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40 pitts structural problem - 12/11/2006 9:21:50 PM   
PJD1


 

Posts: 9
Joined: 11/23/2006
From: houston, TX, USA
Status: offline 		I built and flew a nitro model 40 gas pitts using a tower hobbies 46. On the third flight, the right wing began to flutter uncontrollably, and regretfully the plane was lost. I looked at the rubble, and noticed that some of the attachments that screwed to the wing braces between the lower and upper wing had pulled loose. I attributed this to the crash, but later had another idea. When I put it together, I noticed that the upper wing brace attachment points were about 1" further apart than the lower wing attachment points. Thus in order to get a straight brace between the two wings, the brace had to bend inwardly, thus imparting a stout bending force on the 1/16" ply attachments. Could it be that that force combined with the G forces of flying pulled out the attachments? The nitro model rep. said that it was designed to have a bent brace. Seems kinda stupid to me, all the other biplane braces are straight. I love the plane and want to buy another, but don't want to go thru the same problem. When the flutter began, I was only half throttle.
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RE: 40 pitts structural problem - 12/11/2006 10:45:56 PM   
opjose



Posts: 5830
Joined: 11/26/2005
From: Poolesville, MD, USA
Status: offline 		What engine?



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RE: 40 pitts structural problem - 12/12/2006 5:30:06 PM   
PJD1


 

Posts: 9
Joined: 11/23/2006
From: houston, TX, USA
Status: offline 		I used a tower hobbies 46. The model was designed for a 40, but i only flew at half throttle.

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       Post #: 3

RE: 40 pitts structural problem - 12/14/2006 12:38:01 AM   
kamikazejoe


 

Posts: 16
Joined: 12/11/2006
From: Toronto, ON, CANADA
Status: offline 		just read your post and had ordered the pitts this morning. Do let me know your thoughts on the plane and how it builds and flies before the crash. Do you think some kind of structural strengthing is required that can be done or is it a design flaw ?
KJ

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       Post #: 4

RE: 40 pitts structural problem - 12/14/2006 12:50:36 AM   
PJD1


 

Posts: 9
Joined: 11/23/2006
From: houston, TX, USA
Status: offline 		When you receive your pitts, you will see what I am talking about when you try to connect the upper and lower wings with the 2 braces. The points of attachment for the upper wing and lower wing do not line up and the braces must be deformed to attach them. Writ me back and tell me what you think. If I get another one, I plan to modify the attachment points so that they ar aligned.

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RE: 40 pitts structural problem - 12/14/2006 1:02:44 AM   
kamikazejoe


 

Posts: 16
Joined: 12/11/2006
From: Toronto, ON, CANADA
Status: offline 		sure will do and your point has made me think if i should reinforce that.

How does the plane fly ? I am planning a 70 4 stroke in it

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       Post #: 6

RE: 40 pitts structural problem - 12/14/2006 1:45:52 PM   
PJD1


 

Posts: 9
Joined: 11/23/2006
From: houston, TX, USA
Status: offline 		It flies like a dream. With a 46 2 stroke, the balance point was exactly on 3.1" back measured on the top wing. With a 70 4 stroke you will probably have to add weight to the tail. Also, i hope you dont need a lot of directions to build it, because the instruction booklet leaves a lot to be desired. Good luck. I am probably gonna get another one because the price is right.

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       Post #: 7

RE: 40 pitts structural problem - 12/14/2006 10:51:04 PM   
kamikazejoe


 

Posts: 16
Joined: 12/11/2006
From: Toronto, ON, CANADA
Status: offline 		gr8 news. you can get one from ebay. I got one for $130 shipped

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       Post #: 8

RE: 40 pitts structural problem - 1/12/2007 4:40:39 AM   
opjose



Posts: 5830
Joined: 11/26/2005
From: Poolesville, MD, USA
Status: offline 		I just got mine in.

This is an awfully light plane for a .46, as it calls for a .32 2 stroker.

Is everyone using .46's?

Also what size servo do the aileron's take?

They look like sub-micros. Is this true?

Thanks.



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Build em'', fly em'', crash em'' and build more.

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RE: 40 pitts structural problem - 1/14/2007 1:00:10 AM   
PJD1


 

Posts: 9
Joined: 11/23/2006
From: houston, TX, USA
Status: offline 		The manual call for a 40 size motor or .52 4 stroke. I have just about finished the second PITTS. I used a .46 OS LA engine this time, which is considerably less powerful than the TOWER HOBBIES .46. Also, I am going to modify the way the 2 wing strutts are attached to the lower wing to try to reduce the force on the little tabs protruding from the wing. I used 2 JR MC35 servo's for the ailerons. I had used hi tec HS81's on the first one that crashed. The gears stripped, but that was undoubtedly due to the crash. The JR's seem to work fine. Tell me when you get to the point of mounting the wings what you think about the struts bending like they do.Nowhere did I see any talk of using a 32 size motor. Where did you see that?

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RE: 40 pitts structural problem - 1/14/2007 6:23:44 AM   
opjose



Posts: 5830
Joined: 11/26/2005
From: Poolesville, MD, USA
Status: offline 		THANKS!!!!

P.9 of the manual states under "Warnings".

Gas Pitts - 40 is designed to be powered by 2C: 0.32 ; 4C: 0.52 engine

The two equate fairly well, so the instructions are basically calling for a .32 engine.

The LA .46 would be slightly more powerful than the .32 engine being called for... so it may be a VERY good match.

How does it fit in the cowl? Did you mount it inverted?

I used an LA .46 in a .32 Extra 330 and it rocks in this plane... I just wish that OS had not gone with the cheap bleed needle for the low end. As it is, I cannot lean the low end out enough... but the plane runs VERY reliably... w/NO deadsticks EVER from day one.
So now that you mention it, this may be a great combo!

Re: Bend

I held the two wings together to compare the strut attachment points and you are right.

When the two wings are held so that their centers align correctly there is a 1/2" variance (1 inch total) in the position of the strut mount points.

Now that you've mentioned this I'm going to fix this easily... how?

Take a small 1/2" wide 3" long strip of ply and shape it to go between the strut and the mount to off set the point.

Remove a bit of covering and expoxy the block directly to the wood of the wing.

Apply a small bit of Ultracoat and this "fix" will not be obtrusive. To really hide it, do it to the underside of the upper wing instead of on the lower wing.

A slightly longer bolt will be needed too. Not a biggie once this is pointed out as you have...


- Hey Nitrostaff are you aware of this issue? -


I would advise you to do the same with your "bent" struts. Don't leave them bent... this doesn't sound right.

Real Pitts DO have the struts at an angle though much like what the NP planes would have if the mount points were all properly angled.

I bet someone in the design department made a mistake and didn't want to loose their job....



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RE: 40 pitts structural problem - 1/14/2007 9:56:29 PM   
bkrfrmhell


 

Posts: 85
Joined: 10/4/2004
From: Tofield, AB, CANADA
Status: offline 		This is exactley what I was saying in the other pitts challenger thread i started about engine size. You were telling me this plane could take a 70 plus 4 stroker no problem. I haven't maidened mine yet with the saito 65.....and balance was not a problem with the extra engine weight..I mounted my 5 cell battery just in front of the servos. I am using hs85mg in the wings...should be lots. I think it would be some major wing surgery to relocated the struts mounting tabs. I just epoxyed them real well. Hopefully I have better luck on not loosing a wing. I think an average 40 stroke would be more than enought power for this little plane. Why fly at less than half throttle all the time? Not designed for 3d by any means, just a nice little scale plane. I wish nitrostaff would give his opinion on this strut issue.

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       Post #: 12

RE: 40 pitts structural problem - 1/15/2007 3:09:46 AM   
opjose



Posts: 5830
Joined: 11/26/2005
From: Poolesville, MD, USA
Status: offline
quote:

ORIGINAL: bkrfrmhell

This is exactley what I was saying in the other pitts challenger thread i started about engine size. You were telling me this plane could take a 70 plus 4 stroker no problem. I haven't maidened mine yet with the saito 65.....and balance was not a problem with the extra engine weight..I mounted my 5 cell battery just in front of the servos. I am using hs85mg in the wings...should be lots. I think it would be some major wing surgery to relocated the struts mounting tabs. I just epoxyed them real well. Hopefully I have better luck on not loosing a wing. I think an average 40 stroke would be more than enought power for this little plane. Why fly at less than half throttle all the time? Not designed for 3d by any means, just a nice little scale plane. I wish nitrostaff would give his opinion on this strut issue.



Yeah the 70 equates well with the .40 2 stoker.

My observation above was based upon what the instructions call for.... since they call for a .32 2stoker, yet everyone is using .46's.... which is about equal to your .65 both power and weight wise.

Some .46's put out more power though, e.g. the TH (or GMS .47) is rated at a good 1.75bHp (take that with a grain of salt though), and it lets my NP Ultimate 40S go unlimited vertical on that plane which seems to be heavier but more lightly wing loaded.


Nitrostaff, please chime in about the struts!






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RE: 40 pitts structural problem - 1/20/2007 4:50:41 PM   
PJD1


 

Posts: 9
Joined: 11/23/2006
From: houston, TX, USA
Status: offline 		The best reason for using a 46 size engine is that no weight needs to be added to the nose in order to achieve balance. I have built 2 pitts now using a tower hobbies .46 in the first, and an OS .46 LA in the second one. Both planes balance out perfectly 3.1" back from the top L.E. Both times I mounted the engines at 90 degrees and used a PITTS style muffler to exhaust below the fuselage. As far as the structural problem is concerned, I have finally concluded that the R.H. strut broke at the lower lightening hole just above the lower wing attachment point due to high bending stresses in the region. With no strut to stabilize the upper wing, the wing oscillated violently destroying the ailerons on that side. Examining the L.H. strut, I see cracks in the same area. So all the beefing up of attachment points will do no good if the struts break. The solution I came up with was to remove all bending stresses by shimming the 1/2" difference between the strut attachment locations by using a nylon spacer I got at ACE hardware #6 screwhole in the center, and 1/2" Outside dia. This allows the struts to be perfectly straight. I received a damaged canopy from RAIDENTECH, and e-mailed them a pic of it. They have sent a new one. As soon as it is on, and the OS is purring, I will test fly it. The weather here on the Texas gulf coast has been crappy for a month now.

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RE: 40 pitts structural problem - 1/20/2007 8:02:55 PM   
opjose



Posts: 5830
Joined: 11/26/2005
From: Poolesville, MD, USA
Status: offline 		Yup, that's effectively what I've done, as I posted above...

A "spacer" that is also epoxied to the wood in the wings, to keep the struts straight and add additional re-inforcement.

I did something similiar with my Ultimate 40S' to beef up the supports and this worked great with those planes.


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Build em'', fly em'', crash em'' and build more.

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