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Very Hot Topic (More than 25 Replies) Much loved Comet Hellcat (Read 30730 times)
Konrad1
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Re: Much loved Comet Hellcat
Reply #33 - Jul 11th, 2013 at 4:02pm
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I was able to spend a little time on this ship during my last visit home. I think I have now found out why the tail feathers did hold their trim settings very well.

The tail feathers are built out of 3/32 balsa per the plans. But the sheet parts are cut from 1/16 printed sheet balsa. It looks like I sanded the 3/32 strips to flair into the 1/16 sheet. This would probably have be all right even desirable out at the tips. But I also sanded the 3/32 strips near the center (root area) of the stab to blend in with the sheeted parts found there.

I think I'll rebuild the tail feathers to have full depth #/32 sheeted parts near the roots and then only tapper the strips near the tips to blend into the sheet tips. I might even just laminate the tips from 3 strips of 1/32 balsa.
  
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Konrad1
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Re: Much loved Comet Hellcat
Reply #32 - May 29th, 2013 at 10:52am
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Jim, how long is your motor? Also what does your climb look like? And what are the flight times?
I use to use the motor that came in the kits. Only to find that the models would barely climb. I learned that the more motor the better the climb and total flight time. Now I fear I have gone a bit too far in that the glide is starting to suffer as a result of motor weight and nose weight.

I'm hoping to be able to keep the motor weight much the same and remove a lot of that clay nose weight. I like to see rocket like climbs followed by slow stable glides.

The only time she broke a minute was getting caught in some slope lift.

All the best,
Konrad
« Last Edit: May 31st, 2013 at 4:07pm by Konrad1 »  
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Konrad1
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Re: Much loved Comet Hellcat
Reply #31 - May 29th, 2013 at 10:44am
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Ates,
I wish my plane weighed 18 grams. The 18 grams was just the clay nose weight I had. I'm using the large Peck polymers  gray plastic prop. On a braided motor with 6 strands at a length of about 1.25 to 1.5 times hook to peg length she would climb in a very powerful 60° climb. Unfortunately she'd be down in a minute or less.

I have to ask how do you guys keep the rubber from shifting with those long hook to peg motors?

All the best,
Konrad
« Last Edit: May 31st, 2013 at 4:06pm by Konrad1 »  
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JIM
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Re: Much loved Comet Hellcat
Reply #30 - May 26th, 2013 at 6:36am
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my hellcat flies on 8 inch prop with 2 loops 3/16 rubber.   Grin

JIM
  
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Ates
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Re: Much loved Comet Hellcat
Reply #29 - May 23rd, 2013 at 3:09am
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Hi Konrad,

Great looking airframe, good to hear that you're keen on getting her back in the air.

I read that you had 6 strands of 1/4" rubber in this model which sounds like a lot of motor for a model with 24" w/s and an 18g frame - unless I misread or misinterpreted something. I would think it would be difficult to trim the model with that much power and it would probably go up very fast and then have a rapid descent due to the excessive weight of the rubber motor.

You'll probably finish the model with an AUW of 35-40g, including the the prop and covering, excluding the motor. I would start trimming it with 4 strands of 1/8" rubber (Tan Super Sport or even Sport would work), 1.5x hook-to-peg distance and with a ~7" prop. Moving the rear peg forward by one or two bays is definitely good idea. Once the model is all trimmed, you may be able to go longer motors than 1.5x and still get long motor runs with that shorter hook-to-peg distance.

You may want to take a look at Hodes' rubber motor calculator at the NFFS web site. It comes in handy when trying to choose a motor for a new model.

Short write-up:
http://freeflight.org/DigestOnline/TechLibrary/Rubber%20Motor%20Size%20Calculato...

Excel spreadsheet:
http://www.freeflight.org/DigestOnline/TechLibrary/Hodes'RubberMotorExcelWorksheet.xls

(somehow I couldn't make the second link clickable, please cut and paste the whole line in your browser's address bar to get to the spreadsheet - if you can't I can send it to you, just PM me your email address)

Best of luck!

--Ates
  
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Konrad1
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Re: Much loved Comet Hellcat
Reply #28 - May 2nd, 2013 at 1:09am
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Sky9pilot wrote on May 1st, 2013 at 4:28pm:
Konrad,

My understanding about this, I'm still learning the rubber motors details, is that the fatter/thicker motor strands give  a more powerful burst at the start of the flight with shorter duration.  Maybe higher climb with the initial burst of power.

For more duration the thinner motor is used to pack more winds in a bit longer motor.   

Those with more experience may want to weigh in here!

Thanks,
Tom

That is also my understanding. And it is why I went with the rear most motor peg in the first place. But I fear that the added nose weight is killing any hope of long duration flights, powered or in the glide.

Well, I'm back at work and can't hope to deal with her again for another 6 to 8 weeks.

All the best,
Konrad
  
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Sky9pilot
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Re: Much loved Comet Hellcat
Reply #27 - May 1st, 2013 at 4:28pm
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Konrad,

My understanding about this, I'm still learning the rubber motors details, is that the fatter/thicker motor strands give  a more powerful burst at the start of the flight with shorter duration.  Maybe higher climb with the initial burst of power.

For more duration the thinner motor is used to pack more winds in a bit longer motor.   

Those with more experience may want to weigh in here!

Thanks,
Tom
  

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Your attitude will determine your altitude!- John Maxwell
And ye shall know the truth, and the truth shall set you free. Jn 8:32
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Konrad1
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Re: Much loved Comet Hellcat
Reply #26 - May 1st, 2013 at 3:02pm
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You are correct in that the Invert V may be a concern. As it is now I was able to get 6 strands of 1/4" through the 6th former. And not having anything else to compare it to she flew great.
Moving the anchor almost forward 2 bays and up one set of stringers still gives me more clearance across the mouth of the inverted V than I had in the original location.

Again you are correct the higher rear peg motor mount won't actually change the thrust lines. But should align the motor's length closer to that of the prop shaft. Hopefully putting less stress on the nose block to hold the thrust angles.

Assuming for now that I will use the same amount of rubber (weight) what can I expect with a shorter fatter motor? With the longer motor she would climb at 60° to 70°. Glide was also good and flat just that she came down rather early.

After recovering I hope to have as good a ship as I had prior to taking on this exerersize. I hope she is about 9+ grams lighter and shows a bit better glide. I also hope but don't expect a better climb with the loss of  9 grams but I fear that some of that 9 grams of lost mass might be motor rubber. Cry 
  
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Re: Much loved Comet Hellcat
Reply #25 - May 1st, 2013 at 11:23am
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Konrad1,

I had to re-read your post several times to follow what you were suggesting.  Not you, just me! Embarrassed Roll Eyes

The "Inverted V" of your formers would in my opinion restrict the area the motor would unwind in.  I would like to suggest that if you move one bay forward for the anchor, you might want to move it down one stringer also.  This would take advantage of the more open area of the fuselage for the motor and keep it from hitting the formers and possible bunching from hitting formers and becoming lodged in the restricted area. 

By moving the anchor forward one or two bays (area between formers) it will effect the balance/cg of the model.  It should allow the removal of some nose weight that was previously added to balance the model at the CG.   You could even make two additional anchors, the first one bay forward the second two bays forward of the original location. 

I assume that your reference to the down thrust, was to align the motor to the prop shaft hook after it's adjustment for down thrust (which raises the prop hook towards the top of the fuselage) not really effecting the amount of down thrust.

I hope my explanation makes sense.

Tom
  

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Konrad1
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Re: Much loved Comet Hellcat
Reply #24 - May 1st, 2013 at 9:58am
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Its been over a year since I've found time in my short visits home to do anything with this gal.
The photos show that I've added purchase for the tissue under the windscreen. I've also added some to the top of the fuselage just aft of the canopy. And I hope you can see that I've added some just above the apex of the airfoil on the fuselage side.

I've also added some filler gussets  to the front bulkhead in an effort to bring the covering to the balsa block cowl as smoothly as possible..

I did remove the clay and weighed it. I was amused (shocked) that it weighed 18 grams.

I have yet to move the rear motor anchor (peg) forward 2 bays. Moving the rear peg forward 2 bays should place the rubber motor's mass close to the CoG for the airframe. With the motor centered over the CoG changing motors shouldn't effect the CoG (trim) much.

I hope that the loss of much of this nose weight and some of the motor weight will result in improved performance in the glide, maybe even in the climb.

Next thing on the list is to rework the nose block.

I have to ask should I move the rear beg up one set of stringers when i move it forward? My thinking is that this will bring the motor more in line with the down thrust. My concern is will moving the motor's mass up effect the model's trim?

All the best,
Konrad
« Last Edit: May 1st, 2013 at 3:04pm by Konrad1 »  

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Re: Much loved Comet Hellcat
Reply #23 - Feb 6th, 2013 at 9:58am
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Jim,
"Declage", What do you mean? In my French book Declage means difference. Here I assume you mean the angle difference between the stab and wing.

As most flat bottom airfoils have a positive pitching moment (drive the nose down). The stab needs to produce a downwards force to counter this pitching moment. I set my stabs to be at the same angle as the bottom of the wing (at least) but usually wind up with the trailing edge of the stab higher than the LE of the stab (down force) or up trim. I find that lifting stabs to be rather unstable (aft CoG). I'll take the hit in nose weight for stability.
  
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JIM
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Re: Much loved Comet Hellcat
Reply #22 - Jan 3rd, 2013 at 2:48pm
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Grin  Konrad  Nice looker but as I've said in other forums Comets are tail heavy and need Declage.  You are doing anything wrong, they do need reengineering but can be great flyers with some planning

JIM Grin
  
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Konrad1
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Re: Much loved Comet Hellcat
Reply #21 - Mar 25th, 2012 at 3:39am
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I will keep notes and will post my progress. Unfortunately I only get back home about every 2 month and then it is for only a short time. So it will be a rather slow rebuild.

I can't believe that as a child I ever told my mother that I was bored. I would love to get back some of the time of my mis-spent youth.
« Last Edit: Mar 25th, 2012 at 6:19pm by Konrad1 »  
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C.L. Chennault
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Re: Much loved Comet Hellcat
Reply #20 - Mar 23rd, 2012 at 9:14pm
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Please keep us posted.  I am interested in seeing how this turns out.
  

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Konrad1
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Re: Much loved Comet Hellcat
Reply #19 - Mar 23rd, 2012 at 7:30am
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Tom,
Thank you. I'll try it on some scrap. I too have thinned the glue stick adhesive with isopropol alcohol. I hope it will work with the 18 year old glue on the windscreen.

All the best,
Konrad
  
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