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Author Topic: "Imitation" modifications  (Read 1938 times)

Offline Warren Wagner

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"Imitation" modifications
« on: June 26, 2006, 02:36:38 PM »
The plans for Fanchers "Imitation" have been in my shop since not long after the construction article was published way back in 1979.   When Ultra Hobby came out with a laser cut kit for the "Imitation", well, I had just run out of excuses.....so....."the Imitation is on the bench." 

Ted made a couple of comments about his "Imitation" in the "Unequal panels" thread:

"... My favorite configuration was with the Enya .46 four stroke that was an absolutely perfect combination despite the resulting high weight (about 62 oz with the four stroke up front and the weight in the tail necessary to balance it)..." 

"...All kidding aside, Brett's admonition about the flexible aft fuse is real.  From the pilot's perspective it never bothered my with my original but, from watching others fly theirs, it was very real.  Anything of reasonable weight that can be done to add some torsional rigidity would probably be worth it..."

My intention is to use a variety of engines including an OS 40 Surpass 4-stroke (13.0 oz.), ST46 (11.9 oz.), OS40FP, and perhaps even an electric.

As I am early in the construction stage, I have the opportunity to address both of those problems.

To reduce the nose heavy condition, I was going to shorten the nose approximately 1", but that would cut into the tank compartment that is cut out of the nose.

Then I was thinking about lengthening the tail, 1-2", and moving the stab back a corresponding amount.

But if there is a problem with torsional strength in the tail, perhaps keeping the tail moment the same and covering the rear of the fuse with 1/64", would solve that problem and the nose heavy condition at the same time.

Ted, I need a little help here.   What would you suggest?

TIA

Cheers,

Warren Wagner
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Offline captcurt

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Re: "Imitation" modifications
« Reply #1 on: June 27, 2006, 05:52:39 AM »
Carbon added to the center of a 2 pc sandwich will do a little for torsional rigidity and nearly nothing for bending.

If you want to make a stiffer fuse, without trying to use glass or CF on the outside, the builtup and sheeted fuse is the way to go IMO.  Larry Cunningham details this method well in his Mo'Best series and others are now beginning to use it more often.

I did a profile a few years ago with a warren truss external wrap of 12K CF tow on the 4 lb balsa fuse blank.  It is quite stiff in bending and torsion and has flown quite a few 500pt plus patterns now in many hundred flights.  I would still choose to use the built-up method if I were to do another.

Curt


Online Ted Fancher

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Re: "Imitation" modifications
« Reply #2 on: June 30, 2006, 05:52:07 PM »
Sorry to be a little late here.

I've already sent Warren a comment from a private email he sent.  It didn't mention the desire to use other than the four stroke so I made the simple suggestion of cutting an inche off the nose.  Might not be such a good idea if he uses other engines that weight less an need more gas.

Sort of a conundrum.

Also didn't know it was the .40 Surpass he had in mind.  My experience with the .40 in the Doctor doesn't give me a good feeling about using it in the Imitation.  It is adequate for the the Doctor but nothing excessive.  The flapped Imi at a likely higher weight might make it a bit problematic.  The original flew great with the Enya .46 which was a very powerful engine (I saw the  other day they now make a .53 in the same case and if my engine drawer wsn't already overflowing I'd be tempted to check one of those out.

At any rate, Warren, I'd give a little more thought about the .40.  I think the .52 Surpass might be a better choice if you can "trade in" the .40.

Once again, I concur with the desireability of stiffening the fuse.  My personal preference would be for a built up profile like Larry developed with as much curve as can be worked into it.  I'd then cover that with 3/4oz glass cloth and dope which I think would give pretty good rigidity and not be as heavy as ply sides on a tail this long.

If you did decide to go with the ply I would be tempted to make the tail just slightly bigger which will allow you to fly comfortably stable with a further aft CG which might be necessary.  Not a lot, I think the original was about 22% of the wing area or so; so just increasing the span an inch or two ought to get it up to about 25%.

At the risk of tooting my own horn, I'm glad to see people starting to build some Imitations.  Of all my designs it is by far the easiest to make fly very well and isn't critical on trim.

Ted

Offline Warren Wagner

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Re: "Imitation" modifications
« Reply #3 on: July 02, 2006, 01:00:21 PM »
Sorry to be a little late here.

I've already sent Warren a comment from a private email he sent. It didn't mention the desire to use other than the four stroke so I made the simple suggestion of cutting an inche off the nose. Might not be such a good idea if he uses other engines that weight less an need more gas.

Sort of a conundrum.

Also didn't know it was the .40 Surpass he had in mind. My experience with the .40 in the Doctor doesn't give me a good feeling about using it in the Imitation. It is adequate for the the Doctor but nothing excessive. The flapped Imi at a likely higher weight might make it a bit problematic. The original flew great with the Enya .46 which was a very powerful engine (I saw the other day they now make a .53 in the same case and if my engine drawer wsn't already overflowing I'd be tempted to check one of those out.

At any rate, Warren, I'd give a little more thought about the .40. I think the .52 Surpass might be a better choice if you can "trade in" the .40.

Once again, I concur with the desireability of stiffening the fuse. My personal preference would be for a built up profile like Larry developed with as much curve as can be worked into it. I'd then cover that with 3/4oz glass cloth and dope which I think would give pretty good rigidity and not be as heavy as ply sides on a tail this long.

If you did decide to go with the ply I would be tempted to make the tail just slightly bigger which will allow you to fly comfortably stable with a further aft CG which might be necessary. Not a lot, I think the original was about 22% of the wing area or so; so just increasing the span an inch or two ought to get it up to about 25%.

At the risk of tooting my own horn, I'm glad to see people starting to build some Imitations. Of all my designs it is by far the easiest to make fly very well and isn't critical on trim.

Ted

Ted...thanks for the reply....appreciate your time.

The reason for selecting the "Imitation" with the radial mount nose, was to have a plane capable of easily using a variety of different engines, to include 2-stoke,4-stoke, and possibly electric.

One of these engines is a ST46 which I believe requires 5+oz of fuel,  and the battery packs are rather long, so I would prefer not to cut 1" off the nose which would reduce the tank compartment.

Would it be an acceptable alternative to lengthen the tail instead?   The fuselage is already built, and it would be an easy operation to graft on 1",2", or 3" of fuselage, and then move the tail assembly back a corresponding amount.  What would be the correct amount?

As to stiffening the rear of the fuselage, I have several options, including 0.5 oz. carbon fiber, 1/64" plywood, and 3/4 oz. fiberglass cloth, as you suggested.   I'm in the process of making test samples of these three material to try and get a more objective idea of their relative characteristics.

Again, thank you for your time. 

Cheers,

Warren Wagner
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Re: "Imitation" modifications
« Reply #4 on: July 03, 2006, 08:58:08 AM »
Ted...thanks for the reply....appreciate your time.

The reason for selecting the "Imitation" with the radial mount nose, was to have a plane capable of easily using a variety of different engines, to include 2-stoke,4-stoke, and possibly electric.

One of these engines is a ST46 which I believe requires 5+oz of fuel,  and the battery packs are rather long, so I would prefer not to cut 1" off the nose which would reduce the tank compartment.

Would it be an acceptable alternative to lengthen the tail instead?   The fuselage is already built, and it would be an easy operation to graft on 1",2", or 3" of fuselage, and then move the tail assembly back a corresponding amount.  What would be the correct amount?

Warren, I think you'd be better off to leave the tail as is and accept a little higher wingloading with the engines that need to be mounted further ahead.  You'll note the plans show a hatch in the back for adding tail weight.  That's where I put it when I flew the original with the four stroke.  Brett's ship flies OK with the longer moments but you'll note his last three airplanes didn't copy the idea.

I think you might be better off to sheet the tail to increase its rigidity and add a bit more built in tail weight, leaving the moments as is.  I would couple that with the earlier suggestion to increase the span of the tail to get the total area up around 25%, which will give even greater flexibility in terms of acceptable CG range


As to stiffening the rear of the fuselage, I have several options, including 0.5 oz. carbon fiber, 1/64" plywood, and 3/4 oz. fiberglass cloth, as you suggested.   I'm in the process of making test samples of these three material to try and get a more objective idea of their relative characteristics.

With the length of the tail the plywood still scares me.  I'd still prefer to see the aft fuse "ovalled" as much as possible and covered with something reasonably rigid like 3/4oz fiber glass applied with the minimum amount of thinned epoxy.

Online Brett Buck

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Re: "Imitation" modifications
« Reply #5 on: July 03, 2006, 11:01:14 AM »

Quote
With the length of the tail the plywood still scares me.  I'd still prefer to see the aft fuse "ovalled" as much as possible and covered with something reasonably rigid like 3/4oz fiber glass applied with the minimum amount of thinned epoxy.

     One thing about the 1/64 ply - you wouldn't have to worry about adding weight in the tail.

      1/64 ply is good, in it's element. But it's mega-heavy.  but I think you would do much better with graphite cloth vacuum-bagged on with epoxy, or at least .007 graphite laminate laid crosswise and then covered with .2 oz graphite mat and dope.

      Brett

Offline Warren Wagner

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Re: "Imitation" modifications
« Reply #6 on: July 03, 2006, 01:19:40 PM »
Warren, I think you'd be better off to leave the tail as is and accept a little higher wingloading with the engines that need to be mounted further ahead.  You'll note the plans show a hatch in the back for adding tail weight.  That's where I put it when I flew the original with the four stroke.  Brett's ship flies OK with the longer moments but you'll note his last three airplanes didn't copy the idea.

***Thanks Ted.   I will certainly follow your suggestions, but in the interest of a little design education for us (and this IS the design forum), could you briefly explain the reasoning why lengthening the tail moment is not a good idea in this case.

Cheers,

Warren Wagner

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Offline Warren Wagner

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Re: "Imitation" modifications
« Reply #7 on: July 03, 2006, 01:26:20 PM »
     One thing about the 1/64 ply - you wouldn't have to worry about adding weight in the tail.

      1/64 ply is good, in it's element. But it's mega-heavy.  but I think you would do much better with graphite cloth vacuum-bagged on with epoxy, or at least .007 graphite laminate laid crosswise and then covered with .2 oz graphite mat and dope.

      Brett

Brett,

1)  When you say "crosswise" do you mean in the manner of a Warren Truss structure as Curt illustrated in Reply#2?

2)  Would it be 'overkill' to use the 0.007" graphite laminate and cover it with the 3/4 oz. fiberglass cloth, as Ted suggested?

Thanks.

Cheers,

Warren Wagner
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Online Ted Fancher

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Re: "Imitation" modifications
« Reply #8 on: July 03, 2006, 11:11:16 PM »
***Thanks Ted.   I will certainly follow your suggestions, but in the interest of a little design education for us (and this IS the design forum), could you briefly explain the reasoning why lengthening the tail moment is not a good idea in this case.

Cheers,

Warren Wagner


Brett would be better at a technical discussion.  He's addressed this several times in forum discussions.

My best effort would be to point out that if you went to extremes -- say you had a 60" tail moment -- you'd quickly find the airplane unable to translate the rotation required in a timely fashion.  It would have lots of leverage and not require much deflection to drive the wing to the desired angle of attack but it would simply take to much time to move the tail.  Of course, balance would be an issue at 60" and would become a problem well before you get to that length.

It's kind of a classic case of recognizing that their is virtue in moderation in aircraft design for a given mission.  I can think of no other venue where the admonition avoid the assumption that "if a little is good a lot is a lot better"!  Small changes to a fundamentally sound configuration may improve performance (or might just as easily not).  It is almost certain that large changes to that fundamentally sound design will prove detrimental.

Ted
 
 
 
 

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Re: "Imitation" modifications
« Reply #9 on: July 05, 2006, 09:58:50 PM »
***Thanks Ted.   I will certainly follow your suggestions, but in the interest of a little design education for us (and this IS the design forum), could you briefly explain the reasoning why lengthening the tail moment is not a good idea in this case.

Cheers,

Warren Wagner


Brett would be better at a technical discussion.  He's addressed this several times in forum discussions.

       The quick answer is that while you might get more torque/angular acceleration from a longer tail moment *when the turn starts*, longer tail momentums also limit the maximum pitch rate you can achieve. It's a matter of the tangential velocity of the tail reducing the angle of attack of the tail as a function of pitch rate.

    The longer the tail, the lower the maximum pitch rate. At some point, if you make it too long, it, not the wing loading, becomes the limiting factor in the turn.

     I would note that Frank William's article in the latest SN also addresses this, as somewhat of an aside.

     Brett

     

Offline Terry Bolin

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Re: "Imitation" modifications
« Reply #10 on: November 15, 2006, 08:53:58 PM »
After reading this thread, I want to take my Imitation down, remove the old Fuse and build a new one. The wing was great, I was just concerned about the strenth of the Aft section. I hung it up because the Tail wiggled alot and it bothered me. I was even able to get Ted to send me some pictures of his plane way back when.  Thanks for the info!
Terry Bolin

Offline Jim Pollock

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Re: "Imitation" modifications
« Reply #11 on: November 15, 2006, 09:33:55 PM »
I might point out that the Imitation began a very good lineage of stunters culminating in the current Nats winner in the hands of Brett.

Jim Pollock   #^

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Re: "Imitation" modifications
« Reply #12 on: December 14, 2006, 11:25:19 PM »
This was my version of the Imitation, Stab/Elevator same size as Trivial Pursuit ,
LA .46 Sig Chipmunk cowl , otherwise Imitation , flew very well. 57 oz.

Offline Tom Niebuhr

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Re: "Imitation" modifications
« Reply #13 on: December 15, 2006, 09:52:21 AM »
Walter,
That is as pretty as a profile gets! 
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Offline Derek Moran

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Re: "Imitation" modifications
« Reply #14 on: December 30, 2006, 09:17:14 AM »
This was my version of the Imitation, Stab/Elevator same size as Trivial Pursuit ,
LA .46 Sig Chipmunk cowl , otherwise Imitation , flew very well. 57 oz.

What's the difference between the TP stab/elevator and the stock Imitation- planform area and construction?  Why did you think it was important to make this change?

I've seen Walter's plane and it flies great.  Bashing the Imitation into a Super Chipmunk was very clever and very well done.

Derek
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Walter Hicks

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Re: "Imitation" modifications
« Reply #15 on: January 01, 2007, 02:00:23 PM »
Hi,
  The Trivial Pursuit Stab elevator I believe is more low aspect ratio. Ask Ted about that. The reason I used that was
for aesthetic reasons ! The thing that helps Imitations is Carbon fiber matt and tissue on the fuse which really stiffens the  rear end of it. Mine had very little flex. You cannot go wrong with any derivative of the Imitation. I have a flying partner who just retrofitted a PA .65 with a pipe! Yes anything goes.

Offline Terry Bolin

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Re: "Imitation" modifications
« Reply #16 on: January 04, 2007, 07:04:18 AM »
 ::) #^I have one on the table right now, this inspires me to work harder on it now!
Flying weather is coming and I would like this to be done!
Great Site.
Terry B


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