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Old Apr 09, 2013, 07:37 PM
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Joined Mar 2013
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possible theories on these crash senarios

So lately I have been trying to get an scratch built foamie design to fly for a senior design prototype aircraft for a UAV platform.

Well my first three iterations have had some pretty horrible crashes, which, according to my research now I think was due to a VERY tail heavy aircraft.

From a hand launch the aircraft takes to a vertical climb almost immediately, one time it actually prop hanged for a good while, then it looks to tip stall about the left wing and crash nose first, rather hard I might add. I have no response from my tail surface inputs, which I assume to be due to no airspeed and the bad cg location...

Are these characteristics tell tale of a tail heavy plane, and its just so tail heavy that I don't have the chance to see the extreme sensitivity tail heavy planes normally exhibit?

Any input would be fantastic. Here is a pic of the last iteration before it was united with the earth...
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Last edited by BlackVette2001; Apr 10, 2013 at 09:48 PM. Reason: horrible sentence structure
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Old Apr 10, 2013, 02:51 AM
rookie
wilf370's Avatar
Canada, SK, Saskatoon
Joined Dec 2012
546 Posts
Are you an experienced test pilot. In my experience cg is not that big of a deal in weather you can fly the plane. proper cg makes for a balanced flight. Improper cg makes for an unbalanced flight but still fly's.
your problem seems to be the guy behind the sticks. You need to dial the throws way down and crank up the expo on your transmitter. Also you probly using to much throttle.
Wilf
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Old Apr 10, 2013, 12:50 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,166 Posts
Have you tried one of the CG calculators that take tail size and moment arm into the equation -

http://www.geistware.com/rcmodeling/cg_super_calc.htm

Also as I mentioned in another of your threads, you shouldn't need full throttle at launch, especially with an untried model design, it wont give you time to correct any trim/CG problems before a major problem.
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Old Apr 10, 2013, 01:29 PM
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Wilson, NC
Joined Aug 2009
217 Posts
Hi BlackVette2001
As others have asked, are you an experienced R/C flier? Where is your CG relative to the wing leading edge? A CG location of about 25% of the chord, back from the wing LE, is considered to be safe for a first flight.
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Old Apr 10, 2013, 01:45 PM
dusty bible = dirty life
Majortomski's Avatar
Oklahoma City OK USA Where fakts still exist even if they are ignored
Joined Aug 2000
2,772 Posts
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Last edited by Majortomski; Apr 11, 2013 at 07:38 AM.
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Old Apr 10, 2013, 09:40 PM
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Joined Mar 2013
17 Posts
My friend that was not necessary.

Yes it is a college senior design project, and not in aerospace. Thats why I am having so much trouble, I am in a mechanical and energy major, aerospace was not offered at my college. I have been teaching myself for the last year by reading 20 year old textbooks from a local bookstore, so please, give me a break... no need to troll.

There is little excuse for the english, yes its my native language but I have not been sleeping well lately, 3 jobs and a full course load to get by makes you tired this late at night. I'll do my best to clean it up.

Not that you will care or crack some joke, but I just got a full ride to Purdue today for a masters in aerospace engineering, hopefully in a year or two I'll have the education I need so I wont have to ask a guy like you for any help, but until then, anybody who can point me in a general direction in diagnosing a problem would be kindly received and appreciated. I took on this project because its a passion, just bites that I have no one to turn to at my college for ANY help.




Guys, no, I am not a seasoned test pilot, have a bit over a year of experience on a couple acro planes, a couple warbirds, a flying wing, a gas PC-9, and a few helicopters. Worked on a sim allot, hoped to learn without breaking the few things I have had. I have yet to get in contact with the local rc club, so I am the best my team has.

We are going to work on hand launches tomorrow with my other plan to get the feel down, had my expo at 30, but I am going to try 50, should help. A bit less rudder/elevator throw will also be in order. Replaced my tail section with a 8 inch root to tip v-tail, with the same cord and profile. Figured a better aspect ratio could not hurt.

Thanks for the comments guys, sorry to waste yall's time
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Old Apr 11, 2013, 08:05 AM
dusty bible = dirty life
Majortomski's Avatar
Oklahoma City OK USA Where fakts still exist even if they are ignored
Joined Aug 2000
2,772 Posts
It wasn't a troll, it was judged to be a personal attack.

And I most humbly apologize.


Congratulations on your entry to Purdue, that acceptance is something to be very proud of in itself.

I ask you to take something away from the above; as an engineer you and your work, all of it, the machine you've designed and the words you write about it, are always on public display. You as the engineer MUST always demonstrate in public that you confidently know what you're doing. Make your mistakes in private, make your successes public.

Yes, it appears tail heavy, and you should consider using higher aspect ratio ruddervators. Make them thinner but run the full length of the trailing edge of the fin. Then try gliding flights down a grass covered hill just to see if the plane is stable enough to fly before you try to fly under power.

Best of luck and success on your future studies.

Again I apologize for my harsh words, I could have done a much better job of expressing myself.
Thomas Solinski, Aero Engineer,
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Old Apr 11, 2013, 01:01 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,166 Posts
Just looking at the plane I find it hard to believe it could be tail heavy.

When you consider the length of the fuselage forward of the potential 25% CG, compared to what is hanging out the back aft of the CG........, I just can't see it happening.

There doesn't even see room for a battery aft of the CG to add to any rear CG, yet there is a motor right up front, even if small and light.

Vette, I still say go for a low throttle launch, (with all the throws reduced), even try a glide test. If you have some long grass you can glide it over and into, I would certainly start with just a glide.

If it seems Ok, then the low throttle test.

Nose high climbs from the launch, can be a throttle/lift problem, not just a rearward CG. Many beginners have the same problem with trainers that are set up correctly, but think full throttle is required to fly.
The problem can also be exacerbated if the air foil is cambered, as lift can increase considerably with speed. If the design needs to fly fast, (to perhaps cover a lot of territory), then a fully symmetrical airfoil could be better. Also the incidence angle could have an influence, speed and lift again.

I hope you do succeed, but as often said in many forums, the more information given the easier the answers come.
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Old Apr 16, 2013, 10:40 AM
Thailand
Joined Aug 2010
506 Posts
The tail looks very small and with not enough angle so not much vertical fin area.
V tails should be around 90 to 110 degrees, that looks a bit flat.
Maybe get rid of the V tail and use a normal one?
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