Am I wrong, again? For years I have thought of the thrust line as the line the airplane travels when in straight and level flight. If wrong, what is the proper term for the following line? It is the line from which all incidences are determined.

I swear when learning to fly and when first learning about building and flying model airplanes from plans this line was pretty much universally described as the thrust line.

Mode One,
The more I think I know the more I find out I don't really, really know. Sooooo I would hazard a guess that the center line is the line from where all incidences are measured................and the thrust line is the center line of the propeller shaft or the turboshaft in a jet. The datum line.................wait a minute.....what is a datum?
larry

Thrust line is exactly what it says it is. It's the line the prop pulls everything along. It's the line the jet engine pushes itself along. It basically a line that traces where the prop or jet would go if everything else wasn't along for the ride and encountering the air and it's effects. Of course, neither of those is possible. Except maybe for the GeeBee and the Rata.

Designers usually start with a specific airspeed to work from. It's usually the cruise speed since the airplane's performance (speed and range) for that is the most important. And they want the least drag (usually) at that speed, so they set the datum line of the fuse along that least drag line for the fuse and work from there. They could have the datum line down the center of the fuselage, or they could run it along in space below the whole pile of parts. It can be drawn anywhere, just so long as it's drawn the same on all the blueprints that need to match other blueprints. And they don't have to draw the datum line separate from the centerline, but often do to make it easier to place parts relative to each other.

Sometimes they called the datum line the water line, btw. Funny people, those airplane designers.

Good answer daRock. Also, the datum line is the "reference line" that all other lines and measurements are compared or measured to in the profile view on the plan. So if you need 1 degree of downthrust, the line through the crankshaft would be -1 degree "with respect to" the datum line. Similar story for wing and stab incidence. A centerline most often describes the geometric center of the fuselage as seen in the plan view from above.

"Waterline" is one of those archaic terms inherited from shipbuilding where the datum line is the hull's waterline by historical convention. It's kinda like engine power being measured in "horses". The word "cockpit" is even funnier. Racing yachts in the 19th century had a sunken circular area at the helm where the pilot steered the ship. It so closely resembled a cock fighting arena the nickname stuck.

daRock, Cratecruncher, both have it pegged. If you have these figured out, that's the first step of not only designing and building a plane, but also understanding how, and why, it flys. The thrust line, as stated, is the imaginary line the engine's prop shaft would follow if uninhibited... no matter the attitude of the plane. The datum line, also as stated, is the imaginary line that equally divides the aircraft's balance while in flight, particularly.

Sorry, but the "datum" is the point or line from which all measurements are made. It can be anywhere...below the aircraft, above it, or through it. It is the positional reference line.

Datum Line, Reference Line, water line: different names for the same line. Old school draftsmen use the term "Water Line", a carry over from when ocean going ships were the main form of transportation. The aircraft company I worked in for so many years used the term "water line" on all their aircraft.

\

Rodney,

Did they build Seaplanes?

Charles

Nope, first class fighter planes, both Airforce and Navy along with commercial passenger planes and a few helicopters.

Rodney,

That must have been interesting work.

I passed on a job with the firm that actually took the Republic See Bee, and turned it into a Twin.

No big deal, because shortly after that I was drafted so that decision, unlike some others, had no affect on my life.

Charles

Doesn't speak well for their optimism now does it.....


Modeone, if you have not figured it out by now there is NOline that can be used for a reference forflying straight and level. Why you ask? Because straight and level at max airspeed and straight and level at near stall speeds involves a change in the angle of attack of the wing and by connection the fuselage of a good 7 to 8 degrees. So you can't pin it down to any sort of reference line be it the thrust line, datum line, center line or any other line. In flight the line of flight of the fuselage as well as all those rather important parts connected to it is fluid, very fluid. At best you can find the best angle at any one given speed. This is what airliner designers do. They know the plane will fly at X speed at Y altitude for most of the flight and they choose wing incidence angles such that the fuselage will fly at the least drag aspect for this portion of the flying time. The rest is not a big deal.

Right on. The "datum line" is just a reference line for measuring things against. It israrely a horizontal line while the plane is in flight (exceptbyoccasionalcoincidence). In wrestling with this conundrum I've fallen in line with the wisdom ofother designers. They seem to favor tuning the fuselage to have the least form drag at the "wide open throttle" (wot) engine setting. The idea being that the r/c pilot is trying to go fast and therefore wants maximum airspeed and least drag at that setting. Of course, airliners are running a business and so are looking for the best fuel economy over a given distance and so they optimize everything to that "sweet spot" between speed and specific fuel consumption. I don't know how Boeing designs their planes but it wouldn't suprise me if the plane fuselage isdirtier at maximum design airspeed.

<span style="font-family: Times; font-size: 16px; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; background-image: initial; background-repeat: initial; background-attachment: initial; -webkit-background-clip: initial; -webkit-background-origin: initial; background-color: rgb(255, 255, 255); font-family: Arial; font-size: 12px; background-position: initial initial; "><div>Yes, I think people get confused by the datum line because it really means nothing aerodynamically, but people tend to see downthrust, etc. in relation to the datum line or the appearance of the fuse.  <div>
</div><div>I often see people reducing wing incidence to eliminate "ballooning".  What they are really doing is adjusting the trim between the wing and the stab without considering the impact on downthrust. Best thing is to imagine just the prop, the wing and the stab, and consider the relationships between them without reference to a line or a fuse.  Then it becomes obvious that reducing the wing incidence is exactly the same as reducing downthrust, which is not usually what they should be doing.  That's because reducing the wing incidence means you have to reduce the stab incidence correspondingly, which usually means changing the elevator trim.</div><div>
</div><div>Since downthrust functions by causing propwash to strike the underside of the stab, changing the trim can reduce that effect.</div><div>
</div><div>A plane that looks like it has no downthrust at all may actually have significant downthrust.  Mess with the wing incidence and you can unknowingly eliminate it.  People do this with "old timers" a lot.</div><div>
</div><div>Jim
<div>
</div></div></div></div></span>

The horsepower made a great deal of sense in its time. It was defined by James Watt as 550 foot-pounds of energy per second, and it enabled him to go to prospective customers for his steam engines and say "Now if you buy one of my engines, this is how many horses you can get rid of"...

We know now that the horse or any other biological "machine" really sucks as a standard, because its power output depends a great deal on how it's coupled to the load. When Bryan Allen pedaled the Gossamer Albatross across the English Channel in 1979, he had to AVERAGE 1.3 hp for four hours.

rj

I have read this post four times and I still can find no way to accept any of this.I guess I'm just thick in the noggin' but how does reducing incidence <u>with respect to the horiz. stab</u>, reduce the engine downthrust angle?

Downthrust effectiveness is a result of propwash hitting the bottom of the stab?? Wow! Can you link us to a reputable source which supports this? Why do rear engine canards use upthrust when there is no stab behind the prop?

Please help me understand this new science which you've come to sharewith us!

Propwash and hogwash are both strange words and both often,misunderstood I love datum line tho</p>

To me it's the base referrence line -from which lines/ angles/ /forces are noted or figured</p>

One of our largest airliners had the cruise angle for the fuseselage carefully figgered and the interior floor of the passenger cabin was also carefully calculated so that at cruise - the floor would be -level</p>

Made pushing the service carts easier</p>

only problem was, the calculations were not right.</p>

Not a big deal?</p>

ask the cabin attendants.</p>

</p>

You missed his point. Most people miss the point that changing a wing's incidence isn't just doing one thing.

Most of these discussions really are about shimming a wing. That very often does reduce some trainer's tendency to climb with increased power. And that's pretty much what was the basis of his discussion.

There are THREE important lines that matter (thrust, wing incidence, and horizontal tail incidence) and when you change one of them you're effectively changing more than one relationship. And that's his (and many others) point.

If we change the AOI of the wing, we very often are changing both the thrust line and the tail's AOI because the wing goes to it's needed AOA no matter what.The wingis the player in this game that has all the power.It decideswhatAOA it needs when yousteer you model into the approach and adjust the throttle to get the speed you think is right. If you've suddenly adjust the AOI for that wing and then flown the sucker,that wing is still going to assume the AOAit wants to give you the approach you'redemanding once again. If you're flying that modelat the same speed as usual, that wing is going to assume the AOAit has always needed for that speed. And since it's at a newAOI on thefuselage, the fuselage is going tobe at a new AOA, and the stab and motor aregoing to have new AOA's as well.

Also, change the AOI of the wing and it'sdownwash is going to findthe stab at a different place than before. And the downwash is going to hit that stab at adifferent angle. Not much different, but differentnone the less.

He's sharing with us what a whole lot of people alreadyknow.

<p class="MsoNormal"><span style="font-family:ArialMT">I&rsquo;m sorry to cause anyone to read my writing four times.<span style="mso-spacerun: yes"> </span>I wish I could draw diagrams but I can&rsquo;t, so I&rsquo;ll try a different approach.<span style="mso-spacerun: yes"> </span>Let&rsquo;s put aside, for the moment, the question of whether downthrust works because of the vertical vector component of the thrust acting on the nose of the plane, or because of an interaction between the prop wash and the stab.<span style="mso-spacerun: yes"></span>It won&rsquo;t matter for this discussion.</span></p><p class="MsoNormal"><span style="font-family:ArialMT">I like the suggestion above of looking at the wing as the &ldquo;player&rdquo;.<span style="mso-spacerun:yes"> </span>The AOA of the wing in flight depends on the speed and weight of the plane, the wing characteristics, air density, etc.<span style="mso-spacerun: yes"> </span>So changing the &ldquo;angle of incidence&rdquo; by shimming the wing has no effect on the AOA in flight.<span style="mso-spacerun:yes"> </span>All it changes is the angles of the other components going along for the ride.</span></p><p class="MsoNormal"><span style="font-family:ArialMT">Suppose you decrease the angle of incidence of the wing.<span style="mso-spacerun: yes"></span>Now the whole plane has to fly at a more positive angle to keep the wing at the same AOA. That includes the engine.<span style="mso-spacerun:yes"> </span>So now, by decreasing the angle of incidence, you have raised the angle of the thrust line.<span style="mso-spacerun: yes"> </span>If you started with 2 degrees of downthrust and you decreased the angle of incidence of the wing by 2 degrees, you&rsquo;ve just wiped out your downthrust.<span style="mso-spacerun: yes"> </span>I&rsquo;ll stop for now.<span style="mso-spacerun: yes"> </span>Does that make sense?</span></p>

Let's start by considering that all angles that matter for flying are related to the wing.  The fuselage is just along for the ride and to hold the other bits at the angles they form TO THE WING.  Nothing else matters than the relationship of the thrust to the wing and the tail to the wing.</p>

So now someone wants to correct a ballooning problem by reducing wing incidence.  But consider what it's doing to the thrust line to wing angle.  It's LOWERING the angle so by reducing wing incidence you're also reducing the downthrust angle by the same amount..</p>

As for the downthrust angle acting on the stabilizer I'm afraid I can't buy that one.  It may be a factor but I've seen far too many planes that fly where the prop wash doesn't flow past the stabilizer .  Also a plane in flight is flying with little if any propwash passing the tail since at speed the prop is lifting or screwing it's way through the air unlike the ground based propwash.  So propwash at speed is going to be reduced to an amount that is proportional to the prop's slippage rate.  Typically maybe 15 to 20%.  And what about pylon mounted engines?  Or models with T tails that have the stabilizer well up out of the prop's thrust line?  And then there's pusher designs......  Nope, the thrust line angle works by producing a torque that acts on the wing due to the geometric thrust component that is not in line with the wing's airfoil.</p>

OK, I think Iknowthe source of controversy now. A wing doesnot hunt for anangle of attack with noregard to the other forces acting on the body of the airplane. All the forces acting on the plane must be in equilibrium.I often draw asketch called a free body diagram which displays every force and acceleration acting on an object. That way nothing gets missplaced or forgotten.</p>

If we consider a typical airplane in profile view there would bea weight vector (mg), the lift vector (L), the engine thrust vector (T), a drag vector (D), and possibly a stabilizer negative lift vector (S). All these forces (actually it's their moments) will be in equilibrium when the plane is flying or an acceleration must occur (a pitch rotation).</p>

Rapid gain in altitude with throttle setting indicates that when all the other forces are in equilibrium the wing is at an AOA that generates lift in excess of mg. When incidence is reduced, the planes wing will fly at a slightly reduced AOA at the same speed as before. With less AOA there is less Cl and less lift.</p>

For a moment lets suspend the reality and assume a wing "decides" itsAOA. A trainer with 2 degrees downthrust and +2.5 wing incidence ballons in altitude with increase in airspeed.Now wereducewing incidence (<u>relative to our datum</u>) from +2.5 down to 0. The wing now "causes"the plane to fly at a +2.5 higher AOAat every speed. This implies that now the stab is flying 2" lowerbut not generating any positive liftfor some reason and the engine is higherwhich "somehow" appliesa +0.5 upthrust even though the thrust direction and moment arm have not changed with respect to the airplanes center of mass. If an airplane does that I'll eat my hat.

OK, back to reality.

The wing AOAis a committee decision between stab lift, engine thrust and to a small extent cg because its usually slightly ahead of the neutral point. All these players cause torque reactions that find equilibrium and determine the wing AOA. The wing has virtually no power in deciding it's own AOA because it's lift and drag vectors are so close to the cg that their moment arms are very short.

An example. To trim an airplane that gains altitude what do we do? Put in a click of down elevator right? This changes the camber of the stab creating slightly more lift, creating a slight pitch rotation to it's new equilibrium, which changes the AOAof the wing, which causes it to generate slightly less lift.

Buzzard, the reason it took me 4 tries reading your post is because it was soarticulate and confident and yet completely wrong I thought Ihad to bemissing something. Sorry to be harsh but it was you who "contradicted" me withthis same homespun mythology in another thread while I was in the middle of helping a guy correct an incidence problem. Your post threw the entire thread into confusion andkinda pulled the rug out from under my cred. Next time you do that make sure you're right first ok? There's a sticky at the top of this forum called "Good Reading". Check it out.</p>

So, after 20 posts, it's still not clear to me what the name of this line is.  I guess I will call it the "referance line".  Having beeen technically involved in boats, a waterline is only a straight line in the side and fore and aft views of the vessel.  In the the rest of the views, it is a curved line.  Waterlines were used in to determine offsets so the shape of the hull could be described and reproduced.  The line I'm interested in is always straight.</p>

Cockpit is still the area where the vessel is controlled, unless it has a wheelhouse or pilothouse. </p>

that's what I said

Dick, Your calling it the Datum Line, then?  That, is probably good enough!  

Actually -I am a few thousand years  late  on defining it </p>

 datum (singular )  and  data  (plural)</p>

 Data springs from a datum-point of origin  etc..</p>

 On my kits  from a few years back - I always  made a hotizontal line  such as the canopy floor , which all other  measurements  were tied to.</p>

 Using the  wing as adatum line creates all sorts of  problems </p>

 Being exceedingly lazy -I always tried for  the  most  simple approach.</p>

</p>

</p>

Buzzard bait writes:</p><div><font color="#0000ff">Reducing incidence absolutely decreases downthrust. That's not even aerodynamics, it's just geometry. Bruce, the moderator on the Aerodynamics forum got it right. Read his post. You can also read the following passage and try to picture it. It really isn't anything profound or difficult once you see it...</font></div><div><p class="MsoNormal" style="mso-pagination: none; mso-layout-grid-align: none"><span style="font-family: ArialMT"><font color="#0000ff">Gordon Whitehead, in 'Scale Models for Everyday Flying', discusses the Sopwith Triplane as an example,which rigging diagrams show with the wing at +2 degrees, stab at +1 1/2degrees, and engine at 0. "Note that if we set as per the full-sizeTriplane we have effectively generated about 1 1/2 degrees downthrust on theengine, which will reduce the amount of any extra non-scale downthrust we mayneed to use." Note that he goes by the stab, not the wing, but they are very close.</font></span></p>

</p><p class="MsoNormal" style="mso-pagination: none; mso-layout-grid-align: none"><span style="font-family: ArialMT"><font color="#0000ff">Finally once Bob takes your advice, I suggest he do a simple, honest test. He should go back to the original problem comparing apples to apples. He said, "<span style="font-size: 13px; font-family: Verdana; -webkit-border-horizontal-spacing: 1px; -webkit-border-vertical-spacing: 1px">i can get it to fly perfect at 1/4 throttle then you give it gas and it rockets up at about a 60* angle". <span style="font-size: 12px; font-family: ArialMT; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px">So after making the adjustment you recommended, he should fly the Cub at 1/4 throttle and trim it out to fly level that way, just as before. Now when he gives it gas again, what happens? Does it speed up straight and level, or does it climb again just as it did before?</span></span></font></span></p><p class="MsoNormal" style="mso-pagination: none; mso-layout-grid-align: none"><span style="font-family: ArialMT"><font color="#0000ff">For books I use Whitehead, McCombs and Simmons mainly. What do you use?</font></span></p><p class="MsoNormal" style="mso-pagination: none; mso-layout-grid-align: none"><span style="font-family: ArialMT"><font color="#0000ff">Jim</font></span></p><p class="MsoNormal" style="mso-pagination: none; mso-layout-grid-align: none"><span style="font-family: ArialMT"><font color="#000000">Buzzard your obviously unconvinced by my prior post(s). I decided to move this back to the aerodynamics forum because it's confusing people in the scale forum and further discussion seems more appropriate here.</font></span></p><p class="MsoNormal" style="mso-pagination: none; mso-layout-grid-align: none"><span style="font-family: ArialMT">MYTH: "Reducing wing incidence reduces prop downthrust."</span></p><p class="MsoNormal" style="mso-pagination: none; mso-layout-grid-align: none"><span style="font-family: ArialMT">Airplane pitchin level flight is determined by the summation of moments = 0 about the center of mass (of the airplane). It's pretty much irreleventwhat the wing incidence is (recall wing lift has no moment arm)with respect to the datum line because the moment arm for the prop thrust is stillthe line between the prop center and the airplanes center of mass. The thrust angle has not changed with respect to that moment line when the wing incidence is changed.

Ifyou have a copy of Simons, review pages 1-5 "Fundamentals". Then take a look at the car problem at the bottom of en.wikibooks.org/wiki/Statics/Two-Dimensional_Equilibrium

Then take a look at the diagram I attached. Notice the wing incidence has nothing to do with the angle of fuselage equilibrium. The center of mass acts like a fulcrum between the thrust component in the y direction and the - S lift generated at the stabilizer. From this its easy to see why the downthrust prevents the plane from gaining altitude when the throttle is opened.

When the thrust T increases the plane accelerates to a new constant speed as drag D increases to the same magnitude. However, wing lift L does not increase with speed (V^2) because the y component of thrust increases causing an angular acceleration (a slight pitch rotation) which doesn't stop until the stabilizer counters it with increasing lift of it's own. The only way for the stab to increase lift without a pilot input is to increase the angle of attack (the tail rises) which results in the wing coming to equilibrium at a new (reduced) angle of attack. When the plane isproperly trimmed this results in level flight at all throttle settings.

</span></p></div>