Lofoten

I have a well built Patternplane designed by a friend. Fuselage 195cm, w.span 185cm
It used to have a OS 108FSR (BX1) with a Macs Pipe.
The plane (4,5kg) felt a little underpowered, especially on the tophat and the square loops.
I have now bougth a OS 140 rx that i plan to prop with a 17x12APC. I have now built a DIY Hyde mount and got a DIY nosering made. (http://home.online.no/~ofremmi/HowTo.../HydeMount.htm) thanks Ola
I am now in the process of lining things up and plan to have 2,5 to 3 degrees sidethrust, but I am wondering about skipping the down thrust and instead add a throttle to elevator mix to help on the uplines and downlines.
Has anyone has tried this on a real patternplane, and what effect did you get?
I have a 90 size patternplane where I have about 1 washer of down trust, and ended up with -1% throttle to elevator mix( -1%elevator at idle) to make the downline straight (instead of pulling to the belly.)
All comments and hints are apreciated, hell we might get a good discussion going....

tommy s

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Lofoten

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8178

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There are a lot of designers and flyers that build right thrust into their airframes to compensate for the P factor in some cases the spiral air flow from the prop air flow hitting the vertical stabilizer. I have never understood this compromise because of the impact of what it does when the airframe is inverted and pulling negative Gs. Right thrust settings when you are flying inverted become LEFT thrust! Left thrust really adds to the P factor impact and messes up the flight characteristics big time. I find that “0� right thrust works best for me. Can someone explain why left thrust compromise is good thing when you fly inverted?

EAlt46

Due to the spiral slipstream from the prop, the left side of the fuselage, fin and rudder is always subject to higher pressure than the right side is, so regardless of aircraft attitude, the remedy is the same. Let's say the model is upright and you are viewing things from behind it. The spiral slipstream is pushing the tail to your right and the nose to your left and it's easy to visualize why right thrust would help offset it. Now flip the model over, still viewing it from behind. The spiral slipstream hasn't changed any, i.e., it is still pushing on the left side of the the fuselage, fin and rudder. The only difference is that you perspective has changed. It's still pushing on the left side of the model, which is now on your right, so the tail is now being pushed to your left, making he nose want to swing to your right. The engine right thrust that used to be on your right side from your perspective is now compensating on your left side, still correctly offsetting spiral slipstream by tugging the nose to your left.

Without the upright engine right thrust, most models will want to yaw to your right when inverted during pushes to vertical, during sustained vertical climbs with the belly towards you etc. Believe it, it's true and 100% proven.

MHester

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Well think about it. It's the prop wash hitting the vertical fin and rudder that cause most of the effect, in addition to the side of the fuse.

Now, flip the airplane over inverted. The airflow is spiraling the same direction, except now the vertical fin is on the bottom, backwards.

That's an oversimplification, but it's the easiest way to explain it. It works. But it's a fine line between thrust and rudder trim, and neither is ever perfect. Even with all the mix in the world, at some point in some attitude, you're going to have to get up on the sticks and fly the plane. The thrust just makes it easier on the pilot in most circumstances.

Wing incidence is another area of set up you can discuss all day. Ever noticed how hard it is to get a clean spin break with your wings set at 0? Now give them +.5 deg positive, and voila....sweet spin break. But in FAI, the spins are inverted......OOPS! So once again, well......

Set up is the most personal part of pattern. If you fly 100 peoples planes, you'll experience 100 different setups. Especially as you get closer to the top. Use what works for you, but above al try EVERYTHING. Just don't do anything drastic, so you can get back to where you were if you hate it.

-Mike

8178

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This is how they handle spiral air flow from the prop on some of the full scale aircraft that have that problem http://www.simhq.com/simhq3/sims/air...m/index4.shtml

Lofoten

What about downthrust, will it have an adverse effect to skip it and add throttle to elevatormix.
This for making patternflying easier to get predictable uplines (ex stallturn, going up at 3/4 full throttle, then going to Idle, where a slight heading change towards the canopy as the effect of the engines downthrust has deacreased to a minimum. meaning you have to push a little bit to avoid the plane to flick over.
The same effect will be evident going down. remember the plane has been trimmed to have levelflight at cruising speed. (3/4 full throttle with some downthrust), when going vertical downlines at a few clicks over idle, the positive elevatortrim(or Wing incidence) will cause the plane to pull towards the belly unless youre on the sticks at all time.
My point is: If you skip downthrust and trim the plane for level flight at full throttle, then mix in a throttle to elevator to straighten the uplines and downlines. Has anybody tried this ?

MHester

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I Know, I'm an A&P [8D]

But full scale planes for the most part aren't meant to do what pattern planes are. It is also affected differently due to the sheer size of the plane itself and the rudder.

Lots of very good pilots don't use right thrust, lots do. It's an inherent problem with propeller driven aircraft in general, and there are as many ways to deal with it as there are people. The right thrust is the most commonly used. But if you want a real nasty beast to try and fly, cock your vertical stab 2-3 degrees and then go fly the advanced, masters or FAI pattern I tried it once. Worked ok until I needed a lot of inverted flight segments. Then as the power setting changed, so did the yaw. Mix can't account for every flight condition, just a lot of them. The rest of the time you have to fly it out yourself.

-Mike

EAlt46

Your best bet is to study one of the trim charts that are available, but basically, yes, you do want to adjust engine down or up thrust as part of a complete trimming job. It's an oversimplification, but assuming that you have already got the rest of your trimming reasonably done, you can then start discovering what to do with up or down thrust changes. So, we're assuming your CG is right, your wing incidence is right (elevators are dead neutral for upright level flight at cruise speed), the model only requires light forward pressure for level inverted.

The first, easiest test for up or down thrust is to go to full throttle or a real fast cruise, level & upright, then yank the throttle back to idle and watch what happens. It should continue along for a hundred feet or so, then begin to gradually drop as speed bleeds off. If it drops fast, you need more down thrust and then probably some re-trim for wing incidence until you can once again do the upright & level hands off trick. Keep doing this pull-the-power exercise with any retrim needed until you get reasonable results. If it for some reason tends to zoom up when yanking the power, you need less down thrust or some up thrust. This exercise gets you close to the right down thrust, sometimes nails it.

Next, go into a sustained, high or full power, 90 degree climb and take you hands off. If it shallows, (goes to the canopy), add down thrust. This is fairly typical. Do the opposite if it tucks to the belly. Ofcourse this assume that you have an engine/prop combo that will haul the freight upstairs at a steady clip.

Now you can also get good downliine results by continuing to play with wing incidence adjustments, but it is easy and works well to add a throttle to elevator mix to compensate for slight tendencies to shallow or tuck. If you have to add too much mix, it will screw you up during such things as hammerheads. Again, you would do well to consult a full trim chart and follow it to the letter. I've given you excerpts for a part of the procedure to use. Keep in mind that you should only make one change at a time, observe the effect and move on. Also, many times you will have to repeat earlier trim steps when making changes like I've given you here. Get a trim chart! Better yet, get yourself a few years of Flying Models magazine archives and dig through until you find a series of Dean Pappas's discussion on trimming in his cilumn on Pattern flying. It's a topic he revisits on occasion and he is a master at this stuff.

EAlt46

Oops, said something wrong in the earlier post. In the upline test, I meant to say if it pulls to the canopy, not if it shallows. Anyway, a pull to the canopy does mean it needs more down thrust and vice-versa.

Don Szczur

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Good topic. The general rule of thumb I've found helpful is the trim chart; with the most impacts from engine thrust (to a lesser degree rudder trim) for uplines, incidence for spin entries (left wing or right wing), and throttle to elevator (or if needed aileron) mix at idle for down lines.

I can't explain it from the slipstream perspective, but I carry whatever right thrust and (up or down) thrust to carry a vertical upline with hands off (in calm conditions).

Don