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Old Mar 04, 2006, 04:48 AM
Big gov never Works
St. Martin's Avatar
Skunk Water, Rhode Island
Joined Jul 2002
12,618 Posts
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Skyhawk CG?

This is a pusher jet version of a design by Eric(Airx) that is being kitted by Warbirdkits.com I will also be doing a larger fan version for a 65mm fan.

I'm concerned with the percentage of MAC for the CG. At 25% it is tailheavy. I'm trying to get away with using a 7v speed 400, 3s Hyperion 1500 @ 4oz. The AUW is now at 16oz. This combination is more usable for most people. The pack sits under the canopy, all the way forward. The airfoil is reflexed.

Has anyone any experiance with the flexability of a CG behind 25% on a delta wing planform? I'm searching for all options to avoid adding additional wt. There is a thread in the pusher forum. It was suggested I may get additional input here.

Thanks!

Steve
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Old Mar 04, 2006, 05:04 AM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,585 Posts
Quote:
Originally Posted by St. Martin
This is a pusher jet version of a design by Eric(Airx) that is being kitted by Warbirdkits.com I will also be doing a larger fan version for a 65mm fan.

I'm concerned with the percentage of MAC for the CG. At 25% it is tailheavy. I'm trying to get away with using a 7v speed 400, 3s Hyperion 1500 @ 4oz. The AUW is now at 16oz. This combination is more usable for most people. The pack sits under the canopy, all the way forward. The airfoil is reflexed.

Has anyone any experiance with the flexability of a CG behind 25% on a delta wing planform? I'm searching for all options to avoid adding additional wt. There is a thread in the pusher forum. It was suggested I may get additional input here.

Thanks!

Steve
Nice model.

You didn't say how far behind 25% the model balanced. If it balances at 26% or even 27%, that'd be okay. If it balances at 30% or more, then you'll have to start thinking about moving stuff around or adding nose weight.

Dan
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Old Mar 04, 2006, 05:49 AM
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keven64's Avatar
United Kingdom, England, Burnley
Joined Apr 2001
5,507 Posts
Blimey that DOES look great !

Nice work Steve... and Eric. Bring on the one for EDF...
Scooters are great...

Keven.
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Old Mar 04, 2006, 07:19 AM
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Wareham, Dorset, UK
Joined Dec 2003
1,912 Posts
Quote:
Originally Posted by St. Martin
At 25% it is tailheavy. The airfoil is reflexed.
Steve
Hi Steve

two things (as above) 1. How do you know it is tail heavy? Is it unstable in flight? Have you done the "dive test"? Get it into a shallow dive and see what happens if you neutralize the elevator. If it zooms up it is nose heavy, if it carries on (with the shallow dive) it is neutral.
2. Why do you use a "reflexed" airfoil? That type only makes sense on a tailless plane.

Klaus
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Old Mar 04, 2006, 08:36 AM
Big gov never Works
St. Martin's Avatar
Skunk Water, Rhode Island
Joined Jul 2002
12,618 Posts
The plane balances at 37% with the 1500 pack. 30% with the larger 2200 pack. That is about all I can do without adding wt. AUW without the pack is 15oz. 20oz with the large pack. At that wt I would need the heavier mega mtr 16/15/x. This would require more nose wt. hence a higher AUW. I am thinking now of going with a lighter Just Go Fly 400T motor. 46gr vs 70gr of the 400. At this point the SP-400 is useless. It will not give the desired performance at the balanced AUW.

Klaus, It has not flown. So I guess dynamically, it may not be tail heavy. But it may be, also. I'm trying to find a safe MAC percentage for this plan form.

On the reflex issue, this was designed into it. My RT MiG has a reflex, sort of. It is a proven flyer, altho not by me. If I can prove the CG location on this pusher, it should correspond to the larger EDF version.

I think a small chuck glider is in order.

Hi Kevin, this size should work great for the microfan or Alpha. With the more forward fan location, balancing should not be an issue. Eric plans on doing this. There is a lot of structure, behind the 25% position. And the tail is long.

Steve
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Old Mar 04, 2006, 09:00 AM
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Las Vegas, NV
Joined Dec 2000
2,558 Posts
WOW! That A-4 looks GREAT. Nice job.

Ted
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Old Mar 04, 2006, 09:10 AM
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St. Mary, Maryland, United States
Joined Dec 1996
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Assuming the designer flew it and specified 25% as the balance point you need to add whatever lead is needed to balance it at that point. Once YOU'VE flown it take the lead out a 1/4 ounce at a time until the stability is too marginal for you. 37% ain't ever gonna work 30% might be OK but not on the first flight!
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Old Mar 04, 2006, 09:51 AM
EDF Jet Jam 2015 , May 28-31
Kevin Cox's Avatar
St. Louis Intl, Missouri, United States
Joined Jan 1997
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Excellent work!

25% would be the most aft limit for me. Heck, on a model of this size 25% could be 27+% since you are measuring it by hand .

Good luck either way and remember, a tail heavy model flies once....blah,blah etc.
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Old Mar 04, 2006, 10:35 AM
Big gov never Works
St. Martin's Avatar
Skunk Water, Rhode Island
Joined Jul 2002
12,618 Posts
Quote:
Originally Posted by Chris True
Assuming the designer flew it and specified 25% as the balance point you need to add whatever lead is needed to balance it at that point. Once YOU'VE flown it take the lead out a 1/4 ounce at a time until the stability is too marginal for you. 37% ain't ever gonna work 30% might be OK but not on the first flight!

Thanx for the copliments and support, gentlemen. Chris, this is the prototype. And I always prefer slightly noseheavy. I just picked up a small eflite outrunner. wt is 47gr as compared to 70 of the 400. Will see how this fairs.

Steve
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Old Mar 04, 2006, 02:34 PM
Big gov never Works
St. Martin's Avatar
Skunk Water, Rhode Island
Joined Jul 2002
12,618 Posts
Success!

Thank you for all your input. The small outrunner did the trick. It balances just slightly behind the 25% CG mark or around 26-7 percent, depending on how accurate those marks are. It has much more thrust, and at less amp draw, on a 7-5E prop vs the 6-4.

I'll be back soon with the fan version, which is 128% of this one. It will use a PF shroud with a minfan rotor cut to 65mm, and a HET 2W on a 3s. Or possibly 4s.

Thank You again!

Steve
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Old Feb 17, 2015, 02:51 AM
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United States, OK, Broken Arrow
Joined Oct 2005
33 Posts
Quote:
Originally Posted by St. Martin View Post
Thank you for all your input. The small outrunner did the trick. It balances just slightly behind the 25% CG mark or around 26-7 percent, depending on how accurate those marks are. It has much more thrust, and at less amp draw, on a 7-5E prop vs the 6-4.

I'll be back soon with the fan version, which is 128% of this one. It will use a PF shroud with a minfan rotor cut to 65mm, and a HET 2W on a 3s. Or possibly 4s.

Thank You again!

Steve
Did you fly it? How was CG?
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Old Feb 17, 2015, 03:47 AM
Big gov never Works
St. Martin's Avatar
Skunk Water, Rhode Island
Joined Jul 2002
12,618 Posts
Perfect. This design actually needed up trim, if flown at anything forward of 27%MAC. The jet is long gone, but I still have the larger 66mm version. Important to note, this has a reflexed airfoil.

Fuzz
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