - what do you do when the "theory" and reality clash?
As noted - you try something else (make your own prop).
A lot of the text book prop theory -as seen in th recommended notes - is for large slow turning blades
Drop down to props under 24" diameter -- you can just about chuck the rule book.
Having spent $$$$$$$$ and been fortunate enough to also have many more props provided for evaluation --This is not just an idle comment.
At one time -I thought I could guess how a prop would perform - by just looking at relative blade width, pitch, washout etc..
Nothing more humbling than being proven wrong.
The first real insight I got --(hard cold facts) was done by using a tiny electric motor - a test stand that showed the thrust from the prop on the motor AND a Whattmeter ( an Astro product which reads volts amps and watts)which showed me the battery power being consumed ( energy) at a given thrust.
The props generally accepted as the best were not always better than some considered too small , too narrow etc..
Bottom line -there were many combos which at different rpms gave the same thrust AND using almost the same energy.
The "best" prop --was a moving target .
If you change rpm -you must change prop ---to do the same job (obtain same static thrust) but in each case - consume same energy.
The expression "you must break eggs to make an omelet" is very true.
- unless you see the actual result on all sides of the problem - you may never find out what is the best approach.



/moto

RE: Engine right thrust-why?
 

One thing we can do is study more. It sometimes leads to a more complex and more generally applicable theory.

We really don't disagree much in principle. It just looks like it sometimes.[8D]

RE: Engine right thrust-why?
 

From the experts I've discussed this with, some of which have 30+ years in the "real" aircraft design field, only 20% of any prop at best is actually doing anything, the rest is waste.

RE: Engine right thrust-why?
 

not really waste -
the other 80% provides the 20% which does the hard work - the opportunity .

RE: Engine right thrust-why?
 

So the outer 20% flies the plane. The inner 80 % cools the engine ? :D [sm=thumbup.gif]

RE: Engine right thrust-why?
 

Maybe we are not talking about exactly the same thing here, but according to some formulas I have played with from design books, and converted to software, efficiency factors tend to be in the 70% to 80% range when filled with real-world performance data. That refers to the amount of engine horsepower that is converted to thrust. I'm not arguing here. Perhaps you could specify in more detail what your statement means? Are you talking about efficiency, or how much of the span of the blade is moving fast enough to produce a lot of thrust?

RE: Engine right thrust-why?
 

a funny point - the prop blast does not cool the engine by blowing into the inlets -- it simply can't- there ain't no prop blast at the hub..
however the flow past the cooling outlets does create the flow thu the cowl.
the working section of the prop is a hard thing to define -
If you shove the prop up close to a cowl which is 80% of the prop diameter-which part of the prop is still working?
but -if you extend the prop out waaay out in front of this cowl - is the the work really being done by a different portion of the blade?
If you round the cowl right to the hub - doe this also affect the prop?
Will Buck rescue Dr Huer from the evil Gorp??
Tune in next time ----