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Author Topic: Power trimming question  (Read 1875 times)
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Starduster
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« on: June 09, 2015, 01:27:15 PM »

OK, I am working on getting my Pimenoff #18 and the Taibi Orbiteer trimmed. I guess it's been too long since I've trimmed a high-powered (gas) model. Now, both airplanes have been converted to electric, but the techniques should be similar (or ARE they???)

The balance is good, right at where the C/G should be. The glides look very nice, flat and just a hint of a turn (left for the Orbiteer and right for the #18.

The issue is with the power. I've destroyed two fuselages of the #18 so far. Both airplanes are doing the same thing, albeit in opposite directions:

I set the power for 75%, motor run 3 seconds, and I launch the airplane. Straight out for about 1.5 seconds and then a big ol' wing over, splat into the ground. The #18 to the right, the Orbiteer to the left.

I'm at a loss... do I try rudder tab? Do I try thrust adjustments?

Any help would be much appreciated.

Rich
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USch
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« Reply #1 on: June 09, 2015, 02:35:19 PM »

Hi Iceman
just my 2 penny worth.
Do you have flat wings without warps? The only needed warps are some wash-in on the right wing for the Pimenoff and on the left wing for the Orbiteer. This wash-in can be substituted by equal amount of wash-out on the opposite wing (eventually). As the glide seems ok from your account, if the climb was straight up, add some down-thrust to the motor. And trim the model at full speed, 75% is a loss of time being the flight pattern dependent of the climb speed. And full power sometimes pulls you out of trouble   Cheesy
Maybe use 2 sec. motor run and 3 sec DT, if possible. Hook the DT line to the autorudder arm to have a more precise timing.

Urs
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Hepcat
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« Reply #2 on: June 09, 2015, 05:34:43 PM »

Iceman,
I had scribbled down most of my reply before I saw what Urs had said.  I don't disagree with Urs so I will just briefly say what I intended.
 Look over every piece of the aeroplane and note every warp and offset.  Don't just see them, measure them and write them down and then report them back here.  Don't forget things like the length of cotton pinned to the fin and stretched to each wing tip, and could the fuselage just have an almost invisible warp, sideways or up and down.  Have you put long laths under the wing and tailplane to check the incidence differences.
As you said that you hit the ground in 3.5 seconds I was going to suggest using 2 seconds motor run but Urs beat me to that as well.
It is so long since I did any power flying that I am not the person to advise you but I am just listing the sort of things that a person who can advise you would want to know.
There is a good article on trimming a power model in the latest 'Aeromodeller'.
John
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mick66
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« Reply #3 on: June 09, 2015, 05:56:21 PM »

Hi

I'm not saying my #18 is the definitive trim set up but as per the earlier #18 thread ... how close is your model to:

CG:                         83%
LeftThrust:             +1.5 degrees
DownThrust:           +2.0 degrees (as in down)
Wings:                    +3.0 degrees
Tail:                        +1.5 degrees (giving +1.5 decalage)
PRT tip washout:       +2mm
PRT inner panel:        flat
STB panel washin:    +2mm
STB tip washout:      +2mm
Tip washout measured at TE behind last rib


Lauch at 80 degrees just to right of wind with right wing slightly down .... sort of launching into its pattern.  Quick DT after motor cut is an absolute neccesity.

As Urs says ... anything other than full power is a waste of time.  Also, one of the best tips my Novos clubmate AllanBrown gave me when trimming power models ... in zero wind, before you go to power you must be able to give it a pretty good throw at a spot 50 or 60 ft away and it should pretty much go that way in a positive manner ... no floaty glide rubbish.  Give it a good heave at that spot and it shouldn't float up in a stally manner.  

That way you know your not over-elevated which is what you sound like on the #18.  You should be safe to go as its more likely to fly left than wing over right.

Just my 2 cents.

Cheers

Mike

PS. lol ... its your own fault for going electric anyway.  Stick to using electrickery to light you workshop and stick a motor in it.

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« Reply #4 on: June 09, 2015, 06:46:26 PM »

I too had a lengthy epistle planned, and Urs, Hepcat and Mick have all beaten me to it.
I concur with Mick, it sounds to me like both aircraft are substantially over-elevated (i.e. your set-up is prone to looping).

John
p.s  I don't think, though, that the means of making the propeller turn is significant (unlike Mick).
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danberry
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« Reply #5 on: June 09, 2015, 09:11:49 PM »

Anything less than full power is a waste of time and probably a plane.
I found the electrics don't want so much left thrust. I assume because of the lower RPM.

Is the Orbiteer a high-thrust plane? I thought Taibi's Orbiteer was a pylon design. A pylon must go right unless you're playing with a motor that runs opposite most glow engines.

Don't launch at anything less than 80 degrees vertical.

All the other notes about washin/out apply.
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Starduster
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« Reply #6 on: June 10, 2015, 10:00:25 AM »

Hi

I'm not saying my #18 is the definitive trim set up but as per the earlier #18 thread ... how close is your model to:

CG:                         83%
LeftThrust:             +1.5 degrees
DownThrust:           +2.0 degrees (as in down)
Wings:                    +3.0 degrees
Tail:                        +1.5 degrees (giving +1.5 decalage)
PRT tip washout:       +2mm
PRT inner panel:        flat
STB panel washin:    +2mm
STB tip washout:      +2mm
Tip washout measured at TE behind last rib


Lauch at 80 degrees just to right of wind with right wing slightly down .... sort of launching into its pattern.  Quick DT after motor cut is an absolute neccesity.

As Urs says ... anything other than full power is a waste of time.  Also, one of the best tips my Novos clubmate AllanBrown gave me when trimming power models ... in zero wind, before you go to power you must be able to give it a pretty good throw at a spot 50 or 60 ft away and it should pretty much go that way in a positive manner ... no floaty glide rubbish.  Give it a good heave at that spot and it shouldn't float up in a stally manner.  

That way you know your not over-elevated which is what you sound like on the #18.  You should be safe to go as its more likely to fly left than wing over right.

Just my 2 cents.

Cheers

Mike

PS. lol ... its your own fault for going electric anyway.  Stick to using electrickery to light you workshop and stick a motor in it.



Thanks for all of the info, Gents... always amazed at the depth and breadth of knowledge on this forum. In answer to your query above:


                                             
CG:                         83%                                                Almost exactly the same location (within +/- 1%)
LeftThrust:             +1.5 degrees                                         1.5 degrees Left
DownThrust:           +2.0 degrees (as in down)                     1.5 degrees down
Wings:                    +3.0 degrees                                        +4.0 (per the Zaic and VolLibre drawings)
Tail:                        +1.5 degrees (giving +1.5 decalage)      +1.5
PRT tip washout:       +2mm                                                 Flat
PRT inner panel:        flat                                                      Flat
STB panel washin:    +2mm                                                  Flat
STB tip washout:      +2mm                                                  Flat


I'll work on getting some washin to the STB main panel and add washout to the tips.

Lots of great tips, thanks again.

As for electric...  well...  Wink
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USch
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« Reply #7 on: June 10, 2015, 12:13:21 PM »

A practical tip, to add wash-in on the STP glue a 50mm (2") piece of an inverted TE to the true TE underside (at the DH break). Fast and if the result has to much power you can shorten the piece or plain off some wood to adjust the effect without having to twist the finished wing.

Urs
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Starduster
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« Reply #8 on: June 10, 2015, 12:54:31 PM »

A practical tip, to add wash-in on the STP glue a 50mm (2") piece of an inverted TE to the true TE underside (at the DH break). Fast and if the result has to much power you can shorten the piece or plain off some wood to adjust the effect without having to twist the finished wing.

Urs

Great tip! Thanks. Esp. with a wing with geodetic ribs and (eeek!) plastic covering, trying to get the wash-in to stick can be difficult.
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glidermaster
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« Reply #9 on: June 10, 2015, 02:15:17 PM »

Gosh darn it, beaten to the punch again!

You could probably get by with a Gurney flap on the right wing, instead of getting into steaming warps into previously straight panels.
With a reduction in decalage per Micks post, a Gurney flap would get you moving in the right direction. Washout in the tips is desirable, but you MIGHT be ok without (basically repeating what Urs said).

So far as the power thing is concerned, personally I have nearly always trimmed locked up models by starting on lower power, or by fitting the prop backwards (and then using full power).
If the models are hitting the ground on 75% power because they were over elevated, they will only do it quicker on full power.

Mick's description of a firm toss towards a spot 50 yards away is spot on, the model should continue in the direction of the launch and not recover (or pitch up) at all.

John
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USch
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« Reply #10 on: June 10, 2015, 02:28:23 PM »

Sorry glidermaster   Wink
Next time I will wait a fraction of a second to let you write first. Being at home after a shoulder surgery with one arm completely blocked I cannot even hold the scalpel and cut some balsa. Only writing on the pc is possible    Angry

Urs
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Starduster
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« Reply #11 on: June 10, 2015, 02:40:20 PM »

OK, so now if can only figure out the difference between starboard and port

They're kind of like right and left, aren't they?

 Grin
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« Reply #12 on: June 12, 2015, 09:34:35 AM »

I'm going to jump in and muddy the waters a bit...

Mike's advice about throwing the model unpowered is similar to something I do, but not quite--meaning I do look for a positive recovery.

Here's how I get it without killing perfectly good airplanes (must work, I've yet to have an electric power model go in under power!): We're looking for positive stability--no matter what position you put the plane in, it should be able to recover, be that upside down or whatever. That's from my glider flying experiences. Everyone talks about 0-0. I don't want 0-0, because 0-0 commits suicide in strong thermals and/or windy conditions, and whenever the launch isn't within .00001% of perfect.

Pick a flight direction. With a catapult glider, that's left, with a TLG/DLG, it's left then right. With a pylon power model, it's also right. Put a screw adjuster on your rudder and on your stab. You'll need them. Have your wing flat unless it's not really, really torsionally rigid. No washout, nuthin'. Washout goes nuts at high speeds. Add a washin wedge to the right wing. If the wing isn't stiff, you'll have to build in the washin to the right inner panel because the washin wedge can and will work in reverse in that case at high speeds. Now set the stab just shy of 0-0, and screw in a little right rudder.

Take the model out to the field and balance it so it glides nice and floaty. Get a wide glide circle going using only stab tilt. Now start throwing it--hard. Aim at a point on the ground 60-70 feet in front of you. You want the model to gradually pull out of the high speed descent. Keep adding/removing incidence until it just starts to do so. The model should go straight ahead, then pitch up a little with a slight right yaw/roll, and then stall out, very lightly. Now add some right rudder so that it yaws a little harder--just a little! If there's a fine line between stalling and rolling right into the ground, add more washin. If it takes a whole bunch of right rudder to kill the stall, remove washin. Keep doing this until the model just barely pulls out of the descent, then, slowly pitches up into a gently rolling right turn and circles around back to you, transitioning into a floating glide. If it stalls in glide, add noseweight, or remove it if the glide has become non-floaty. Bear in mind that if you've had to move the CG back, the stab tilt will become more effective and will need to be reduced. Also, it's best to tilt the wing, not the stab, because shimming the stab will change your stab incidence.

Start with 2 degrees left thrust and full power. Use the shortest power run you can (3 seconds if possible) and fast DT. Even better, buy a Lemon RX receiver for 6 bucks and hook your ESC and DT servos into it, and control that via a cheap radio, like the little 4 channel ones available from Horizon Hobby for $30. You can hook the DT to the aileron channel and set it so that you flick the aileron stick to trigger the DT.

I launch into the wind with only slight right bank. If the model shows any tendency to go left, you don't have enough right rudder. If somehow the model straightens out of the right bank, the right rudder should be strong enough to roll it into a spiral climb as it attempts to loop (should never go more than 10 degrees past vertical). If you've trimmed the glide as above, the model should need only some fine tuning, such as to correct an excessively tight spiral or to correct a stall at the end of the motor run. So...if you have to spiraling too tightly, decrease incidence, and expect to need to tweak the rudder to push it into the glide. Spiralling too loosely, try adding incidence. If still no effect, add a touch of right rudder. I should also add that I throw my models pretty hard. I've found that in windy weather, some of my models will pull very hard to the right, going almost level before pulling away into a perfectly fine climb if I just release them in the traditional manner. Throwing the model puts it into the correct pattern from the start (one turn in 5 seconds in the case of my E-36, which is trimmed only for 5 second runs and hence needs a faster turn than is typical).

Bear in mind that I use 0-80 screws for adjustments and that I only go 1/4-1/2 turn of adjustment at a time unless something is very obviously way off.

Also, if there is any flexing of the wing on the way up, I cannot help you. Flexing wings will throw all of this out, and I know one of the models you're working with is of higher aspect ratio than what I normally work with.

I'm pretty sure based on your original post that the models are over-elevated, but without video, I'm hesitant to make that claim.

A lot of good advice has been given...your mileage may vary with mine. It's worked for what I fly, but I'm pretty picky about building airplanes that I'm certain will respond to my flying philosophies.
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« Reply #13 on: July 05, 2015, 11:54:56 AM »

OK, guys, I guess it really does pay to listen to the pros now and then.

I had a very successful morning with the #18. I used Mike's set up, and it worked beautifully.

The suggestion to use a very short motor run and a short DT worked out well. I started out as suggested by test gliding fast and to a spot 50-60 feet away. The airplane showed no sign of stalling.

I started the power with a 1.5 second power with a 3 second DT. The airplane showed a slight right turn in the power, so I added a slight amount of left rudder (by adjusting the Dan's timer) The power was a little straighter and so I went to 2.5 seconds power and 4 second DT. I think it needs a little more left rudder for the climb, but it looks very, very promising.

But now the bad news: I made a serious error when I built the airplane. I used hard balsa for the main wing spar. Ummm... no.... The wing folded. I was kind of expecting this, so I'm not too upset. I just ordered spruce from National Balsa and will re-build the wing. No other damage from the flight (again, that quick DT is a life saver, even when the wing folds)

Luckily, I have another kit from Bob Holman ready to go, and it is a pretty easy build, so I'm expecting to be back in business with a week or two.

Thanks again for all the help. I'm actually excited about getting the airplane back into the air.

Rich
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« Reply #14 on: July 05, 2015, 01:36:32 PM »

Happy that you had some successful trim flight, Iceman, much  less about the broken mainspar  Undecided Yes, short Dt's are hard on the wing and such a model needs a spar made from pine or spruce or some black magic (carbon) running on top and bottom of a balsa spar. Don't forget to build the wash-in into the new wing right from the start, avoids messing around later  Wink

Urs
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Starduster
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« Reply #15 on: July 12, 2015, 03:57:49 PM »

Hey, guys...

Wow, did I have a great day flying the Number 18!

Thanks for all of the great suggestions.

I completed the new wing this past Thursday. I used a spruce spar and a spruce leading edge. I was able to reuse the wing tips, so the re-build went very quickly. I also built in the wash-in to the right wing.

After a quick couple of test glides, I set the throttle at 100% and 2 second run. DT was set for 4 seconds. First powered flight looked very very good. Just a slight right turn into the glide.

After a couple more test flights, I set the motor run for 4 seconds and 6 second DT. Beautiful spiral into the glide.

After a few more flights, I got the power to 6 seconds and the DT at 12 seconds. Again no problem at all.

Finally got to 6 seconds, 120% power and 30 second DT. Man o man, this is really fun! I did have to use four of the five rudder positions on the Dans timer.

First position was initial power, about 10 degrees left rudder. Second position was 5 degrees left rudder. Third was 5 degrees right rudder a half second after motor cut-off, to bump it into the right turn in the glide. The last setting was the glide turn, 10 degrees right.

This airplane is a real performer (surprise, right?).

All in all, I must have flown the airplane 24 times.

You guys are the best!



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« Reply #16 on: July 19, 2015, 08:03:14 PM »

I had another fantastic morning flying the #18. I feel like I've really got the airplane dialed in. I'm up to 8 seconds for the motor run. It's fun to see it accelerate as it climbs away. After the first 10-20 feet the electric motor starts to really unwind and the airplane really accelerates. It got as high as I was comfortable with at 8 seconds.

One thing I did have to change in the timing was initially (see my post above) I had a right rudder, thinking it would bump it into the right glide. What I found instead was that it entered the right turn too fast and lost a lot of energy in a right spiral before slowing for the glide. Now the first four setting are less and less left rudder. At the top of the power, it has a touch of right turn, and now it very smoothly transitions to the glide.

I just could not be any happier with this airplane. Probably one of the best Freeflights I've ever had.

It's just too darn bad that there are no contests in the Northeast US that has an event for it.

This is a really fun airplane to fly. One thing that I love about electric is that there were several times this morning where, because there was very little wind, I would walk over to where the airplane landed, pick it up, reset the timer and fly it again.

Thanks again for all the help.
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« Reply #17 on: July 20, 2015, 07:51:09 AM »

It's just too darn bad that there are no contests in the Northeast US that has an event for it.

Perhaps take it to the AMA Nats sometime? You could fly it in B electric; it would no doubt be quite competitive. Some guys in the Maxecuters are flying higher end electrics now as well.
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« Reply #18 on: July 20, 2015, 11:55:52 PM »

Am I reading correctly that you are moving the rudder during the power phase?
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« Reply #19 on: July 21, 2015, 06:08:52 AM »

Am I reading correctly that you are moving the rudder during the power phase?

Yes, the initial rudder setting (on launch) is about 10 degrees left. After 3 seconds it goes to 5 degrees left and after 6 it goes to 0. The reason is that the sub-rudder on this airplane if very ineffective at low airspeeds, but as the airplane starts to accelerate, it really takes hold, so it does not need as much rudder. It's kind of cool, actually, to see the airplane start to arc over to the right, and then correct itself. At the top of the power phase, it is in a very slight right bank, which places it very nicely into the right glide.
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« Reply #20 on: July 21, 2015, 09:34:46 AM »

I wasn't sure if I was reading correctly.
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