thrust angles, important? why?
 

i'm thinking of putting together a hanger 9 pulse XT 40

i've seen one where the 2 stroke engine was mounted 90 degrees so exhaust exits under fuse and it looked alot cleaner. i was told by LHS that if i did that it would change the thrust angle and could cause problems. i have no idea about thrust angles and wonder why they are so important?

the only reason i want to have it at 90 degrees is for looks, but i'd rather have a good flying model.

RE: thrust angles, important? why?
 

*blink*

Did your LHS explain how, exactly, rotating the engine would change your thrust angles???? Because I'd SURE like to hear THAT one.

Go ahead and rotate it...the engine's going to point the same direction regardless.

And ya might want to look into finding a new LHS *heh*

RE: thrust angles, important? why?
 

I'm with gboulton on this one; you need a new LHS...:eek:

Thrust angles are important for a number of reasons and can affect flight characteristics. It's my guess that you're not really interested in that right now, nor am I really wanting to try and explain it all right now.

Suffice to say the manufactures have done flight testing on their models and established thrust angles which work best for that particular model and recommended engine range. Rotating the engine does nothing to affect thrust angles as gboulton said since the engine's centerline does not change in relation to the firewall.

RE: thrust angles, important? why?
 

Your LHS rep is an idiot! "and you can quote me on this." This leads me further to say if you need something, ask us here first before you buy something you don't need

RE: thrust angles, important? why?
 

Since we're all sitting here bashing your LHS...in whom you may have faith...it's probably best if one of us delve at least a bit into where angels (and zeeb) fear to tread. *heh* So, without getting TOO far into the physics and mechanics of things, here's a little "thrust angle primer" so to speak.

Let's take any airplane you wish to name. It has a "true center", or 'reference line'...that is to say that, if balanced correctly and trimmed for level flight, it will travel forward directly on that line. it is the 'center of flight' if you will. There are bits of the airplane above, below, and to the sides of this line, but this is the center line of the flight path taken by the airplane. (Yes, the actual definition of the reference line is considerably more complicated than that, but that works for this discussion, so go with me)

Now...everything would be hunky dory if we could mount our motor such that it provides thrust directly along that line. It pulls, in other words, EXACTLY in the direction the airplane wants to fly, imparting no other forces except straight thrust along that line.

However...'tis not to be. SEVERAL things impact its ability to do so, but the 2 most common are "P-Factor" and "Torque"

Torque is pretty simple. The propeller is a rotating mass, spinning in a clockwise direction (usually) from the pilot's point of view. Ok, fine. But, Newton was right...for every action, there's an equal and opposite reaction. So, as the prop rotates clockwise, the airplane tries to rotate counter-clockwise, or roll to the left.

P-Factor is a bit more complicated....in its simplest terms, one side of the prop-disc (the imaginary circular disc created by one rotation of the prop) produces more thrust on one side than the other...the side where the blade is traveling DOWN rather than UP, or, usually, the right side. This means that the motor is basically pulling the plane with more force on the right than the left...introducing a tendency to yaw to the left.

Finally, motors are quite frequently mounted such that the prop's center is not in line with the reference line, due to the shape/size of the cowl, motor, etc...frequently it is above or below it, meaning, again, with respect to the reference line, the airplane gets more thrust from below or above, leading to a tendancy to pitch up or down.

You combine all of these things, and what you get is an airplane that WANTS to fly in a particular direction, but an engine that it attempting to pull it slightly away from that straight line.

Many folks try, thus, to correct this with 'thrust angles'. They mount the motor slightly "tilted" in one way or another (or multiple ways) to offset these issues. For example, to combat P-factor, (that tendency to yaw left), they'll introduce "right thrust"...which i to say, they'll mount the engine pointed very slightly (usually a degree or two) to the right...P-Factor tries to yaw the plane left, but the engine pointed slightly right tries to yaw it right...the hope being that these two effects "cancel each other out". You'll also hear folks talk about "up thrust" or "down thrust" to combat a plane's tendency to pitch one way or another, depending upon motor location.

These attempts to manipulate the motor's physical placement so as to direct its thrust elsewhere are commonly known as "thrust angles". Quite often these days (as is the case with the Pulse XT) ARF manufacturers have (at least in theory) tested their airplanes to determine the 'correct" amount of thrust angle in various directions, and build these angles into the firewall/motor mount. You'll probably notice that the firewall on the Pulse seems to be 'pointing right'...it is.

And yes, depending on the airplane, these angles can certainly have a rather significant impact on how the aircraft flies, so, especially for a newer pilot, it can be pretty important not to change them if the manufacturer has set them. As you can see, however, simply ROTATING the engine can't possible change these angles. Now, if you need to MOVE it to rotate it...say, up, or left, or whatever, then yes, it might...but simply twisting it 90 degrees has no impact on the angles whatsoever.

Now...having said all that, I'll depart a bit from 'common wisdom' and say this:

A growing number of people are starting to realize something Chip Hyde knew a LONG time ago...that ALL of the "variables"...P-Factor, Torque, "Spiral Slipstream", etc etc...that we try to 'correct' with thrust angles are just that...VARIABLES. Torque and P-Factor, for example, are MUCH more pronounced at slow speed, high power, high AoA attitudes than they are at higher speed/lower AoA attitudes. As such, NO "thrust angle" is going to be correct for all situations. it may "exactly" offet a variable at 3/4 throttle, level flight, but then be well off the mark in a high alpha situation, or steep climb from slow speed.

As such, many of us build airplanes with "0-0" angles...in other words, mounted straight ahead, right on, or as close as we can get, to the reference line, with no up or down thrust, and no right or left thrust.

YES, that makes your airplane react to all the "variables"...but the reaction is, in a manner of speaking, "consistent", and thus, at least in the opinion of some (myself included), easier to trim.

Anywho, hope some of that helps. I'm sure it'll be disagreed with by several, but hey, that's half the fun. :)

RE: thrust angles, important? why?
 

I learned about thrust angles and wing incidence the typical way :Din the field . A very simple blus max high wing easy to fly sunfay flyer .It had down thrust build into the firewall . So while flying around I was wondering why I am fighting to keep this plane level and all it wanted to do was clime out of sight .Ok , maybe it needed more down thrust so , we added washers to the top two engine brackets , nope still climed , ok maybe the wing that was designed and set at the factory on a set position on the fuse was wrong ???? Himmm so I added in test increment 1/4 balsa pieces to the fuse where the wing was touching the fuse ---wala , perfact hands off flying .
That plane was a great teaching tool to 1)check every angle , every measurment , take wing foil configaration into effect and you will spend so many more hours at the field simply flying .
I went out and bought a wing incidence meter from GP , I check every surface , I also now measure the firewall for thrust angles , note what you find so if you see really negative habbist at the field you know where your plane well enought to tell someone at the field " he do you think it needs right thrust ect ect "

RE: thrust angles, important? why?
 

Thanks gboulton for the time you took for that post, very informative.

So i can rotate the engine and still have a good plane. I couldn't see why it would be a problem.

That's the thing when you're new to something, who's advice do you take and who's do you ignore? As I've said before, I'd rather know the "why" behind peoples advice, rather than just follow it blind.

RE: thrust angles, important? why?
 

Have to agree with LDM...as you progress, you'll find...or at least I have...that all those "geeky gadgets" like incidence meters, throw meters, etc, while perhaps not "necessary" actually provide useful information. Now, whether you use those particular tools (for example...I don't own a throw meter...just measure it with a ruler...and I use a laser level and protractor to measure incidence) isn't the issue...but understanding WHAT they measure, and WHY and HOW those numbers/measurements impact the plane in flight can go a long way to troubleshooting problems.

SOOOOOOOOOO many times, on here, out at the field, and in bull sessions at the LHS, you see/hear folks leap to "obvious conclusions" based on some aircraft behaviour or another. It climbed? MUST BE TAIL HEAVY! It rolled? MUST BE OUT OF TRIM! The reality is that many folks have what I call the "bag of answers"...and it's a small bag. *heh* For them, every aircraft behaviour can be explained by CG, Trim, and Thrust angles. These are frequently the same guys for whom every crash is either "a radio hit", or "that *&&$^#& [$BRAND_NAME] servo. I'll never use [$BRAND_NAME] servos again!!"

As LDM points out though...something as seemingly "minor" as wing incidence can RADICALLY change how a plane flies. One of my first kits was so sensitive to it that simply changing the length of the nose wheel gear by 1/2" made the plane go from "will NOT take off at any speed" to "comes off the ground at reasonable speed". Now, granted, the reasons it was so sensitive all had to do with me being a new, and thus VERY poor, builder...but it shows how many things can interact to dramatically change a plane's behaviour.

So...again...I agree with LDM. Learn EVERYTHING you can about the airplane...both how it flies, AND why it flies that way. That way, as you progress to bigger/better airplanes, when one does something funny out of the box or off the table, you'll be well armed with possible solutions.

Finally, I'm gonna toss this out there. I do this pretty regularly, and am frequently ignored...but hey...I'll keep doing it anyway. :)

It is my own personal opinion that ANYONE who wants to progress from "guy who herds an airplane around the sky" to "guy who can FLY" should read Wolfgang Langewiesche's Stick and Rudder: An Explanation of the Art of Flying. Yes, it's over 60 years old...and yes, it's about scale flight. But beyond pursuing a degree in Aeronautical Engineering, it is, imo (and the opinion of thousands of scale pilots as well) THE best education you can buy on what airplanes do, why they do it, and how to be in charge of it.

Just my $0.02

RE: thrust angles, important? why?
 

Thanks GB. You've thrown that title out before and when I wanted it, I couldn't find it. This time I wrote it down! Ought to make for some fine winter reading whiling away the hours waiting for resin to set.

RE: thrust angles, important? why?
 

:)

Glad you "found" it again. *heh* it's readily available on Amazon, so not to hard to find the book. When ya get done, give me a PM or something...would love to know what you think. :)

Oh...and no, the wheels aren't supposed to fall off when you land...they're supposed to do that during the take-off. ;)

RE: thrust angles, important? why?
 

Hey neat...just found this...apparently the whole text is available on Google Books....so here's your chance to get a bit of "light reading" done on the pc :)

http://tinyurl.com/36mwmp

(malavivius....sorry about the threadjack. :))

(EDIT : Correction...the whole book ISN'T there, just select pages of it...enough to whet your appetite)

RE: thrust angles, important? why?
 

Yup. That's were I found it too. Showed wifey dearest and told her I wanted it for my birthday. 40 years old Monday and I own more toys now than when I was a kid! Thanks again GB.

RE: thrust angles, important? why?
 

hey don't appologise, i'm going to find this book too.;)

RE: thrust angles, important? why?
 

I have engines upright, sideways and inverted. FYI, inverted is no problem as long as you lower the fuel tank. if you do not lower the tank, you wil have problems.

RE: thrust angles, important? why?
 

Right you are Gordon. A long time ago, I finally decided that adding offset thrust and down thrust was a crutch. IT enables more people to fly model airplanes without learning proper engine management, and without having to learn to properly utilize the control surfaces of the plane. How many full scale airplanes use right thrust, and/or down thrust? Fly the airplane, and leave the engine at 0/0. Plus you won't have funny looking airplanes with the engine sitting cockeyed in the airframe.

Bill, AMA 4720
WACO Brotherhood #1

RE: thrust angles, important? why?
 

Adding right thrust to a model to enable it to fly straight is a crutch.
Give us a break!!

RE: thrust angles, important? why?
 

GB, thanks for posting that link! After reading about 10 minutes worth on the net, I ordered it. Can't wait to get it!

RE: thrust angles, important? why?
 

*heh*

I dunno that it's so much of a "crutch" as it is a "baseline" if you will. I mean, the "range" of tendencies will still be the same in either case...it's just that with thrust angles in the motor that range can approach true symmetry at some specific condition of flight. Granted, we don't ever STAY in that condition long, so hey. *shrug*

To me, it's not that using thrust angles is "bad"...you just have to understand what you're doing and why. I've watched guys take a new plane out, fly it, and notice that it's "pulling left on an upline...must be thrust angle!". So, they go change it, and go back in the air, and now the airplane is "drifting right" during level flight...so, of course, they fix THAT with trim. Obviously, they say, the thrust angle is right because it's straight in the upline!

:eek:

It's just a matter of where you want your baseline of "0 pull" so to speak...idle on the ground, or at some precise condition X in the air. Since we so rarely achieve, and aren't likely to stay in, precise condition X in the air for long, I choose idle on the ground.

============

BendR...when ya get it, holler at me :) As I said above, I'm always game to discuss that book. *heh*

RE: thrust angles, important? why?
 

2-3 degrees isn't much and I offset the mounting of my cowl a tad and can't even notice it.

RE: thrust angles, important? why?
 

Heh, heh, heh.......:)

See what you started GB?

I apologize if I offended malavivius but I never know if a new guy is really interested in what's going on with an airplane. Since I have a commercial pilot license as well as A&P licenses, it's not that I "fear to tread" but more that I sometimes tend to offer more information and in a more technical manor than what is required.

I was also a bit miffed yesterday morning after finding out I wasn't going to get my new X9303 this month since I'd ordered it minus the servos I couldn't use anyway. There were no sets shipped that did not include servos so I get to wait another month.

Any way, you did a great job of explaining things. I'm even considering saving it for others who ask....:eek:

RE: thrust angles, important? why?
 

In effect, they all do. Most use vertical fin offset to the left to accomplish the same thing, but of course when they throttle back the airplane turns right until the speed stabilizes. And most older twins had the engines turned out by a very noticable amount. Check out the upthrust on the engines of a 727 or a DC9.

Jim

RE: thrust angles, important? why?
 

You'll find many people here continuously whine and complain about how poorly their engines run or airplanes fly. it isn't that they "don't get it." They just choose to ignore it. If these new guys want to start off in a Corsair, I say go for it, nothing like dropping $500 in the toilet in one morning, thus lesson learned.

RE: thrust angles, important? why?
 

As I said....to me, it's not so much a "crutch" as it is personal choice. IMO, it still comes down to the idea that, at some point, the plane will fly straight, and everywhere else it won't *heh*. Just depends on where you want that "point" to be. For me, I'd just as soon trim and mix to overcome any turning tendencies, and then not sweat dealing with offset and thrust angles.

And as jrf points out...some full scale aircraft DO have accommodations to help offset various torue, p-factor, spiral slipstream, etc tendencies. Of course, they have an advantage. In their case, they CAN pick a specific flight condition (cruise flight) and "tune" to that condition...since, unlike our models, they will spend a large percentage of their lives in that exact condition.

Of course, as CH says...2-3 degrees ain't much, and it's not like it's "difficult" to put it in, so hey...whatever works. This is very much a cat that can be skinned a few dozen ways...so long as he stays skinned. :)

In any event, I think all of us agree that simply rotating the motor isn't going to change the thrust angle, and that OP is pretty safe mounting his engine that way. :)

And zeeb...feel free to share that if you wish. Just remember...YMMV. :)

RE: thrust angles, important? why?
 

Not offended at all Zeeb, but yes i am interested in the technical side of things, eventhough i may not understand all of it yet.

Cyclic, I'm not sure if you're having a dig at me, but i was recomended this plane ( Hanger 9 Pulse XT 40) by a number of people who said it would make a good step up from a trainer.

GB, I ordered the book too.

RE: thrust angles, important? why?
 

Nope, no dig here, still back on the thrust angles. Any plane other than a scale or sportscale would make a good second plane.

Along with an earlier reply here somewhere. I don't mind basic technical aspects of things. Problem is I am a "basics" sort of guy. I could care less about the scientific explanation of a HUD, I learn how it operates and just want the darn thing to work and if it doesn't , that is not my area to fix it.

I listen to people tell others here how to tune a 2 stroke. good lord, can they possibly ramble (as I am now) on any more than they have too? Get to the point already.