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Author Topic: Miller "Little Gem" build for FAC Goodyear mass launch  (Read 5918 times)
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flydean1
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« Reply #100 on: August 20, 2017, 10:45:17 PM »

Stall tuck under is usually a symptom of inadequate decalage and too aft a CG.  You've run the numbers and McCombs is a good authority.  Might be some of the stab is blanked out.  The Gurney flap on the stab might restore some effectiveness.
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Crabby
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« Reply #101 on: August 21, 2017, 12:48:21 PM »

Mike, that plane is really mid-winged and not low wing... in trimming it does make a difference. I wish I could expand on it some... the best I can do is let the cat out! It may have been flydean who enlightened me when I was trimming my diels Wildcat... one observation...It looks like your troubles begin at the transition where the knots are moving beyond the cg. Might be a thought to move the peg noseward a bay or so.
« Last Edit: August 21, 2017, 01:19:00 PM by Crabby » Logged

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BG
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« Reply #102 on: August 21, 2017, 02:24:13 PM »

I like Crabby's suggestion. I tend to move the rear peg forward rather than add nose weight. Also serves to improve the tail moment situation.
bg
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MKelly
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« Reply #103 on: August 21, 2017, 10:25:34 PM »

Crabby or FlyDean1, I'd be most interested to hear your techniques for trimming mid-wing models.  My experience is limited to low-wing warbirds and a couple high wing peanuts (which flew or not based on their own merits as built, not any trimming I attempted).  Had some discussion on the FaceBook Flying Aces page that suggests increasing the stab size.  I'm tempted to try moving the cg forward a bit to get more stability before starting to append area on the stab - if it doesn't work I can always move the cg back to the McCombs position.  Moving the peg forward isn't an easy option either, there would be some surgery and re-covering required.  Probably won't get to try anything before the weekend.

Cheers,

Mike.
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flydean1
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« Reply #104 on: August 21, 2017, 10:43:16 PM »

Re:  Crabby's comments.  Many years ago, we had sort of a one design FAC event featuring the Grumman Wildcat on the anniversary year (50th I think) of its' first flight.  We had about a half dozen entrants at KOI that year.  I had a slightly enlarged Stahl design.  There were at least a couple Diels kits, and at least one from Mike Midkiff's plans.  We discovered that in trimming, all of us assumed it should go to the "safe" direction to the left with torque.  All of us had trim problems until, mostly in desperation, we started turned them to the right.  Presto!!! they all started to fly.  All of us circled to the right, despite the origin of the design.  The F4F was a mid-wing airplane with the thrust line on the wing leading edge, and the stab slightly elevated above the wing.  If you enlarge the stab, go with a bit of span first.  More effective than chord.
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dslusarc
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« Reply #105 on: August 21, 2017, 11:37:23 PM »

I bought one of these kits from Shortys as well, any chance on sharing the artwork file for printing the tissue? I like the black and white color scheme!

Don
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MKelly
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« Reply #106 on: August 22, 2017, 11:01:47 AM »

Don,

Happy to share the graphics.  I'll need to clean up the working file - it's a 23 MB Powerpoint with lots of extraneous work-in-progress stuff.  Do you have an 11x17 printer, or should I drop the graphics into a letter-size format?

Flydean1, thanks for the trimming info - I have the Diels Wildcat in the stash for a future build.  I'm going to try a few things on this one before changing to a right turn, but I've got that in my pocket now in case things don't work out.

After thinking through all the comments, looking at the model and watching the videos some more I think I'm going to do the following for my next trimming session (intent is to increase stability to discourage the tuck and counter the torque to handle a bit more power)

1. Add a thin Gurney flap to the upper TE of the stab

2. Move the cg forward about 1/16" (put it just ahead of the spar)

3. Trim glide as required by adjusting the stab

4. Work up in power again, adding right thrust as necessary to allow more turns/torque without plowing in to the left at launch.

This week is busy, and the weekend may bring rain (which we need) - not sure how soon I'll get to try this approach.

Cheers,

Mike
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Crabby
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« Reply #107 on: August 22, 2017, 11:29:28 AM »

How are you going about moving the cg up?
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MKelly
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« Reply #108 on: August 22, 2017, 04:40:22 PM »

Add a little weight to the nose.  I need to tighten up the noseblock a bit, so I'll do that first then ballast as required to get the balance where I need it.
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dslusarc
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« Reply #109 on: August 22, 2017, 05:59:55 PM »

Don,

Happy to share the graphics.  I'll need to clean up the working file - it's a 23 MB Powerpoint with lots of extraneous work-in-progress stuff.  Do you have an 11x17 printer, or should I drop the graphics into a letter-size format?
Mike

Yes I have a large format printer. I bought it for printing tissue :-)   A 23MB file is fine for my email server and I have Powerpoint as well.  I sent you an email. Thanks for sharing the file!

Don
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Hepcat
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« Reply #110 on: August 22, 2017, 09:00:41 PM »

Response to #106

Mike,
You obviously have your trimming problems well in hand although I am afraid some scale models will always be difficult.  There was just one further thing that I think should be said. Don’t always think that the equations of Bill McCombs and Jim O’Reilly are perfect. I think what they did was very sensible. They made a graph with CG position on the X axis and Tail Volume Coefficient on the Y axis. They then found the TVC and the CG of a lot of successful models and marked them on the graph. They then drew a straight line as near as they could passing through the middle of this group of dots.  The slope of this line expressed as an equation is what decides the CG position by their method. 
I have already said that is sensible and often helpful but there are a couple of things that should be kept in mind.  Calculation of the TVC requires the Moment Arm of the tailplane which is the distance from the lift line of the tailplane to the CG of the aeroplane (and is probably the most important factor in the TVC). However McCombs and O’Reilly don't know the CG position because that is what they are trying to so they decide to call the distance between the quarter chords of the wing and tail as the moment arm, obviously wrong and could be miles out.  The 2011 Sympo had a graph by Jim O’R of his method dated 1982 and had ‘dots’ for ten or more models by top flyers of around that time (35 years ago) and as far as I could see were all large competition rubber models. I think scale models need their own equations with some new constants.
John
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tom arnold
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« Reply #111 on: August 22, 2017, 10:33:16 PM »

Good point, John. I have used the McCombs formula very successfully but there have been a couple of models that would not cooperate and acted too tail heavy in spite of my very careful calculations and drawings on the plans. I solved the problem by moving the CG a bit forward but I bet that Moment Arm difference is what caused it. Thanks for the explanation and suggestion and I hope somebody smarter than me will come up with that scale graphing and adjustment.
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MKelly
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« Reply #112 on: August 22, 2017, 10:45:41 PM »

John, true words.  For this model I keep looking at the cheeks and thinking that's a lot of planform area well ahead of the CG that's not included in the McCombs calculation.  That sure seems like it would reduce stability. 

While the stab may be small, it's a lot easier to move the CG forward than to increase stab area, and if it doesn't work probably the worst I have to face is repairing the wheel pants (again).

I do think this model has a lot of potential - it feels like it wants to fly, right up to the point of the occasional suicidal dive...

Cheers,

Mike
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BG
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« Reply #113 on: August 23, 2017, 11:32:32 AM »

Thanks for that explanation john. I did wonder at the choice of 25% wing chord to 25% stab chord for the moment arm. Perhaps we should start a new thread for this topic?

On the Little Gem: Perhaps some trial and error with some stab tabs would be good? I solved a tail volume problem with a cat glider by adding masking tape tabs to the stab until she started to behave. It worked like a charm and I was able to add and take away easily. If you do this you might be able to identify just how much extra area you need for a more reliable flight pattern. You could use light balsa tabs too so that you don't mess with the cg too much.

Speaking of CG did you move the rear peg forward?

B
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MKelly
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« Reply #114 on: August 23, 2017, 02:03:50 PM »

BG,

I didn't move the peg forward.  It sits about halfway between the wing TE and the LE of the stab - next bay forward would have put it right at the TE of the wing.  As it sits the rubber runs about 3" in front of and about 5" behind the cg.  My trimming flights so far have been made with a single loop about 1.5x the hook-peg distance, and I'm using piece of 1/4" tube with the ends flared (see pic) as a bobbin on the motor peg (which is 3/32" OD tubing) to let the rubber move about and hopefully minimize bunching.

Stab expansion will be the next thing to try if cg adjustment doesn't tame the beast.

Cheers,

Mike
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Re: Miller "Little Gem" build for FAC Goodyear mass launch
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MKelly
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« Reply #115 on: August 23, 2017, 02:46:34 PM »

Don,

Happy to share the graphics.  I'll need to clean up the working file - it's a 23 MB Powerpoint with lots of extraneous work-in-progress stuff.  Do you have an 11x17 printer, or should I drop the graphics into a letter-size format?
Mike

Yes I have a large format printer. I bought it for printing tissue :-)   A 23MB file is fine for my email server and I have Powerpoint as well.  I sent you an email. Thanks for sharing the file!

Don

Don - file sent.  Let me know if it didn't come through.

Cheers,

Mike
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ZK-AUD
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« Reply #116 on: August 23, 2017, 03:33:47 PM »

Mike just looking at your rubber issue a couple of posts back.  Even on these littlies I go with a braided motor now.  if a loop of 1/8 would suffice I use 4 strands of 1/16 braided. If you have access to a stripper you can halve anything.   For my bobbins I've been using the plastic outer tube of R/C snake with a sliver of fuel tube stretched on either side of the rubber.  really light and I will make up 4 or 5 motors and number them so I can keep track of how many flights each has done and rest them.  with the braiding i find you can get a really long motor in and keep it all even
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« Reply #117 on: August 23, 2017, 09:26:50 PM »

Some indoor duration flyers use tubing at each motor hook on their stick models and it seems to work well.

Fred Rash
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MKelly
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« Reply #118 on: August 31, 2017, 05:18:49 PM »

Got a bit more trimming in today - https://www.youtube.com/watch?v=h8Z1zje57kQ.  Moved the cg forward about 3/32" for these flights, flying weight with the longer (14") motor was 24.1g.  I did not put a gurney flap on the stab as glide and flights didn't indicate it was needed.  Things were getting a bit breezy at the end, so I wrapped things up.  Longest flight was about 31 seconds, but I was looking more at flight dynamics than duration today.

Model is still really sensitive to thrust adjustments - don't think I really have the down and right thrust correct yet, but it flies better than the previous sessions.

Cheers,

Mike
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MKelly
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« Reply #119 on: September 13, 2017, 10:32:57 PM »

Took Little Gem out with a new motor today - 2 16" loops of 1/16" rubber.  I also trimmed off about half the left rudder flap.  Flights were a bit better, with three flights lasting between 35 and 40 seconds, and no instances of tuck-under.  With the reduced left rudder and longer motor I was able to put in a lot more turns without plowing in to the left at launch.  I added a little left wingtip weight, which improved the transition from power to glide, and added a bit more right and down thrust.

The model responded pretty well to the reduction in left rudder trim, but still seems to be fighting the left turn.  I think next time out I'll try trimming for a right turn under power, as Flydean suggested earlier.

One of the longer trim flights is included in this video:  https://www.youtube.com/watch?v=rvzBorDE5UE

Cheers,

Mike
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