Engine right thrust-why?
 

And/or why do we add down thrust? What happens if a particular design calls for right and down thrust and I get one or the other off a bit? I guess said another way, how do you know when you've got it right, or wrong. I think it has something to do with the advancing propeller, but beyond that I'm clueless. Then, does a 3 blade prop affect the equation somehow?

RE: Engine right thrust-why?
 

A lot of our models for whatever reason tend to climb & turn left under power, also more power = more up & left. This could be trimmed out at any one speed ---- but then a dead stick will leave U trimmed to go down & right. Thrust line ofset use to
correct this up & left tendency varys the correction with the amount of thrust & leaves a straight ahead trim if U dead stick. A lot easier to deal with when U are already busy.

RE: Engine right thrust-why?
 

Unfortunately the thrust "correction" is power sensitive -- more power = more effect. If you adjust thrust angles for cruise, or full throttle, there will be trim changes at low power settings. There are no free rides.

RE: Engine right thrust-why?
 

This link should help http://www.rcuniverse.com/forum/m_3683941/tm.htm

RE: Engine right thrust-why?
 

Thanks, I should almost qualify for a PhD if I can assimilate all that! Should have known that had been discussed in depth, but maybe some others will also benefit.

RE: Engine right thrust-why?
 

What is the size of your plane ?

Do you have a C O G, or Balance Point for it ?

RE: Engine right thrust-why?
 

Mainly downthrust is to compensate for the fact that most of us are too lazy to use the elevator trim control as a primary control much like they do on full sized stuff. Add enough downthrust and the thrust vector makes up for the extra speed and the model only wants to climb a bit at full throttle and go into a powered glide at low throttle rather than try to loop or to dive like it would if the downthrust was not there and you didn't constantly play with the elevator trim.

Right thrust is just a way to make up for the torque reaction.

If you don't quite get the angles right don't sweat it. But I would not suggest taking it all out or you'll find that you NEED to learn to use the rudder and elevator trims as a basic and constantly used flight control.

Full sized stuff sets the throttle and basically trims the plane and flies for hours at a time in one direction and one trim. However our models are all over the sky and the throttle is on and off like a light switch at a rock concert. So we try to compensate by using thrust angles.

RE: Engine right thrust-why?
 

The reason is P-factor, the torque produces by the prop blades and the slipstream from the prop hitting the left side of the vertical fin/rudder. Twist a door knob to turn it, and the door knob will want to twist your hand the other way. For every action, there is a reaction. Twin engine planes with counter rotating props do not tend to turn left. Also jets do not tend to turn left. Do a google search for "P-factor" to get more detailed explanations.

RE: Engine right thrust-why?
 

Cyclops: it's a Ziroli Stuka, building with a 'short' kit. The plans give me a CG, and calls for right thrust. The way the firewall was precut, it appears to have a bit of downthrust-though not specifically mentioned. Bill

RE: Engine right thrust-why?
 

B Matthews told it straight -
to add-- all of these "side thrust /down thrust angles are simply efforts to compensate for a model which is likely going to fly a rather high angles -getting going -or slowing down
Compensation is the key word
IF the model was extremely light (extremely low wing loading) non of this would make any difference.
When the prop (a spinning thing) is forced to slide as it spins-- slide up or down or sideways , it resists this motion --you can test this for yourself with any small electric model held in your hand - or using a toy gyroscope top you will see one of the forces involved
so --IF the plane could always fly at a very low angle of attack - these forces would never amount to a hill of beans
Scale warbirds typically end up with high wing loadings - -and so the plane is often operating on the ragged edge of flying well - -it is at these angles -the prop skidding forces are at maximum.
If flying speed is kept up and th plane is gently maneuvered -th forces are at minimum.

RE: Engine right thrust-why?
 

Down and right thrust is mostly a carryover from free flight where there is no control available to correct for changes in flight condition. A free flight model had to climb under full power as long as the engine was running, then transition to a slow glide with minimum sink rate after the powered phase of flight. It further had to glide in a circle to keep it in the vicinity as it came down. The down thrust prevented too steep a climb (or even looping) when under power. Since the airplane was trimmed to circle in the glide, the side thrust made the airplane circle the opposite direction under power. Using power to circle under power in the opposite direction as the glide helped in avoiding a spiral dive onder power since there was no elevator control. A skillful freeflight modeler, by adjusting thrust and some other tweaks could reproduce a consistant flight pattern flight after flight.

With radio control, where all axis control and trim is available, side and down thrust is not actually required. BMatthews answer is right on the mark. It can be helpful in flying a model that wants to climb or turn excessively but the angles are not critical.

RE: Engine right thrust-why?
 

You must have not followed up on http://www.rcuniverse.com/forum/m_3683941/tm.htm

RE: Engine right thrust-why?
 

I'm going to put my two cents in by saying that a prop creates three things known generically as p factor. They are differential blade thrust, slip stream over the tail and gyroscopic precession. These three forces act on the plane depending on your flight configuration Without going into a long winded discussion, the right thrust helps compensate for these factors and generally you will have an easier time flying your plane. Its like putting washout in your wings; Don't have to have it, but it makes the plane easier to fly. If you want to know how a plane fly's, read "Stick and Rudder", an old book but probably one of the best on the subject.

RE: Engine right thrust-why?
 

P Factor is another name for Asymmetric Propeller Loading.

There are four turning forces: Torque, P Factor, Spiral Slipstream and Gyroscopic Precession, all of which have different causes, and can act in different directions under different conditions.

In some texts, they are all referred to generally as torque, which is technically incorrect, but it is sometimes done in the interest of simplicity. I have never seen all four forces referred to as P Factor.

RE: Engine right thrust-why?
 

If P stands for -- phorse- then it all works ---
I think that is a phine way to look at it
(you may say --phooey)

RE: Engine right thrust-why?
 

Then the fact that a WW II fighter had a huge fan slamming air on 1 side of the fuselage as the prop approached the upper vertical part of the fusulage ( the pilots left hand side ) and creating a vacumm on the opposite side after it passed the vertical, has nothing to do with pulling the nose ? That fusalage areas may not be equal above and below the prop centerline.
The larger area could be above or below.


Ok, Cyclops. Then why do radial nosed planes suffer the same fate?

Why is it a 10 : 1 or greater geared glider can be almost impossible to fly if torque is not the major factor.

Yet a ducted fan model that is equally over powered has no noticeable torque effet or " P " factor ?

RE: Engine right thrust-why?
 

Does the above mean to imply that the spiral component of the slipstream is very compact or dense compared to its aftward component?

RE: Engine right thrust-why?
 

If you are referring to torque as the force causing the plane to rotate counterclockwise if the prop is rotating clockwise, it is a force although I really don't know if right thrust would do anything to help compensate for it although it is so minimal, it may not be an issue. I realize my jargan is not completely right but I do understand how it works. I have also heard the forces related as torque but by my above statement you could see how that would be very confusing due to most peoples understanding of what torque is. Thats why I refer to it as P-factor although not technically correct. Right thrust help make plane go down straight down runway and track good through air to put it in an easy to understand way. :D:D:D

RE: Engine right thrust-why?
 

Mesae. I do not know.
But I do know all motion pictures of WW II fighters flying away at full power and pulling up steeply show the nose way off center to the right constantly.

RE: Engine right thrust-why?
 

I've done a lot of pull-ups, pull-downs, push-ups and push-downs in Decathlons and I don't understand what you are referring to.

RE: Engine right thrust-why?
 

As you know, by the rigorous definition, torque is a rolling reaction, so right-thrust doesn't help with that. I was trying to be general, yet somewhat complete IAW aeronautical texts.

RE: Engine right thrust-why?
 

In the 3 blade prop area, you have broken up the air with the previous prop a bit more so, the thicker the air, the more effiecient the prop, it's kind of like putting 2 motors and props operating independantly just behind each other, the rear prop only has the left overs from the front one, this is why you don't see them too pushed with RC plane designs. The same principal is demonstrated with RC combat where the front plane prop washes the other, causing the opponents plane to fall out of the sky since it doesn't have dense enough air to pull in enough air to transfer to it's prop, that and of course the instability caused by the wash.

RE: Engine right thrust-why?
 

That does not actually happen in flight & only occurs under particular conditions on a test bench.

RE: Engine right thrust-why?
 

Mesae. The planes are pulling a vertical climb and the nose is constantly off center to the right.
I have seen it on a couple of clips. The plane is crabbing or cat walking to the right. The engines were at or close to full RPM's.

RE: Engine right thrust-why?
 

If a plane has enough power to maintain speed the need for any offsets is lost.
Not to poo poo the old fighters -they were the king of their day - current unlimited aerobatic stuff tho is far more powerful -power to weight
Example -- a Pitts 12 will pull away almost vertically from a vertical stopped position.
On models of bipes we designed for TOC- one problem was thrust setup - some needed up thrust - why?
simple - not enough power -- they were always slowing on verticals and requireg a bit of upTRIM -even tho it was not readily apparant.
add more power - the need goes away . We did this year later using more powerful engines - not available in 1990

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 ----