Thrustlines.......... A curly conundrum.

[PJ Rowland]

I read with interest Al rabe's posting about generally accepted rules of Stunt design, with over 40 years of design experience, countless wins at Nats level, his words should be taken strongly as law.

I dont dispute any of it, but it did get me thinking about one aspect of it:

" Any downthrust will create some asymmetry in lift. Asymmetry isn’t necessarily all bad. "

I agree with the above statement, its not possible to disagree..

Perhaps for the extremely technical amongst us :
My question is this:

Can you measure the Thrustline? What exactly is the force of that measurement. When I say force.. If referring to how you measure it, and what force it is in relation to.

Obviously the first answer is - yes you can measure the thrustline as the line running through the centre of the Prop blade.. Which is fine, I have no problem with that. However, is thrustline arbitory, or is a measurable, tangible force?

IF ITS a measurable tangible forces, is must be subject to the other forces working against it.. forces such as gravity.

How strong is the thrustline force vs the force of gravity, this is in reference to, if you run a Zero degree Thrustline, and an inline tail, or inline wing, with the hopes of have neutral thrust axis, does gravity effect it enough to nolonger be tangent with the orginal established line?

Converserly, if you run downthrust, that effect is gravity having on that upside down?


Is the thrustline tangible in any capacity as a force? and if so, what is required to calculate its effect. Think of a bullet being fired from a gun approx 36" from the ground, it takes approximately 104 Meters to fall to the earth. It will fall due to the laws of gravity.  I cannot work out how to write the formula here - But you CAN calulate it fairly accurately.

Can one calculate the effect gravity has on the Thrustline? Or is it of such tiny consequence that its not relivant to any aircraft discussion. ?

[Serge_Krauss]

P.J.-

This thread probably should have been started in the engineering forum, to avoid unfriendliness directed toward those who try to provide answers. Oh, well...

The thrust line is just an imaginary line that passes through the source of thrust (center thereof) in the direction of thrust. In geometry, lines are straight, infinitely long, and have position and direction. They are descriptive in this case because of their position and direction. This is important, but not the whole story; it is not sufficient to tell the story quantitatively.

In particular, thrust is a force, a push or a pull. To understand the model's behavior, we need to know how much push or pull there is and where it acts. Forces are quantified by how much they can accelerate a given mass: F = ma, by Newton's second law of motion. That is, the force necessary to accelerate a given mass by a given acceleration is equal to the the product of the mass and the acceleration: F = m x a.  Forces have magnitude and direction, which makes them what we call "vectors". They also have points of application. So to a certain extent, by showing the line through which thrust acts, the "thrust line" helps describe the effect of a model's thrust.

Thrust is the force that overcomes aerodynamic drag, accelerating the plane in level flight, until the drag rises (as the square of the speed) to equal the thrust (net fore and aft force equals zero), at which point the plane reaches a constant "top" speed (fore/aft acceleration is zero). That speed provides the air's interaction with the wings to provide lift. When the plane is pitched upward, the thrust provides some of the lift (i.e. the vertical force), while gravity joins the induced drag (drag from lift) to reduce thrust available for propulsion. If thrust equals or exceeds the weight of the plane plus the parasite and induced drags, then the plane can climb vertically, without losing speed..

So where does the thrust line come into this discussion? Several ways. Al covered one in his discussion of down thrust, where he discussed aerdonymaic effects. There are a couple other basic areas, which I think are at the root of your question. One is that the direction of the thrust (down thrust, or the familiar out-thrust for line tension) affects how much thrust is available for propulsion. The other is the positioning of the thrust line, which concerns moments (leverage to rotate the plane) about the plane's center of mass, or "c.g." as we call it. This is an important consideration in down-thrust and out-thrust too. In all of these cases we must talk about components of thrust in directions of interest.

I have to leave here in about 20 minutes (I hope!). So cutting this short (!), the diagrams below may suggest answers to your questions. The first one simply shows the components of a thrust vector in two perpendicular directions - like "forward" and "down". You can see in it that tilting the thrust vector downward diminishes the forward thrust slightly, while creating a downward force component. The second shows how a high thrust line can create a moment (torque) about the c.g. to, in the case of the Sterling Yak-9, tend to pitch the plane down. Such a torque must be countered by drag forces above the c.g., up elevator deflection, and Al's gyroscopic precession. The torque is defined as the product of a force (thrust here) by a lever arm, the distance it acts above the c.g. in this case. The torque or pitching moment here is defined as the thrust multiplied by the perpendicular distance from its line of action ('thrust line') to the c.g. This analysis works with out-thrust to show how it helps by bringing the thrust line inboard of the c.g. to rotate the plane's nose outward, if the c.g. is in a reasonable place (not too long an inner wing, enough tip weight...). The c.g. must be far enough out that the thrustline passes inboard of it. Gotta go!

(Edited later to embellish some and correct a typo; 'down' and 'up' where interchanged. Also diagram corrected to return c.g. to original height; add 16 to number of views)...and I'd thought I'd typed "precession".

SK

[Steve Helmick]

Cross-control yeilds stability, in general. Like "toe-in" in the front suspenders of your car or truck.  Steve

[M Spencer]

What happens if the Propellors at the Back ? ,

incedently the Do 335 was quicker on the rear with one motor out .

[john e. holliday]

Did you also notice that some of his air foils are not full semetrical.   

[Douglas Ames]

Since engines are fixed, so is the thrustline (angle) relative to the fuse. centerline. It won't change, however downthrust becomes upthrust when inverted. This is my take on Al's statement - "Any downthrust will create some asymmetry in lift"...
Another variable between models is the angle of attack need for level flight - upright or inverted. This is where any downthrust would have different affects depending on overall design - aifoils, moments, etc.

[John Miller]

Some time back, here on the forum, we had a long and interesting discussion regarding this subject. I would recommend anyone truly interested try and find the threads, and read them over.

Down thrust is one of those items that has complicated effects, and the most useful is to counter prop effects. The best thing is that it works either upright, or inverted agianst those prop effects.

Also, remember that when using symetrical airfoils, you don't have lift until you have some angle of attack with the airfoil. With a 0-0 ijncidence between the thrust line and the airfoil chord line, you wind up with upthrust that also magnifies the prop effects trying to force the nose up in level flight.

To each his own, of course, I used to expound on a 0-0-0 set of incidences, thrust, wing chord, and stab. Today I see it much differently. I advocate, and use about 2 degrees of down thrust  0 degrees incidence in the wing, and about 1 degree positive incidence in the stab.

[Brett Buck]

Since engines are fixed, so is the thrustline (angle) relative to the fuse. centerline. It won't change, however downthrust becomes upthrust when inverted. This is my take on Al's statement - "Any downthrust will create some asymmetry in lift"...
Another variable between models is the angle of attack need for level flight - upright or inverted. This is where any downthrust would have different affects depending on overall design - aifoils, moments, etc.

   Al, and everyone else, is looking only at the air. The most obvious thrust line effects are the relationship of the thrust line to the CG, not how it effects the airflow.

   Brett

[Serge_Krauss]

What happens if the Propellors at the Back ? ,

incedently the Do 335 was quicker on the rear with one motor out .

Any pitching moment from high or low thrust lines will not change with choice of fore or aft propeller location. However, inertial and propeller effects on aft-mounted control surfaces can change considerably. I can see why the control quickness might change markedly with the Do 335's prop stopped. That's also a pretty big gyroscope to be jerking around by a tail with such a short "handle".

Since down-thrust can change the pitching moment resulting from thrust lines that do not go through the c.g., height of the thrust line might affect the amount of down thrust preferred. The exaggerated drawing below shows how the moment arm of the thrust can change with increased down thrust angle compared to the earlier drawing (Apologies for inaccuracies in these drawings. They were done using Microsoft Word drawing tools, which do not allow all lengths and angles).

SK

[Howard Rush]

Force normal to the free stream from a prop at an angle to the airflow is a significant effect.  Engine offset on a combat plane causes a lot more line tension than T sin (offset).  Anybody know how to calculate that?

[PJ Rowland]

I still didnt get an answer : Can one calculate the effect gravity has on the Thrustline? Or is it of such tiny consequence that its not relivant to any aircraft discussion. ?


The closest I saw was : The thrust line is just an imaginary line that passes through the source of thrust (center thereof) in the direction of thrust.

Which indicated that the thrustline is simply an arbitory line, with no real " force " hence unmeasurable.

Thrust itself surely must be measurable, what and how do they take into account thrust vectoring?  There would have to be something thats of measurable qualities.

This has no impact on anything Im designing or trying to accomplish, I just read with interest Al's postings, and pondered something that seemed of interest to me and some other like minded people.

[ash]

Your question doesn't actually make a lot of sense the way it is worded.

The thrust line is just a imaginary (but not arbitrary) line. Gravity doesn't affect the imaginary.

Gravity also doesn't affect the force that acts along that imaginary line, it only applies a force to mass.

If you mean to ask, "can one calculate how to adjust the designed thrust angle to compensate for the effects of gravity on the model?" then that is a different thing.

The simple answer is "yes". The doing of it is not so simple. Trial and error is much more practical for most of us. The determination of all the contributing factors and their values is much harder than just trying a shim under the engine mounts and seeing what happens.

[Allan Perret]

Gravity affects the plane as a whole, or as a system. 
It doesnt affect the thrustline specifically relative to the wing, fuse, or any other component of the plane.
Thrustline is simply determined by engine mounting,  it is same as the centerline of engines crankshaft.

[PJ Rowland]

Thanks ash and serge. Both were very infomative. I did want to further understand the relation to thrust, and the thrustline.

The main issues were : When you design inline setup ( not that I do ) If you cannot properly calcultate the effect gravity and drag have on the thrust direction then how can it truely be considered "inline"


Im just deviating from BOM and Nats discussions. I find it fascinating.

[Peter Nevai]

When speaking of gravity, you have to fall back on basic physics. There thrust lines and such have no bearing. Gravity exerts 1g at all times, regardless of thrust angle. The airframe and power plant will always weigh the same and have the same terminal velocity regardless. Energy is expended either against 1g or not. In this case, the quantity of thrust available in any direction is most significant.

A spinning prop will pull anything attached to it in the direction it is pointing. Either it is accelerating towards the ground or fighting against gravity. Either you have enough lbs of thrust (move more weight in air than the model weighs) or not. Thos is a real generality but I'm typing this on my phone and is as comprehensive as I can stand.

[john e. holliday]

The only time I worried about thrust angle/line was when I was flying RC.  Even had arguments with guys on the planes with full semetrical airfoils.  I set every thing on a zero reference plain and adjusted as I flew the plane.  In control line was worried about keeping everything in line with a little out thrust.

[ash]

The main issues were : When you design inline setup ( not that I do ) If you cannot properly calcultate the effect gravity and drag have on the thrust direction then how can it truely be considered "inline"

I think you've managed to misinterpret something quite badly somewhere along the line. I don't know what it is, though. You need to go back to "first principles" to understand how all these factors interact and it looks like you're kind of diving in at the deep end with this.

Thrust is the force provided by the propeller and it acts along the axis of the engine crankshaft. That axis is referred to as the thrust line. Think of the thrust line as simply the direction the prop is blowing air. That's it.

Gravity acts on the mass of the model to make a force called weight. For mathematical and conceptual simplicity, this mass can be assumed to be a point mass equal to the total mass of the model, located at the CG. Gravity doesn't affect anything but the mass (aka weight).

I think what you're getting at is the effect of having the wing and tailplane offset from the thrust line. That's where drag comes into play. Drag is the force induced by the air moving over the solid structure. Notice how these are all forces? They don't affect each other, they simply add and subtract to/from each other as they are different flavours of the same thing.

If you have a force offset by a distance, it's called a moment (consider it a twisting action). We all know about nose and tail moments (ignoring the (ir)relevance of the common terminology). Just like these moments from nose to tail about the CG, there are moments above and below. Lets assume the thrust line passes through the vertical CG. That's the force pulling the model forward. The forces holding the model back are mostly the wing drag, the undercarriage drag and the tail drag. For the model to fly at a steady speed, all that drag adds up to exactly equal the thrust. They cancel each other out mathematically.

Gravity has no part in that basic equation in level flight. Gravity ony affects the basic lift/weight balance which can be considered independently for the purposes of basic calculation.

So, all those drag forces below the thrust line will induce a nose-down moment. All those above the thrust line will induce a nose-up moment. We prefer the up/down moments to be equal otherwise we'll have to apply some degree of control to fly level. Inline designs assume that reducing those offset distances to zero will balance the moments above and below... or at least reduce their effect.

Perhaps this is what you're getting at. If the engine centre line, the wing centre line and the tailplane centreline all line up, it's inline. End of story. That's all the definition "inline" means. Of course, those aren't the only drag forces acting on the model in level flight, nevermind in a turn! So, no... inline geometry doesn't guarantee a balance of drag induced moments above and below the CG. So what? Who's calculating it? Fly it and trim it. This is exactly what we're correcting when we adjust the handle and elevator/flap neutral to get even level flight.


The fact is, this is a super simplified look at the forces in play. Once you introduce acceleration (eg, climb and rotation) things get a great deal more complicated in the calculation department - although still not impossible.

It's very late here and I'm losing track of what I'm saying so I'll come back to this another time.