How to trim without subtrim
 

Hi,

I have a hanger 9 Saratoga with O.S .46 AX that requires a good 5-6mm of up elevator to maintain levelflightat 60%-80% power. I have added 60gm of weight to the tail in 15 gm increments and found it balanced and flew better with 30-45gm on the tail but it still required a large amount of that up elevator. This is creating a lot of drag and makes inverted and knife edge type flying very difficult.

Any ideas on how to level the plane out without adding large amounts of subtrim and hence adding lots of drag etc.

Thanks in advance

Stix

RE: How to trim without subtrim
 

Just some background.

On the engine: The Saratoga is designed for an Evolution 46 / 52 or a Saito 82. Those engines, as well as your OS 46 AX, total about 17.5 oz. (or 500 gm). So nothing unusual about the "expected" weight as compared to "actual" weight of the engine.

On CG: The manual states that the CG be at 70 mm back from the LE (range being 64 to 76 mm). You added 60 gm (about 2.1 oz) to the tail created a tail heavy (but flyable) airplane. I know that the 70 mm is a good starting point for a static CG before you test in-flight trim. What is the final CG?

On wing incidence: The tail feathers are bolted in place rather than being glued. The 2-piece wing is bolted together then the dowels on the LE are inserted into a former at the top front of the wing saddle and the rear of the wing is bolted down thru the canopy "hatch". The manual does NOT specify a desired wing or stab incidence angle. You got what they give you. So, do you have (or have access to) an incidence meter?

On elevator trim:Per manual the low rate is 22 mm and the high rate is 29 mm. The 5-6 mm up elevator is about 1/4" of trim. Where have you set the low and high rates?

RE: How to trim without subtrim
 

What you are adding is not subtrim. It's just trim. Subtrim is all about getting your servos centered properly before you start trimming your plane. It is also handy on transmitters with analog trims to get everything right electronically and fly with all the levers centered.

Now for your issue, without seeing your plane I would guess you have a wing incidence issue. In the case of needing a lot of up elevator that would indicate too low an incidence angle compared to the horizontal stabilizer. Before you go changing it though, you need to understand that there are 3 different adjustments that affect pitch issues. CG, wing and stab incidence, and thrust line. Since the stab is permanently glued in, there's not much point in trying to adjust it although that may well be the cause of your problem. How exactly are inverted and knife edge flight very difficult? If you are needing a lot of down elevator to maintain inverted flight, you may still be nose heavy, but a glide test should let you eliminate that. If the plane tucks in knife edge (I would expect toward the canopy if it's an incidence issue) getting all 3 right will certainly reduce it but may not eliminate it. Before messing with the incidence, make sure the CG is right to give you a fairly shallow glide until you start stalling, then test the thrust line (there should be no pitch change when going from full throttle to idle or vice versa). Only then can you set the wing incidence, but then that will probably require you to go back and adjust the other items.

RE: How to trim without subtrim
 

Ok let me try and answer these questions as best as i can.

CG is currently 2.85" back from LE, i don't hyave an incidence meter but may be able to borrow one. Low rates are 22mm and high rates 30mm,

Yes i need a lot of down elevator for inverted flight and yes it does tuck during knife edge.

The way it is trimmed at the moment i have 7mm (I measured properly) of up elevator and if i reduce power to idle and centre the stick it dives steeply, not a gentle glide. Increasing power from 70%-80% to 100% starts quite a steep climb.

Thanks for the help guys, this stuff is a bit beyond my skill level but i love this plane and want to get it flying right, not with a heap of drag from the elevator.

FYI it does not glide well at all, it does not like slow landings and even a little bit of flaperon induces stalls at a low speeds. It does not handle like the reviews of this plane state, it is more of handful in the air than my .40 sized extra with o.s .55 in it.

Thank in advance

Stix

RE: How to trim without subtrim
 

H9 has pretty nice planes. I have had and still have several. That being said, any manufacturer can have a lemon come off the line. Check for any obvious construction errors with the fire wall. Your discription of the flight characteristics is indicitive of a thrust line issue or a very nose heavy plane. You may also have an assembly issue. If you added resin to fuel proof the fire wall and have a blob of it behind the mount causing the engine alignment to be off. Engime mount screw loose? Check all these and anything you can think of that would throw off the engines allignment.

RE: How to trim without subtrim
 

I'm not very familiar with thrust lines etc but i did notice from the start there is a 2mm gap between spinner and cowl at the top and a 3.5mm gap at the bottom.

I fuelproofed after i mounted the engine mount so i didn't mess with alignment. If my engine is pointing "up" to much wouldn't that give me the opposite problem to what i'm having?

Thanks in advance

Stix

RE: How to trim without subtrim
 

I'd add support to the suggestion to check the incidences and engine thrust angle. I'd also return the CG to the prescribed point until the other issues can be resolved.

Post the results of the incidence and thrust readings and clarify whether the stab is bolt on or glued. Also to help clarify for those of us not familiar with the plane... is the wing foil symmetrical or otherwise?

RE: How to trim without subtrim
 

It is really hard to say without knowing the numbers. An example of the difficulty is that when a lot of up elevator trim is needed for powered flight trim... it can produces a too shallow or too steep of glide slope dependent upon the stab/elevator proportions and authority of the stab or elevator. What I'm saying is if the stab has a lot of authority and it is positive (as might be the case), it will produce the effect you noted of a too steep glide slope. If the elevator has more authority, the up trim that you describe would produce a too shallow of glide slope.

RE: How to trim without subtrim
 

i have a few arf planes with the engine thrust angle "built in" but still had to add a small amount of down thrust to the engine

RE: How to trim without subtrim
 

There's a guy, Dean Pappas, that has published a great 3-piece (pdf files) article on "trimming" - starting with static "on the fingers" to dynamic "perform this maneuver and observe behavior" discussion.

Ground Up Part 1

Ground Up Part 2

Ground Up Part 3

HTH

RE: How to trim without subtrim
 

Everything the OP has said so far indicates the airplane is nose heavy. The other trimming suggestions and articles are of little help until the CG is set correctly.

RE: How to trim without subtrim
 

I have a Saratoga 40 with a .46 Thunder Tiger Pro engine.  Very nice flying model.  

I try to set up all my planes with minimal digital trimming.  That is - by adjusting the control linkages and leaving the servos at their natural zero. 

I did set up the Saratoga's C.G. a hair (1/8") behind the manual's suggested range. 

Years ago I was shown a way to tell if your C.G. is good.  Fly upward at about 3/4 throttle on a 45° upline.  Roll to inverted and, after the line is steady release the sticks.  If your nose falls you are nose heavy.  If the nose rises you are tail heavy.  If it holds a climb for three or four seconds before dropping that's about perfect.  This is best done about halfway through a tankful of fuel.

It's been a reliable method for me.

Note that you can also raise or lower both ailerons to change the effective angle of attack of the main wing.  If you are needing to add a lot of trim elevator try lowering both ailerons a tiny bit (a few mm at a time - two turns of the clevis).  The bad news is that adding very much will make the model throttle sensitive (like a flat wing).

RE: How to trim without subtrim
 

Cheers for that charlie, i will give that a go!

Stix

RE: How to trim without subtrim
 

I agree that everything I've read here sounds nose heavy. You're toward the front of the CG range, so move it back a 1/4 inch and see what that gets you.

On the thrust line, the relation to the cowl isn't important aside from looks. Put a level (incidence meter is more accurate and easier to use) on the horizontal stabilizer and shim it on your workbench to get it level. Then use your level to see how far off of 90 degrees your prop is compared to the stab. I would expect 2-3 degrees of down thrust and maybe 1degree of right on a model like that. If you have up thrust that would explain your diving glides (or at least contribute to them) as your tail trim would be compensating for the thrust pulling the plane up. Measure it (and keep good notes in case you need to go back) and only fix it if you truly have up thrust. Then do a thrust line test as described in the trim manual.

RE: How to trim without subtrim
 

I'm not getting how everything the OP has said indicates a nose heavy condition. I've reread the OP's post and he says it is balanced at 2.85" aft of LE. That point is confirmed to be the published CG by John Gilmore.

RE: How to trim without subtrim
 

Was the model ballanced properly? Perhaps the OP should get with a more experienced modler to verify the existing ballance is as stated.

RE: How to trim without subtrim
 

The plane is being pulled down... rather than up.... hence why it needs more than 1/4" up elevator trim. The climb at more than 80% power might be due to the gross amount of up elevator trim simply having more authority at greater speed and engine thrust.

We only know a few facts so far.

1. It is balanced on the prescribed CG
2. It requires gross up elevator trim for level flight at 80% power
3. It dives at low power
4. It climbs at more than 80% power

We don't know some critical facts

1. The decalage relationship (incidence numbers)
2. Engine thrust angle
3. Airfoil symmetry (symmetrical or asymmetrical)

Thrust angles have nothing to do with a plane that is diving at low power unless the elevator is trimmed down to counter powered flight lift and that is not the case... so set thrust angles aside for the moment
CG is correct... so set it aside as the cause of the low power diving.
With the elevator trimmed grossly up and it still dives at low power... that points to what you suggested.... the most likely explanation is a decalage problem, ie that the stab is too positive.

RE: How to trim without subtrim
 

Doing so will make worse the existing off power dive angle problem.

RE: How to trim without subtrim
 

I'd never argue with verifying the facts. Speedracertrixie is quite correct that the first effort in trimming is setting the balance point and you are correct in that is should be done accurately. I simply take the OP's word that he has checked the CG and it is on the prescribed point.

RE: How to trim without subtrim
 

Stix... given that the Saratoga has a bolt on stab, it is very easy to adjust the stab incidence. The conditions you describe indicate that it is too positive. I'd loosen the stab mounting bolts and start with a 1/32" shim under the aft end of the stab. Make adjustments in no more than 1/32 at a time.

RE: How to trim without subtrim
 

AA5BY is likely right, but I'm also going to suggest measuring the incidence before doing anything. Having a set of hard numbers to work with lets you really know what the problem is and lets you put it back exactly as it was if your adjustment doesn't help. It also lets you make fewer adjustments as you can get a feel for how much difference a given change makes. If the budget doesn't allow for buying an incidence meter now, you can mark the center line of your airfoil forward and aft and use a ruler to measure the difference. Then divide your chord by 360 to find out how much difference in height a degree makes, and divide that number into your measurement to get the incidence.

So for example, if your chord is 7 inches and you measure a 3/32 difference in height, you'd start with 7/360 which gives you .019444. 3/32 as a decimal is .09375. .09375/.019444 is 4.82 degrees of incidence. To do this right, you have to block the fuselage so that it cannot move at all, and do your measurements on the exact center line of the airfoil. Leave the marks you make until you're done with adjusting so you get accurate measures of the changes you make. Things can be complicated a bit by the movable control surface on the stab, so you have two choices to handle that. You can clamp a straightedge on the elevator, always in the same place, and measure at the back, or you can measure the thickness at the trailing edge in front of the elevator and make your measurements at the top then adjust for the thickness. So doing it that way, if your stab is 1/4 inch thick at the TE and you measure from the top, you'll add 1/8 inch to your measurement to be at the center.

Of course option 2 is to buy an incidence meter and attach it to the surface, then read the dial to get your incidence!

RE: How to trim without subtrim
 

I have checked and re-checked the CG and it is right. I think trying to measure it manually is a bit beyond me but i will try to borrow an incidence meter.

Thanks for all the help guys it is greatly appreciated, i will report back with numbers when i have them.

Stix

RE: How to trim without subtrim
 

Ok, how about a way of determining difference in incidence without a meter.

Set the stab to 0 degrees. Set you plane on a table. Use a small level and place it on the stab (I hope that your plane's stab is flat). Use "stuff" under the tail to raise the tail until the stab is level. Cool, it's now at 0 degrees.

Measure the height of the center of the trailing edge (TE) of the wing from the table. Measure the height of the center of the leading edge (LE). What's the difference? What's the length of the wing's chord (distance from the LE to the TE)? I just measured the angle where the chord is 12" and the difference is 1/8" - the angle is 1.3 degrees.

If you give me the length of the chord and the difference in the height between the LE & TE I'll give ya the angle. I'm using a tool that lets me adjust the three points of a triangle and it will display the angle.

Yea, I'm being a butt head by using inches and there is a trigonometry formula to determine the angle.

RE: How to trim without subtrim
 

Ok, if the chord is 300 mm and the difference between the LE & TE is 5 mm, the angle is 1.3 degrees.

RE: How to trim without subtrim
 

Ok i can do that, where do i measure the wings? tip, root or middle?