Hi there,

I'm flying a pattern plane that is verly new, so I'm in the state of trimming. I'm using the trim-charts that are around on the web. My plane has got one strange habbit, and that is that in a vertical (power off) dive, the plane pulls towards the canopy.

Using the trim-charts I get confused, some say that I have to reduce the incidence, others say to increase it.

For me it's more logic to decrease the incedence, because it appears that the plane is having to much 'lift'

Has somebody some more trics to trim these 'power off' kind of things.

I already played with the CG, I moved it to the rear, and now my Knive Edge is much better, only a slight amount of pulling to the canopy.

Any help is appreciated.
Winfried

Here is a trimming guide which came with a kit I purchased. I hope the author doesnt mind me publishing it.

Give this a try.

Peter




Setting a patternship right, is a very complex procedure, here is a description of what we suggest to do. There will be other opinions around, and these might be as well useful and right, but for sure our suggestions will lead into a perfect flying pattern plane, and you can follow up the actions step by step.

Basically there are several momentums in flight, which influence the flying. First the CG, which is the center of gravity. Secondly, the CL, which is the center of lift. Both points are not the same. Moving the CG forward or back, wich change the distance between center of gravity and center of lift. The center of lift cannot be changed, the center of gravity can…

Then there are the angles of attack of both wings and horizontal stab. Both need to be right, but most important is the difference of both to each other, not so much the single angle of attack.

Last but not least, there is another variable, this is the thrust line of the engine. This is important, and should be measured in relation to the wing. It makes things easy, and we will refer in this section all angles of attack to the wing as a zero point.

Get the plane on your table, and first set the wing incidence to Zero. Then the stab should be negative (nose down) about 0.2-0.3 degree. You will need an incidence scale to measure this correctly. In relation to the wing, the down thrust should be zero. And the CG should be then exactly at the wing retaining bolt.

With these basic settings get on with you first serious trim flight.

First setting adjustment will be down thrust (and right thrust at the same time). Take the plane to a high level, and push into a vertical down line, throttle AT IDLE. At exactly 90 deg down you release all sticks. The plane should now continue to dive exactly vertical. If the plane goes to the belly, you need to uptrim, if it goes to the canopy, you will need down trim on the elevator. Do this maneuver several times, and set the elevator trim so that the plane goes exactly vertical down.

(In this maneuver there is only the incidence difference of wing of stab influencing the flight. No thrust line is involved, as engine is at idle, no center of gravity in relation to center of lift is involved, as there is a vertical downline without any momentums of both at all)

With the trim setting you found, the plane might dive or climb a little bit in horizontal flight, but please completely disregard this for this trim flight. The next step is, to fly vertical uplines with the SAME TRIM SETTING. At full throttle pull vertical up, and see if the plane goes to the belly or to the canopy. If it goes to the belly, you have to add upthrust, if it goes to the canopy, you have to add down thrust. It is that easy, and you cannot fool it. Do not change your trim setting at this time, land, and adjust the thrust line accordingly. After adjustment, make another test flight, to see if it improved. You should stop when the plane in vertical up AND down lines goes absolutely straight, without trim changes from your side. Then your down thrust (up thrust) is set right. Of course at the same time, you will find out about your right thrust, and you can correct at the same time, too.

Now the effects of the thrust line are eliminated, and the angle of attack issues relative to the thrust line and relative to both wing and stab are eliminated, too. Now you can continue to find the right CG settings.

Fly horizontal, and NOW change your elevator trim so that the plane flies straight and level. If your CG is close, you should only need one or 2 clicks of up trim. If you need more, this is a hint for a nose heavy setup. Flip the plane inverted, and see if it now dives to the ground severely. If it does, your CG is definitely too far forward, and you should move it back.

Try the same maneuver again, after you moved your CG, and you will find an improvement already. Set it so that the plane only dives very little bit inverted, when if flies with the same setting absolutely straight upright.

Do not worry if the elevators are not 100% neutral for all this testing. After you confirm everything you have flown, and can confirm that the incidences, the thrust lines and the CG is 100% correct, THEN and ONLY THEN, you can go ahead and change the incidence on the stab (and only the stab) permanently. Measure the trim you needed for perfect level flight at the trailing edge of the elevator. Then remove the rear anti rotation pin from the plane, adjust the hole, and reglue. You will have to change the trailing edge of the stab about the same amount as you needed trim in your flight testing. E.g. if you had to trim 1.5 mm down for level flight, you will have to rotate the whole stab so that the trailing edge comes down this same amount of 1.5 mm, then set the trim to neutral and fly again, to verify. Usually it matches with the first attempt.

Last but not least, you should test the balance around the rolling axis. First make sure that both right and left stab have the same angle of attack, and the deflections of your elevators are exactly the same on both sides. Then fly and pull up to a loop. Is the plane moving to the right during the pull, your right elevator might travel a bit more than the left one, or your right wing might be a little bit heavier than the left one. If it turns to the left, it is vice versa. Please try it many times, and confirm the direction to which the plane pulls. You might have forgotten even after your landing. Confirm it several times.

Then do the same thing inverted, and push up to a tight loop (negative loop). If one wing is heavier than the other, then the plane will now pull to the opposite direction than in the upright flight (positive loop). If it is indefinite, a slight adjustment on the elevator deflections might be necessary. If it is clearly pulling to the opposite direction, it will be most likely that both wings are not 100% balanced.

On a side note, it might be that 2 or more slight mistakes might eliminate each other, or add on to each other. To find out, you will have to make a lot of “dial in flights�, and you might have to do the procedure over and over again.

Looking at the pull to canopy in downlines only, you would increase wing incidence maybe 1/2 degree to start. Here's why it works: the increased wing incidence will give it a higher angle of attack in level flight - hence it will climb. To get back to level hands off flight, you will add down elevator trim. Now, since you had pulling to the canopy in downlines - you now have the "fix" (down elev trim) set into the airplane for you. Moving the cg rearward will also have the same trim affect.

I agree with UAL based on my limited personal experience,
I had the same thing going on and what you said is exactly what cured pulling to the canopy in the downline on my prophecy. Eliminating up elevator trim was the cure, and the way we achieved it was by adding positive incidence so you can level out the elevator trim and still fly level hands off on horizontal flight. This can also effect where the CG needs to be. Its an art for sure, Im glad I have friends who are experienced and REALLY good at trimming to help me.

If I understand correctly if the plane is built right you want to adjust CG and wing incidence so that the plane flies flat and level through the bulk of the range of the throttle with zero trim in the elevator. If I understand correctly if you achieve this you will not only have straight downlines but virtually no need for mix on knife edge.

A friend who flies advanced, designed and built his own plane and actually has zero zero, no trim anywhere. It is almost perfect, really cool to see how smoothly and effortlessly it flies. It will 4 point with virtually no rudder input.

I think you should test a vertical dive with full power.... or in a vertical up (safer)

if you get the same canopy pitch, so, work on wing incidence...

if you had change in pitch between these two situations (full and iddle) , try to work on engine down incidence

There is a VERY important principle here to consider....ALL trimming of a modern pattern airplane is choosing the compromise that you can live with!!! There aren't magic bullets, or "formulas" or "tricks". Sorry to put it so bluntly, but when one starts cranking on things on a design that already has the best compromise that the DESIGNER intended, it's a slippery slope, and you can EASILY end up with an airplane that is so terrible to fly that you HATE it.

The MOST important consideration in ALL your trimming efforts - is establishing the optimal CG. This is the one where YOU are "okay" with the elevator trim and knife-edge pitch characteristics that YOU can live with.

AFTER that, comes wing incidence, and thrust changes. BE extremely careful here...remember... a wing incidence change will require an elevator trim change. This will affect BOTH knife-edge *AND* downline pitch. Upline pitch is pretty much a function of engine thrust line. Most folks (including some REAL "name" FAI-types) get the airplane as good as it can be on everything, THEN....

They mix a tiny amount of DOWN elevator with low throttle. And leave it on all the time.
Unless you are a very patient and methodical "trimmer", follow that trim article above, or Mike Chipchases', or the NSRCA's, or *mine* (and Verne Koesters')....all of them agree. CG first, then your taste for elevator trim, then mixes - basically.

Hi there

Thanks so for for all the advice, coming weekend I have a competition, so I'm not gone chance anything before that. But aftre that we have 2 months of competition holidays so I will spend time trying to solve this 'habbit' and let you know the outcome.

Thanks so far
Winfried

trimming all begins with how you fly. do you trim the model to fly hands off upright, or as you fly across the flats, hold a small amount of up elevator, then, when inverted, the same amount of down?? this, i believe, is the starting point.

Good question, I think I trim for zero gravity. meaning, I have to give a slight amount of up to fly straight. With todays programs the amount of straight flight and upside down flight is almost equal. So there is no real advantage of flying the normal parts 'hands off'

I will watch myself a bit more coming flights to see if I really fly with this small up, or still try to fly hands off...

I will let you know

Winfried

if this is so, you have too much down thrust. your probably trimming the model with some positive incidence / up elevator to counteract the down thrust. therefore, when you do a downline with the motor at idle, the positive incidence is pulling the model to the canopy. set your elevator at zero. perform a high verticle dive power off and watch what the plane does. adjust your incidence from here till you have a perfect downline. form this, adjust your thrust so the model performs a perfect verticle upline. you will more than likely find, if your model has the wing below the thrust line, the motor will be carrying upthrust!! this is the step 1 and 2 of trimming.

Hi Andrew,

This sounds logical. As i said, comimg sunday I have my competition, after that I will start playing with these things.

In short, you wright the same test as the first reply to my Post.

Thanks again,

I will let you know.
Winfried