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Author Topic: Trimming Discoveries  (Read 1580 times)
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Maurice
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« on: May 10, 2010, 08:30:19 PM »

Thought I might just throw my 2 cents worth into the ever expanding dark and mysterious world of trimming rubber powered free flight models. Let it be said right up front that I am no expert and I crash and bungle my way through the process! In this discussion I will talk about my "discoveries" with the F4F Wildcat from the Dumas kit. When I trim other planes I will attempt to pass on my findings and my failures.

When the big test glide day came I removed the prop and added weight to the aircraft (in the nose this time but it has been the other way around) to balance about 1/3 rd back from the LE. This is always approximate. I checked the washout (about 6mm) and the tail alignment and it seemed OK. I tossed the bits of tissue and stick into the where there might have been some breeze only to watch it (in great dismay and terror) roll in on the left wing and impact! SO! I tried more washout (heated it up to do that) and launched it again. It wobbled a farted around (sorry about that word) and flew, wing over, into the ground! Right, said Captain Nohope, more dihedral (which I did, I increased it about 1/2" or so under each tip). Powered up the plane a let it go; what a disaster! Over on one wing and SPLAT! Australians are good with expletives and I am not an exception to this!

OK, getting really cheesed off with the beast so calm engineering thinking needed free from emotives outbursts! So the model was returned to the shelf where I could glare at it hoping to intimidate it into behaving while I cooled off. The next thing I tried (it had this Dutch Roll kind of behaviour) was to increase the vertical fin area and it seemed reasonable but not smooth so I figured it was something to do with the decalage (this man is slow) so I tried bending down the elevators and BINGO! Oh yeh! On rails! SO then I "un-bent" the elevators and packed up the LE of the tailplane with a 1.5mm packer and now it works and it works well! There will be video proof soon. I was even able to remove some nose weight. This is good. Hope no one is too confused with this rambling and I will attempt to explain more in detail if needed.

I will make a spreadsheet of my trimming behaviours per model and hope this might help.

Cheers,
Maurice
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Maurice
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« Reply #1 on: May 11, 2010, 04:32:10 PM »

Here is the trimming data on the Grumman; it is what I had to do to get it to fly! It works. It is not perfect and I am sure there is a lot of room for improvement but it is a good ball park figure. Even putting the data together in some form of matrix helps with the thought consolidation!

Check it out
Maurice
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FLYACE1946
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« Reply #2 on: May 11, 2010, 07:43:20 PM »

Thanks for the chart. I guess I better start one for my models.
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crashcaley
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« Reply #3 on: May 11, 2010, 08:27:12 PM »

Maurice, Don't worry about the colourful words, etc. Yours are mild compared to mine, or are they?  Grin Very neato way of keeping a record on your model. I've seen some out in Perris that are religious(?) about keeping trimming records and other data on their models.

But back to your model trimming exploits. Very happy it survived the rough edges and is now trying to behave.

Caley
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What's stall speed?  Undecided
ironmike
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« Reply #4 on: May 11, 2010, 08:48:38 PM »

Maurice

Great thought process behind your chart

Now: How do you define and measure the wing incidence, is it the bottom surface of the wing or another line thru the wing airfoil measured against what?
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Maurice
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« Reply #5 on: May 11, 2010, 10:30:38 PM »

Hello Ironmike.

Ah hah! You know if I tell you THAT secret I'm gonna have to kill you (no, not true, my humour is weird). When I build the plane I run a line from where the nose button is installed to the tail. I usually do this with some piano wire so that it is reasonably rigid and I mark a reference point at the tail (usually just a dot with a marker of some sort) then I put some marks along the fuselage that I can measure to later using the wire as a guide. Now what I believe is that I have a zero thrust line (in reference to the motor hook being at zero; does that make sense?) and then when I install the wing (not covered at this point, I place a wire from the LE to the TE (through the midpoints of the LE and the TE) and tack it in place with the smallest possible drop of super glue so that it does not move. I offer the wing up to the fuse and secure it in place with pins (or whatever) and I measure the LE and TE of the "wing reference wire" in respect to the zero thrust line I described earlier. For zero, the measurement has to be the same for the LE and the TE. I also set the tailplane at that value as well (well, most times when I don't stuff up) and there you have it!

For determining angular thrust I use sine calculations for 1 degree, 2 degrees etc based on a length (say 3") so that I know what the opposite value (that is the opposite side to the angle of interest)will be (in mm's or inches) and there you have it. If someone wants a table of this I could knock one up. I make shims to this formula so I know how much to pack in. I have a thrust guide built into the back of my block that I use to determine down and side thrust and also provide rotational stability for the drive system. If someone would like a photo of this I can provide that.
Who let the dogs out?

Cheers,
Maurice
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Maurice
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« Reply #6 on: May 13, 2010, 12:42:39 AM »

Some more "thought" on what happened with flying this morning. The effort was focused on the F4F and as the video later will show Matt spent considerable effort in getting the model to track properly. Later I will list "the procedure" but for now it will be an "on field" discussion (rambling). OK, the model flew fine without power. This is done by removing the prop, balancing the plane and test gliding for smooth consistent performance; this was achieved. What this tells me is that anything else after that is to do with dynamics, i.e. spinning prop, thrust, lift etc and not the plane (because it glides, right?)

The model didn't climb significantly and still wanted to "wing over" on power (but it didn't do this on the glide, right?) so there was a thrust problem. Doing one thing at a time we worked on the turn. We ended up putting a tab on the low wing (the one that the plane rolls over onto) and the problem went away, HOWEVER, we have determined that it is a torque reaction problem (the banking turn that is) but now it is flying flat and not climbing SO more downthrust was added. The plane flew well, It climbed and turned but the tabbed wing was dragging. The tab was like an aileron dropping down on the wing. NOW, killing lift is easier than making lift and a dropped trim tab is a big drag doohickey so we transposed the tab to the right wing and set it UP (the opposite to the other side). Now the model climbed amazingly well BUT when the power came off it flew to the right. Well of course it would because the lift was being killed on that wing SO what that means is that MORE right thrust was needed and VOILA, the plane was trimmed. The tab was removed. Does that running commentary make sense? I will do spread sheet later.

Gotta go
Maurice
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ironmike
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« Reply #7 on: May 13, 2010, 10:45:19 AM »

Maurice

You are getting into the black art, BE CAREFUL.

Your wing reference line is a line thru the center of the LE passing thru the center of the TE?
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Maurice
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« Reply #8 on: May 13, 2010, 09:57:35 PM »

Hello Ironmike, yes that is what I try to do. There are probably better ways but seeing as most model airfoils are Clark Y (well I think so) it sort of works. It is a reference point to start with. Yes it is the dark side!

Cheers Maurice
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JetPlaneFlyer
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« Reply #9 on: May 14, 2010, 01:50:45 AM »

Maurice,

Setting the model up with negative decalage (i.e. the tail at higher incidence than the wing) really is, at best, operating on the ragged edge of stability. You may still have a little stability margin due to the setting of the wing the chord line (LE to TE) rather than the zero lift line... But the fact that the wings have washout, so most of the wing actually is set negative to the fuselage, diminishes any residual decalage even further.

Are you sure that you are measuring accurately? If so and if you really do have negative decalage then I think you might have more issues to come in the trimming process, like poor recovery from a dive. It sounds to me like the problem you had at step one of the trimming process was simply that the CG was too far back, '1/3rd chord' does not work for all models.

Steve
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FLYACE1946
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« Reply #10 on: May 14, 2010, 11:19:30 AM »

ONE RULE OF THUMB IS TO MAKE ONE small adjustment at a time. Maybe this was said earlier but it bears repeating.
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Art356A
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« Reply #11 on: May 14, 2010, 12:44:52 PM »

I had put up this pic on SFA some time back. These sticks are held on with rubber bands, but narrow masking tape works better. Using a gelpen, mark a pair of dots on the cardstock for each stick, take the card to the drawing board, and work out all the angular differences. Know the difference between your actual mean chord and the flat bottom of the foil and figure it in.

In this case I have only 1° of decalage, but there's 1½° more within the airfoil, so I net 2½° like it should be.
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My arms are so weak, it's like that pushup I did last year was a total waste.
Maurice
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« Reply #12 on: May 15, 2010, 08:48:18 PM »

Art, that is so cool! Great ideas there. I can and will see a use for what has been shown here in your photo; nice work. The "art" of trimming a model is one fraught with many variables. The best aid for trimming any models is long grass.

Gotta go now the internet kiosk is just about done,
Maurice
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Maurice
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« Reply #13 on: May 26, 2010, 10:42:57 AM »

Apologies for the length of this grand epistle but what I find is by putting my thoughts on paper and displaying it to the modelling world I can (selfishly maybe) garner a whole bunch of fantastic input from other crazy people who do this hobby. Now, I am not perfect and have a lot to learn and am still a babe in the woods but the attraction of this hobby is that you never stop learning. So, with my heart in my mouth, here is the first installment. I base all of this on what experience I have and have applied this interpretation to it. It should be regarded as a live paper. Input is welcome and encouraged.

Just some thoughts on trimming OR What to do before you install the rubber.
1.   Always make sure that the model is square in respect to the tail plane, wing relationship and that the rudder is vertical
2.   Ensure that the wings are not twisted
3.   Determine if there is sufficient washout (this may not be the same on either wing depending on what flight pattern you build into the model)
4.   Remove the propeller
5.   Determine the CG of the model (from the plan if available or 1/3 the mean chord; a starting point)
6.   Choose an area with some long grass if possible
7.   Wait for a relatively wind free day
8.   Rub some talc powder on your launching fingers
9.   Locate a point to aim for during the launch
10.   Launch the model from shoulder height with determination NOT with aggression
11.   Observe the flight
12.   Repeat the launch as often as needed to determine the flight characteristics of the model
13.   Only make changes one at a time!
14.   If the model stalls add nose weight BUT if you have to add way too much then it is not a CG issue but a decalage problem. This could mean that the LE of the main plane has too much UP and stalls because of the nose of the model pitching up because of too much lift. Adding a lot of nose weight is indicative of this OR the tail plane’s TE is too much UP. If this is the case, raise the LE of the tail plane and launch the model again. Often it is a combination of main plane and tail plane angles of attack that need to be determined. The lower the angle of attack of the main plane the less likelihood there is to raise the LE of the tail plane
15.   If the model rolls to one side or the other then the wings may need adjusting. If there is a slight curve to the right (looking from behind after launching) then that might be OK (for a low wing trimmed to fly left)as the torque reaction might compensate for this but it is not really kosher as that slight roll might become a death spiral from altitude. (If the model tucks into a turn when under power and becomes a lawn dart then it is too nose heavy)
16.   So try to change the wash-out incrementally to get the model to glide straight without porpoising, spiraling, rocking from side to side or stalling
17.   If the model rolls from side to side (oscillating continuously) then there may be a need to increase the vertical tail plane’s height/size. Add a small tab on the vertical plane, launch the aircraft and see if the rolling goes away. If it still does that then the wings might not be square to the fuse or, worse, they are not glued on properly and are moving in flight) Check this and rectify.
18.   If the plane curves left, or right unpredictably try launching the model with the wing low. If it continues to curve off in the direction of the low wing then there is not enough dihedral for recovery. Try it with the opposite wing to the former test and it if does the same as before but in reverse then it proves the dihedral is insufficient. You might wish to try adding winglets to both tips of the wings at perhaps 45 degrees and if the problem goes away then you have a choice. You either leave the winglets on (ugly) or you reset the dihedral (soul destroying and time consuming; your choice)

Woooo, need to stop!
Cheers,
Maurice
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