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Ok p51

Old 04-27-2015, 11:51 AM
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mashp39
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Default Ok p51

I have an OK Model P51.Does anyone know the control throws and CG for this plane? I got it at a swap meet and it is 46 size. I tried to take it off but it seemed to be very tail heavy and the take off was aborted. I used the CG and throws for a Great Planes 40 size P51. Someone told me later that the CG for the OK model needed to be set about 2 inches from leading edge of wing at fuse side. This seems to be too far forward but I will try it next. All help and advice will be welcome! Thanks.
Old 04-27-2015, 01:30 PM
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scale only 4 me
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If the wing is the same scale shape and size,, Make doesn't really matter for determining CG location,,,

Also, I don't know what you could have possibly felt on the ground that would make you think it's tail heavy??
Old 04-27-2015, 06:14 PM
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mashp39
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It had a long take off roll with plenty of speed but did not want to lift off. When the nose came up it did not want to take to the air.I checked the balance at 5 1/4 inch and it balanced with a slight nose down but at as little as 5 inch it went very tail heavy.I checked it at 2 3/4 inch and it seems it will need much weight.What is the factory spec?
Old 04-28-2015, 03:34 AM
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Are you balancing the plane inverted with no fuel? Not wanting to lift off, or Rotate, is more a symptom of a nose heavy or under powered.

The specs of the GP plane are listed here, http://www.greatplanes.com/techsupport/technotes.html you can confirm those numbers here http://www.scaleaero.com/CG_Calculator.htm

If that doesn't work I suggest you ask for help from a club member who has experience with "scale" planes, Not the guy that said 2",

good luck
Old 04-28-2015, 03:55 AM
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mashp39
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I soloed in 1978. This is the first plane I have seen with this problem.I did have a very experienced club member attempt the take off I described. He aborted the take off when the plane was about 5 feet in the air.Only damage was bent retracts. About 8 other club members agreed the plane seemed tail heavy.The plane was balanced inverted, empty fuel tank, and retracts up.The guy that said 2 inches was at another forum in RC Univerce that was also trying to be helpful.The motor is a 46,well broke in, Super Tigre with plenty of power.I would like to know the factory specs for a OK Models P51. I new CG, control throws andweight. Any INFO wil be appreciated. Thanks.
Old 04-28-2015, 07:54 PM
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Hmm 1978 was a good year. OK mashp39 if you get actually get an original copy of the this rather vintage ARF's manual with CG info then please disregard anything I have to say. My apologies, I do not have any specific numbers from that manual or am I going to try to suggest using info from a similar sized airplane.

Instead what I am going to offer a very simple system that just might preclude you from ever having the need to go on an internet search for exact info. from some long since gone manufacturer agine. This system works very well with most warbirds and even most sport airplane for that matter. What it provides is a safe place for a starting CG with any unknown airplane or design. It works best with monoplanes that can be single tapered or double tapered even elliptical in wingform. Obviously it wont work on canard or delta. but just about any conventional configured monoplane.

mashp39 all you need to do is find a point halfway between the fuselage side and the wingtip. Now at this point half way out there measure the distance between leading and trailing edges of the wing. Take this distance out there that you have measured and divide it by four, now take that sum and measure that distange back from the leading edge out there at the half way point and note this position.

From this position all you need to do is project this point inward toward the fuselage and this is you target CG that you want to use to have the airlplane balance level at. Do not balance it slightly nose low for mother and country if you do that then you have not balanced the airplane at that target point but a point further forward.

John
Old 04-29-2015, 04:02 AM
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mashp39
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Thanks for your info.I used your method and the number is 4 inches back from wing leading edge at the side of fuse.This seems more right. World models P51 calls for 5.5 inches and Great Plains calls for 5 3/8 inches. The 4 inch mark will be used and the control settings will be used from the other manufactures.
This plane came from someones back shelf and with no info I had to start somewhere. The 2 inch advice came as a suggestion to keep out the tail heavy as a start.These older ARF planes were not designed as todays are.Maybe that is why they are not here now.
This is not a major project now but I will post when flight is tried again.Thanks for all the input.
Old 04-29-2015, 05:31 AM
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What ever it works out to that is what it works out to and I could care less what the numbers are for World models or Great Planes are, different airplanes and different airfoils.

This is a very old method that is very simple and has been used by desiginers since the beginning that requires no computers or programs but just simple common sense. It is not rocket science nor is it an exact science but what this simple mid span averaging method does provide a rudimentary average 25% mid span chord. Balancing at this 25% will provide a safe and practical starting point to fly fly the airplane at initially, perhaps a few percent too far forward but in almost every case a safe known position that can be worked out with slight changes on subsiquint flights..

In Virtually evey case almost every day someone appraoachs me at the field sitcks some unknow typical airplane that I described in my first post above and wants to know what it should be balanced and in almost every case with a simple yard stick of tape measure a safe balance point can be worked out.

Now that we have a reasonable CG worked out for some unknown desdign how the heck do we set up the throws.Well onnce agin its not rocket sicience but perhaps more of a simple experience thing. This common sense thing would suggest never more than sixty degrees on highs and never less than about 25 degrees on lows. Yes I do suggest the use of expo. for any typical warbird and about negative/positive as what ever is required for your system at forty percent on pitch and about 30 % on roll (ailerons). Not critical numbers but agine practical starting numbers for most warbirds as well as many sport aircraft.
Everythng in my two previous posts has worked very well for me over the years. If any of this works out for your mashp39 wold appreciate you returing with your success story or your failure storys

John

Last edited by JohnBuckner; 04-29-2015 at 05:37 AM.
Old 04-29-2015, 05:16 PM
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mashp39
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OK, I will return but it may take awhile as this plane is not my main interest at this time.I know how it behaved at its last flight attempt and couldn't just leave it alone.I believe this will work a lot better.I am going to use this method and check some other planes as compared to their factory specs.Should be interesting.Again thanks.
Old 05-03-2015, 01:19 PM
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THe calculator URL doesn't seem to work anymore??

Last edited by Gray Beard; 05-03-2015 at 01:21 PM.
Old 05-04-2015, 02:42 AM
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mashp39
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What is calculator URL?
Old 05-04-2015, 04:21 AM
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Try this one:

http://adamone.rchomepage.com/cg_calc.htm

Or this (simpler):

http://www.scaleaero.com/CG_Calculator.htm

Last edited by Top_Gunn; 05-04-2015 at 04:24 AM.
Old 05-04-2015, 08:22 AM
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What Gun just posted. Either I typed it in wrong or the one I have is out of date. The ones gun posted are working and they are what I use on unknown planes. I even used it on a two meter pattern plane and it was so dead on I never moved the CG during test flights. Very easy to use. Just type in your measured info and push a button.

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