A question of weight....again
08-24-2006, 01:58 AM
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A question of weight....again
Ive been reading the posts for a long time now and have learned a great deal. I finally got the nerve to submitt a question and look forward to the response. Ive been flying my Model Tech P-51D for over a year now. Its is powered by an Evolution .46 and all standard servos as recommended in the manual. However the finished dry weight of the airplane was 7Lbs 4oz. (The manual specs estimated the weight to be 5.8 - 6.2 Lbs.) Even so the plane flys great. No bad stall tendencies and lands relatively slow.
I decided to install a 4c and purchased an ASP .65. I installed the ASP (after having to make modifications to the motor mount and catching heck tying to get the throttle linkage around the fuel tank). With the new engine the plane will weigh around 8Lbs. Ive mounted the receiver and battery rather nicely and do not want to move them unless it will adversely affect the flight charateristics. I believe I will still have to add 4oz to the tail in order to balance it at the forward recommended CG. How much will this additional weight make and will it potentially over stress the airframe?
I decided to install a 4c and purchased an ASP .65. I installed the ASP (after having to make modifications to the motor mount and catching heck tying to get the throttle linkage around the fuel tank). With the new engine the plane will weigh around 8Lbs. Ive mounted the receiver and battery rather nicely and do not want to move them unless it will adversely affect the flight charateristics. I believe I will still have to add 4oz to the tail in order to balance it at the forward recommended CG. How much will this additional weight make and will it potentially over stress the airframe?
08-24-2006, 02:31 AM
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RE: A question of weight....again
Glad you posted! Welcome!!
You're not gonna want to hear it, but the "prime directive" is "Don't add weight if you can move weight." If you did a clean radio installation forward, I suspect you can do one aft
... Adding dead weight to an already overweight airplane should be avoided like a date with your sister's homely best friend...
If I may crunch some numbers, you're adding 12 ounces in this version... My calculator says that's a 10% increase in weight on an airplane that is already 17% overweight... total obesity is about 27% above the max target weight... That's a lot... (I must admit that am unfamiliar with "real world" weights for this airplane - hopefully someone who flies model will chime in, but those numbers got my attention, and I have nothing at risk)
The new engine doesn't appear to add power commensurate with the additional weight, so the power loading (hp/oz) is gonna go down, the wing loading (oz/sqin) is gonna go up... trends that make me nervous...
If it were my airplane, I'd be looking to reduce weight, not increase it... [
] HTH
Cheers!
Jim
You're not gonna want to hear it, but the "prime directive" is "Don't add weight if you can move weight." If you did a clean radio installation forward, I suspect you can do one aft
... Adding dead weight to an already overweight airplane should be avoided like a date with your sister's homely best friend... If I may crunch some numbers, you're adding 12 ounces in this version... My calculator says that's a 10% increase in weight on an airplane that is already 17% overweight... total obesity is about 27% above the max target weight... That's a lot... (I must admit that am unfamiliar with "real world" weights for this airplane - hopefully someone who flies model will chime in, but those numbers got my attention, and I have nothing at risk)
The new engine doesn't appear to add power commensurate with the additional weight, so the power loading (hp/oz) is gonna go down, the wing loading (oz/sqin) is gonna go up... trends that make me nervous...
If it were my airplane, I'd be looking to reduce weight, not increase it... [
] HTHCheers!
Jim
08-24-2006, 09:24 AM
#3
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RE: A question of weight....again
Let me heap some more bad news on ya'
Sorry, but that model is a .46 size. And it comes from the mfg pushing the scales. I've got a bunch of .46size and have weighed them all. (If you are already squirming in your seat from reading this far, then don't read farther.)
The engine you've retrofitted will probably pull the airplane as well as the Evo did. You can usually estimate what size FS you can retrofit by taking half the displacement of your 2stroke and adding that back. You started with a 46. Half of that is 23. Add 23 to 46 and you got 69. A .70FS is about equivalent. So far so good.
But most 46size airplanes that fly good are usually under 6 pounds. A lot of people argue that wing loading matters. It does. But it's not the only thing that matters, nor is it the most important. Overall weight is really more important than wing loading. Yeah, yeah, it's splitting hairs, but it's those hairs that affect landing speed, climbing ability and some other stuff.
My heaviest 46size is under 6 pounds. It flies good. I've flown other guys 46s that were around 7 pounds. I was flying them because they didn't want to. OK, that's a rough way to put it, but basically true. They wanted me to figure out what was wrong with the airplanes basically.
Where am I going with this? I think I forgot.... grin......
You don't want to add weight to that airplane. No way. Don't. It's so dead simple to move a battery, that ought to be done without thought. If that's not enough, move the RX, but don't expect miracles if it's a new one and an ounce or less. Standard servos are usually a bit over 1.5 ounces each. A lot are close to 2 ounces. Move a couple.
If I were you, I'd look around for somebody to sell the P51 to. My Tiger2 is the heaviest 46 in the hangar and it's "only" 5lb12oz and flies great. But I can definately see it's weight working when flying it. I just finished a KYOSHO P40 and put retracts in it. It weighs 5lb7oz and flies good.
Sorry, but that model is a .46 size. And it comes from the mfg pushing the scales. I've got a bunch of .46size and have weighed them all. (If you are already squirming in your seat from reading this far, then don't read farther.)
The engine you've retrofitted will probably pull the airplane as well as the Evo did. You can usually estimate what size FS you can retrofit by taking half the displacement of your 2stroke and adding that back. You started with a 46. Half of that is 23. Add 23 to 46 and you got 69. A .70FS is about equivalent. So far so good.
But most 46size airplanes that fly good are usually under 6 pounds. A lot of people argue that wing loading matters. It does. But it's not the only thing that matters, nor is it the most important. Overall weight is really more important than wing loading. Yeah, yeah, it's splitting hairs, but it's those hairs that affect landing speed, climbing ability and some other stuff.
My heaviest 46size is under 6 pounds. It flies good. I've flown other guys 46s that were around 7 pounds. I was flying them because they didn't want to. OK, that's a rough way to put it, but basically true. They wanted me to figure out what was wrong with the airplanes basically.
Where am I going with this? I think I forgot.... grin......
You don't want to add weight to that airplane. No way. Don't. It's so dead simple to move a battery, that ought to be done without thought. If that's not enough, move the RX, but don't expect miracles if it's a new one and an ounce or less. Standard servos are usually a bit over 1.5 ounces each. A lot are close to 2 ounces. Move a couple.
If I were you, I'd look around for somebody to sell the P51 to. My Tiger2 is the heaviest 46 in the hangar and it's "only" 5lb12oz and flies great. But I can definately see it's weight working when flying it. I just finished a KYOSHO P40 and put retracts in it. It weighs 5lb7oz and flies good.
08-24-2006, 09:52 AM
#4
RE: A question of weight....again
Welcome to RCU!
I have been known to fly a few planes that, like myself, carried a few extra pounds, and here is what I would do...
Move the battery as far back as you can without having to jump through too many hoops. Then add whatever weight is needed.
Now FLY THE PLANE!
Hell, I've had a few planes that flew BETTER once they got heavier.
Now this is not always the case, and you may find that you need to jump through a few more hoops later to lighten it up a little more. But why go through all that if you don't need to?
I have been known to fly a few planes that, like myself, carried a few extra pounds, and here is what I would do...
Move the battery as far back as you can without having to jump through too many hoops. Then add whatever weight is needed.
Now FLY THE PLANE!
Hell, I've had a few planes that flew BETTER once they got heavier.
Now this is not always the case, and you may find that you need to jump through a few more hoops later to lighten it up a little more. But why go through all that if you don't need to?
08-24-2006, 12:59 PM
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RE: A question of weight....again
Thanks guys for the responses!!................... this is a great forum,
I'll take the advice and move the battery and rx back as far as reasonably possible to balance her. I'll causiously fly her and see how she does. I have a Giles and an Extra 300s for aerobatic fun flying. I only fly the P-51 for high speed passes and an occasional roll or loop so if she doesn't have a very good climb rate it will be OK. But if it becomes a brick I'll make some major weight saving changes.
BTW, I switched to the 4S for the scale-like sound, not so much to improve the power or performance.
Thanks again
~J
I'll take the advice and move the battery and rx back as far as reasonably possible to balance her. I'll causiously fly her and see how she does. I have a Giles and an Extra 300s for aerobatic fun flying. I only fly the P-51 for high speed passes and an occasional roll or loop so if she doesn't have a very good climb rate it will be OK. But if it becomes a brick I'll make some major weight saving changes.
BTW, I switched to the 4S for the scale-like sound, not so much to improve the power or performance.
Thanks again
~J
