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Old Jun 19, 2012, 01:11 PM
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Thrust angles & relative factor's

I decided to start this thread due to many threads with contradicting statements.Thrust angles on a puller are a given.I'm more interested in pushers.Almost all commercial high mounted pushers have the prop end up a little.I have heard this is mainly to counteract nose up during throttle or to lessen the distance between CG and thrust angle,therefore reducing pitch change.However I have also read that having horizontal stab in the prop wash on a high mounted pusher has a greater nose up effect than just pointing the prop end up.Im still a noob and always learning.I'm mainly writing this thread because I finished a scratchbuilt pusher which has an extreme thrust angle(16deg),due to location of tail boom.It was originally intended to have an EDF on top of the wing so prop clearance wasn't an issue before.I think the lift factor from the wing and the downforce on the tail from the small t-tail section will counteract the nose down tendency on my high mount pusher.When perched on a CG stand she noses down with throttle so I'm hesitant to give her a glide test because I'm scared she'll nose in.I followed formulas on adamone rc and used CG calculator's so I think I got everything else right.Just curious,does she look airworthy, even with the thrust angle?Any suggestions,or opinions are greatly appreciated.While I'm here,which way should a pusher prop mounted above CG point?Prop end up or prop end down?
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Old Jun 19, 2012, 02:00 PM
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The pitching moment caused by the thrust line is exactly like turning a bolt with a wrench. Think of your hand as the motor, the wrench as the pylon, and the center of gravity as the bolt. By looking at it this way you can see that an engine on a pylon directly over the CG is the worst case. You can not fix this by tilting the motor. The only thing on an airplane that can counter the pitching moment from this is the horizontal stabilizer. Making the HS that you have won't be very effective during takeoff because there's not much airflow over it yet. What you need to do is strengthen the vertical fin and move the horizontal up int the prop wash.

http://www.rcgroups.com/forums/showt...5#post11005529
http://www.rcgroups.com/forums/showt...24#post7092431

--Norm
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Old Jun 19, 2012, 06:23 PM
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Norm, considering moving my h stab up would require some unwanted surgery,do you think if I made the top t-tail section bigger and with the negative incidence already on it,maybe I could get the level power off glide Im looking for.Then once it gets a little airspeed and elevator authority I can start to attempt my takeoff climb.In other words would just making the top t-tail section bigger give me enough downforce on the tail to achieve level glide until elevator authority increases?Sorry Im just really trying to avoid major surgery if at all possible.Thanks for the insight so far, I believe your post about propwash on the h stab is the one I originally spoke of.Hopefully you can feed me some more info on this theory.
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Old Jun 19, 2012, 09:01 PM
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Still confused??????

Just when I thought I was beginning to figure things out I see this picture.I do believe a full T-tail would solve my scratchbuild problem because the elevator in the prop wash would make the elevator immediately effective.I must say Im suprised to see an airplane with a more extreme thrust angle than mine.Yet I'm guessing it flies fine.Any theories?..........(crickets chirping)
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Old Jun 19, 2012, 10:57 PM
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Increasing the size of the tip plate on your vertical fin would be a whole lot more efficient than tilting the motor as much as the plane in post #4.
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Old Jun 20, 2012, 04:35 AM
An itch?. Scratch build.
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Having a look at some of the articles on pylon mounted motors, (addition of power to glider, and some references to flyingboats etc). The trust angle on quite a few of them is 'up trust', i.e. the angle is up at the front.

There are reasons for that, position relative the the CG, angling the air flow down onto the horizontal stab etc.

But where is the change over point between up thrust and down trust ? (relative to the thrust line being above the CG as in the model shown and various pylon mounts).

I know it can also be influenced by other factors such as wing incidence, wing section, the amount of thrust etc.
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Old Jun 20, 2012, 06:40 AM
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So many variables

It sounds like setting up proper thrust angles on pushers is a fine art with many things that need to be taken into account.Judging by what's been said here,it seems like angles change slightly with each airplane based on first and foremost the design of the airplane.Slight changes like wing incidence or movement of horizontal stab could have different effects on the needed thrust angle.Well considering my motor pod's already glued on my scratchbuilt,I think I'll make my t-tail section bigger.If it still needs a lot of reflex(up elevator) to glide level than I'll probably switch over to full functional T-tail.It seems like the change from upthrust to downthrust or vice versa starts occuring at or near the CG.Moving the trust line in front of,behind,above,or below will obviously result in needed adjustments.I think if the thrust line is directly through the CG or center of drag then you can get away with mounting perfectly level.Geez,where's all the aerodynamic engineer's when you need one!
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Old Jun 20, 2012, 10:24 AM
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There is also the situation of the pylon-mounted motor located in front of the wing. I have such an airplane with such a motor, the Sea Cub II, which is based on somebody else's design.

Note that the motor has up thrust, which does two things: It provides a small thrust vector up to counter-act the nose-down tendency at low speed, and it provides some down wash on the horizontal tail. This seems to work well enough in flight, though ground take off is a long time coming. In flight, trim change is needed from low throttle to high, and vice versa.

To reduce scraping damage to the underside of the nose, when taking off from pavement, I added a small cheater nose wheel, in case I apply too much throttle on take off.

Then there is the case of the motor pylon mounted behind the wing in a pusher arrangement, like the Dynam Hawk Sky. This set-up seems to have neither up nor down thrust. So there isn't much corrective wash over the horizontal tail. When hand-launching, it needs less than full power, otherwise it will dive into the ground. Once it's up to speed, full power can be applied. I forget whether a trim change is needed from high to low throttle and vice versa.

Jim R.
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Old Jun 20, 2012, 04:37 PM
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After taking a closer look at thrust angles on the Hawksky and the blue and white plane I posted, they seem to have identical design characteristics.The blue and white one is smaller I think,but other than that the only differences I see are thrust angle and the hawksky seems to have a little more positive wing incidence.I can't yet say I fully understand why two planes with such similar characteristics need such different thrust angles.That's why I'm glad sites like this exist.Great input so far folks,keep em coming!!
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Old Jun 21, 2012, 12:48 AM
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My TLAR senses tell me that for the sort of layout you have that your motor angles are not far off at all.

As for the test gliding situation you do that power off anyway. Set the glide FIRST and then if the model pitches under power you angle the motor as needed. Never compromise the power off glide settings to allow for an odd motor angle. If you do you simply endup with a terrible flying design.
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Old Jun 21, 2012, 08:41 AM
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Just curious,what does TLAR mean?
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Old Jun 21, 2012, 09:57 AM
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TLAR: That Looks About Right.

I'm no expert in pylon-mounted pusher motor set-ups, like the Hawk Sky's, which is behind the CG, so I'm asking: to correct for pitch down with power up, do you angle the thrust line down toward the CG? Is that what I'm seeing in the diagram of Post #6?

Jim R....
...who wrecked his arm decades ago, so his pitching...
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Last edited by JRuggiero; Jun 21, 2012 at 09:59 AM. Reason: extended question
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Old Jun 21, 2012, 12:00 PM
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Jim and Nomadic. With a high thrustline the dominant force at work is a pitch down one from the thrust. That is why the engine or motors need to be mounted so they point up on a "front" mount setup and angled down towards the front on this rear style shown on Nomadic's model.

In Nomadic's case if there is still a tendency to pitch down when under power then he would need to angle the mount so it points down towards the nose even more radiclly.
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Old Jun 21, 2012, 12:42 PM
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BMatthews,

Thanks for clearing that up. I depend on guys like you and Brandano and Sparky Paul (where are you?) to keep my head above the intellectual waters of Modeling Science.

Jim R....
...whose science hopes were...
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Old Jun 23, 2012, 03:33 AM
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Two points to bear in mind that don't seem to have been mentioned so far:

i) What do you actually want your model to do when you open the throttle? For an aerobatic model, we usually want no pitching effect, whereas for a powered glider we normally open the throttle when we want to climb.

ii) Normally the throttle is increased in order to gain speed so, in addition to the pitching effect from the increased thrust (which is what has been discussed so far) the increase in speed will also almost certainly have a pitching effect of its own. This is why modestly powered models can get away with little or no downthrust even with a pylon mounted motor. Often these are beginners models and have a slightly forward cg position so that the increase in speed creates an upward pitching moment which is enough to counteract the downward pitching moment of the thrust/drag forces. If you were to fit a more powerful motor to such a model and open the throttle sharply, the downward pitch mentioned by an earlier posting would of course dominate.

The worst model I had from this viewpoint was one of those foamy flying hydroplane things. All the drag (and there was lots of it) was from the sponsons and the thrustline was of course very high. In the cruise it was fine but if you got into difficulty and banged the throttle open, it would head for the deck and when you throttle back to land, it would stand on its tail. Great fun, especially when the sponsons had taken on water. . .
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