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Question about Model Airplane Weight Limits

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Question about Model Airplane Weight Limits

Old 12-21-2013, 08:20 AM
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gregoryshock
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Default Question about Model Airplane Weight Limits

I have read that model airplanes tend to have more power per weight/size then a full size airplane. For an example I read that a 40 size trainer plane like eagle 2 was originally powered by a 20. If a person is using the smallest engine stated by the kit specs, how much over the weight limit can one go? My kit says it should weigh between 7 and 9.5 pounds.
Old 12-21-2013, 08:47 AM
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jester_s1
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That all depends on what model you are talking about and how you want it to fly. Yes, models are much more powered per weight than full scale planes, but it's not because modelers just want crazy amounts of horsepower. It's because airfoils become less efficient as they get smaller, therefore more power is needed to make the planes fly. So don't worry about that. If your plane is a warbird that is going to be heavily loaded anyway, you won't like it overweight and under powered. If it's a 4 Star or Ugly Stick, it will tolerate some extra weight. Just don't expect it to pull through big loops and such. If it's a trainer, extra weight won't hurt its flying characteristics, but underpowering it will make it more of a challenge to fly on windy days.

So what model are you building and what engines are you considering? And what are you planning to do that will add extra weight?
Old 12-21-2013, 09:07 AM
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gregoryshock
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Originally Posted by jester_s1 View Post
That all depends on what model you are talking about and how you want it to fly. Yes, models are much more powered per weight than full scale planes, but it's not because modelers just want crazy amounts of horsepower. It's because airfoils become less efficient as they get smaller, therefore more power is needed to make the planes fly. So don't worry about that. If your plane is a warbird that is going to be heavily loaded anyway, you won't like it overweight and under powered. If it's a 4 Star or Ugly Stick, it will tolerate some extra weight. Just don't expect it to pull through big loops and such. If it's a trainer, extra weight won't hurt its flying characteristics, but underpowering it will make it more of a challenge to fly on windy days.

So what model are you building and what engines are you considering? And what are you planning to do that will add extra weight?
The model is the F4U Corsair by Top Flite

http://www3.towerhobbies.com/cgi-bin...&I=LXHU80&P=ML

We are using the engine size suggested by the manual and they say it has plenty of power. However, on the towerhobbies website it says in the specs that it should weigh between 7 and 9.5 pounds. We are looking the model over and we know we did about one thing like use a heaver battery pack then perhaps they did, But other then that we have stayed as close to the manual as possible and can not figure out how even without the retracts they suggest that you could come anywhere close to 7 pounds. Currently our model is weighing about 10 pounds. We don't have it covered yet but we weighed everything and the covering that will go on or in the model.
Old 12-21-2013, 03:45 PM
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j.duncker
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OK at 10 lbs + you want an engine from the top end of the range, in fact I would consider an engine like the OS 95 AX. I used to be asked to test fly a lot of models and the ones that gave me the hardest time were the overweight, underpowered WW2 birds.

They would have to treated very gently on lift off and babied around until it got ' on the step '. Even then a pulling any G often resulted in a snap roll.

How do you build light models. It starts with wood selection. Apply glue sparingly especially epoxy. Weigh things like wheels and batteries and look for the lightest alternative. When finishing avoid brush painting at all costs, spray and only the minimum amount to cover.

Light models fly better. However you will learn a lot from flying something a little underpowered and a little overweight.
Old 12-21-2013, 04:24 PM
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Nothing worse than an under powered airplane. With that goes the old saying altitude and speed are life.
I take it you're interested in being more scale like.
IMHO go with the largest engine recommended. Now throttle is not an on off switch so experiment with different power settings.
Old 12-21-2013, 07:48 PM
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JohnBuckner
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[QUOTE=gregoryshock; If a person is using the smallest engine stated by the kit specs, how much over the weight limit can one go? My kit says it should weigh between 7 and 9.5 pounds.[/QUOTE]


Welcome to the world of manufacturers advertising Fluff. If you use the smallest manufacturers recommended engine displacement and are over the 9.5 with that type of airpane then it is going to be a dog. Remember the higher the wingloading is for that type of airplane the skill level to be successful with it just as mentioned by Jester is going to be much greater. That has a way of shortening the lifespan of many warbirds.

I have a Corsair that is not Top Flite but I just went out and checked the wingspan it is 63 inch virtually the same size. I have no idea what it is but I use it for arrested landings. Someone had originally put a Enya .91 four stroke in it and I actually did fly it with that one once and only once. it was a dog and from all the missed approachs it would be doing its life would be short so I dumped that engine and replaced it with an OS 1.08 two stroke and now the ship is a delight to fly, Just flew yesterday. I honestly cannot remember what it weighs but if you are interested I will weigh it tomorrow when I get back from the field.

John

Last edited by JohnBuckner; 12-21-2013 at 07:50 PM.
Old 12-22-2013, 01:20 PM
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gregoryshock
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Originally Posted by JohnBuckner View Post
Welcome to the world of manufacturers advertising Fluff. If you use the smallest manufacturers recommended engine displacement and are over the 9.5 with that type of airpane then it is going to be a dog. Remember the higher the wingloading is for that type of airplane the skill level to be successful with it just as mentioned by Jester is going to be much greater. That has a way of shortening the lifespan of many warbirds.

I have a Corsair that is not Top Flite but I just went out and checked the wingspan it is 63 inch virtually the same size. I have no idea what it is but I use it for arrested landings. Someone had originally put a Enya .91 four stroke in it and I actually did fly it with that one once and only once. it was a dog and from all the missed approachs it would be doing its life would be short so I dumped that engine and replaced it with an OS 1.08 two stroke and now the ship is a delight to fly, Just flew yesterday. I honestly cannot remember what it weighs but if you are interested I will weigh it tomorrow when I get back from the field.

John
I am not saying this as an argument against you or anyone else here. But let me add this thought, I already got this plane set up for the .61 FX. I suppose the whole engine mount will need to be changed (I'm not sure if that's possible anymore, one might need to redo the firewall) I may need to buy a new cowl. Anyways back to what this manufacture says about all of this. Starting on the towerhobbies page specs. It says

.60-.80 2-stroke
But the manual says .60 - .90 2-stroke
http://manuals.hobbico.com/top/topa0101-manual-v3_1.pdf
* As far as weight goes the manual says nothing so I had to go to the towerhobbies page to get that. You can read the towerhobbies specs here
http://www3.towerhobbies.com/cgi-bin...&I=LXHU80&P=ML

For the weight I decided to compare this kit to the New ARF version. I noticed they changed some of the specs. My plane has a Wingspan: 62" Wing Area: 700 sq in and the ARF is Wingspan: 62.5" Wing Area: 699 in² On the low end they both use the same engine. .60 on the kit and .61 on the ARF. What is really interesting is the weight difference. The kit starts at 7 pounds and the ARF starts at 8.5 pounds and they both end at 9.5lbs. And then on top of all of that, in the kit manual it says this.



* I emailed top flite and still waiting for their response to this. When or if I receive one I will post it.

John I am always interested in what other people did on their models.

Last edited by gregoryshock; 12-22-2013 at 01:25 PM.
Old 12-22-2013, 02:36 PM
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JohnBuckner
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OK just weighted my ship and the all up weight ready to go minus fuel of course 9.7 pounds it was a dog with the 91 fourstroke and it took the 1.08 two stroke to come alive.

Trouble is the fellows start layering on all the details and those original factory recommendations are only so much fiction and worse there is a point in every design where going bigger often heavier engines not only won,t help but makes things worse. And always as one goes up the wing loading scale the skill required is goes up at even a greater rate.

Sorry but you did ask.

John
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Old 12-22-2013, 03:40 PM
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invertmast
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It'll fly fine with the .61

i had a Top Flite 60 size P-47 with all the "fixins" (cockpit, flaps, concealed muffler, retracts, gear doors, dummy radial and three drop tanks). It flew fine. Takeoffs with no flaps were about 50-60 feet longer than ones with flaps, landing roll outs were a bit longer, but mostly because i would do wheel landings with no flaps and 3 point landings with flaps.

It was a pig at 11 pounds!

you corsair will be fine, just dont expect to fly it like a sport model. Your flying will have to be smooth and use energy management for large manuevers. Dont try and jerk it off the ground on takeoff either, let it get the tail up and to the speed you Think it should fly, then let it go a few more seconds Then smoothly lift it off the ground and do a nice gradual climbout (none of that 45* to vertical stuff).

Then on your first flight get up high and slow the model down to see what speed it stalls at.


90% of the people now a days over power stuff because they can and bc they dont know how to fly a heavy-ish model with modest power. When i got started in the hobby 25 years ago, you learned to fly the plane, not the engine.
Old 12-22-2013, 05:25 PM
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j.duncker
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Originally Posted by invertmast View Post
It'll fly fine with the .61

i had a Top Flite 60 size P-47 with all the "fixins" (cockpit, flaps, concealed muffler, retracts, gear doors, dummy radial and three drop tanks). It flew fine. Takeoffs with no flaps were about 50-60 feet longer than ones with flaps, landing roll outs were a bit longer, but mostly because i would do wheel landings with no flaps and 3 point landings with flaps.

It was a pig at 11 pounds!

you corsair will be fine, just dont expect to fly it like a sport model. Your flying will have to be smooth and use energy management for large manuevers. Dont try and jerk it off the ground on takeoff either, let it get the tail up and to the speed you Think it should fly, then let it go a few more seconds Then smoothly lift it off the ground and do a nice gradual climbout (none of that 45* to vertical stuff).

Then on your first flight get up high and slow the model down to see what speed it stalls at.


90% of the people now a days over power stuff because they can and bc they dont know how to fly a heavy-ish model with modest power. When i got started in the hobby 25 years ago, you learned to fly the plane, not the engine.
I am going to disagree with your first sentence It will not fly fine, it may fly if flown with care from a tarmac strip but the climb out will need to be flown with care and there will be no 'get out of jail' reserve power. On a grass strip take off will be dubious affair.

The good news for the OP is that the OS 95AX fits the same bolt holes as the 61FX I guess the cowl may need some extra attention to allow for the slightly larger engine.
Old 12-22-2013, 06:01 PM
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Originally Posted by j.duncker View Post
I am going to disagree with your first sentence It will not fly fine, it may fly if flown with care from a tarmac strip but the climb out will need to be flown with care and there will be no 'get out of jail' reserve power. On a grass strip take off will be dubious affair.

The good news for the OP is that the OS 95AX fits the same bolt holes as the 61FX I guess the cowl may need some extra attention to allow for the slightly larger engine.
that is fine, but like i said, im speaking from experience with a similar model that weighted more. Its not really flying with care, its "learning the model", which is the entire purpose of maiden flights. You need to learn the slow speed characteristics of a model.

You dont need a ton of power to fly the thing. I have flown a 10' wingspan all flying wing that weighted 36 pounds with a CG 2" to far forward on Only 12 pounds of thrust from two EDF units. While its performance wasnt spectacular, of flew and flew well. When we upped the power to 24 pounds of thrust, it didnt fly any better, it just flew slightly longer each flight.

I still stand by my ground that it will fly fine with the 61 as long as the OP learns the model. Granted more power is always better, sometimes it isnt possible.
Old 12-22-2013, 06:50 PM
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Flying fine is a very subjective opinion, it all depends on what your accustom to flying and or what your flying skills really are. As others have already stated, your best all around remedy for building airplanes that fly forgiving, and with authority on smaller power plants is to learn to build and finish just as light as you possibly can. JMO though.

Bob
Old 12-22-2013, 08:08 PM
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OK let me see if I understand, Forget what the manufacturer says for just one second. So far the airplane is to be powered .61FX so far so good great engine. But the airplane currently weighs 10 pounds and it is not even covered yet much less all the little tid bits that we all miss when doing those kinds of projections when we are at the same stage?

John
Old 12-22-2013, 11:08 PM
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I once test flew an electric Hots that a buddy had built. It turned out very nicely done, but had so little power that it would not take-off from an asphalt runway. (This was in the golden era of electric flight, with a ton of NiCad's, which when coupled with a direct drive motor and high wing loading was destined to be a very nice three wheel car.) So we did what any modeler would do, we hand launched it into flight. Fortunately, the launcher was tall at about 6-2, because it never got any higher. I flew one circuit around the field to land it successfully, all the while on the edge of a stall. We three (all engineers) decided that more power was unavailable, so the next best thing would be more wing area. About a month later we returned to the field with his new "U-2 Hots" with about 50% more wing area. Flew great! So high wing loading and low power is not a good combination.

Other possible problems - a .61 runs pretty well with a 11-7 or 11-8 prop, also pretty good with a 12-6, but you might like the looks of a 13-6 or a somewhat bigger prop because of the radial cowl. Well as you pile on more load, the rpm goes down, and so does the power output. Maybe you are slightly nervous about getting off the ground, so you tweek the needle valve just one more click. Bigger engine will turn the bigger diameter, and you can ease off the needle because you can blow more air past the cowl. As was stated above, you want to get the tail in the air and gain flying speed while still on the runway. Most clubs have at least one pilot that is up the task, though figuring out how that might be can take time. If nothing else, just add weight to your friendly flying sport model and see how it does with a 33-35 oz wing loading.
Old 12-23-2013, 03:46 AM
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Extra HP in some cases gets it done but only masks the higher wing loading problem. HP is not an antidote for a properly loaded wing and never will be, any airplane that snaps over like a trip hammer when you pull a little to hard back due to this problem is in many cases destin to a short lifespan especially for those with finicky engines... Just my own opinion.

Bob
Old 12-23-2013, 04:32 AM
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I had the same Corsair with an OS91 in it. The plane performed great. Lots of power, lots of torque, and great authority in the air. I had a 15x8 prop on it. Great climb performance and aerobatic abilities. Got off the ground in 70 feet or less. I think that was a perfect engine and prop combo for the plane. It was never sluggish in any maneuver.

I also have a Cermark F-18 (prop-jet) that had an OS91 in it too. While the 91 gave the F-18 lots of performance, it also caused too much vibration. So I switched the engine to an OS .61 FX. Top-end speed is still the same as the .91 provided, but acceleration is slower, and climb performance is not nearly the same. But I got rid of the vibrations the .91 was giving me. The Corsair does not have a vibration problem with a .91 because of the large firewall. However, the F-18 did not have a wide or large firewall, so the F-18 would vibrate with a .91 engine.

All in all, the Corsair will be marginal with a .61 engine. The performance will suffer, acceleration will be slower, and the Corsair will "feel" heavier. Also Remember this ... The Corsair has lots of drag created by the very large cowl. So a small engine will not be able to overcome that issue. You will notice much better performance with a .91 vs. a .61 in the Corsair. Your Corsair will also have a better chance of survival with a .91 engine.

Don't always listen to the manufacturer's engine recommendations. Sometimes, they are wrong. Using the smaller end of the engine range is usually NEVER a good idea. A .61 in this Corsair will be sluggish, and you won't enjoy flying it. The .91 will make all the difference in the world and you will really enjoy this plane.

PS
No way this plane will ever weigh only 7 pounds when finished. Not even without fuel. I build light, but strong. My Corsair was 9.5 lb. WITHOUT fuel, had a 14 oz fuel tank, did not have retracts or flaps, was made with real aircraft grade plywood bulkheads, was covered in Monokote, and had Dubro Low-bounce rubber wheels (the best wheels). The alien pilot I used was heavier than normal (5 oz). And granted, a .91 engine is about 14 oz heavier than a .61 engine. But use a .91 engine in this plane. The size of this plane justifies the engine. It will still land a slow speeds, and be stable.

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Last edited by Airplanes400; 12-23-2013 at 05:23 AM.
Old 12-23-2013, 08:42 AM
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flycatch
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A lot of suggesting going on. The OP fails to tell us exactly what this model is and I'm assuming it is a high wing trainer. So, OP tell us what you got.
Old 12-23-2013, 08:48 AM
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AMA 74894
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Originally Posted by flycatch View Post
A lot of suggesting going on. The OP fails to tell us exactly what this model is and I'm assuming it is a high wing trainer. So, OP tell us what you got.
ummm... if you read posts #3 and 7 you'll see the model is in fact:

"The model is the F4U Corsair by Top Flite"
complete with the link to the manual at tower.
Old 12-23-2013, 11:10 AM
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gregoryshock
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Originally Posted by Airplanes400 View Post
I had the same Corsair with an OS91 in it. The plane performed great. Lots of power, lots of torque, and great authority in the air. I had a 15x8 prop on it. Great climb performance and aerobatic abilities. Got off the ground in 70 feet or less. I think that was a perfect engine and prop combo for the plane. It was never sluggish in any maneuver.

I also have a Cermark F-18 (prop-jet) that had an OS91 in it too. While the 91 gave the F-18 lots of performance, it also caused too much vibration. So I switched the engine to an OS .61 FX. Top-end speed is still the same as the .91 provided, but acceleration is slower, and climb performance is not nearly the same. But I got rid of the vibrations the .91 was giving me. The Corsair does not have a vibration problem with a .91 because of the large firewall. However, the F-18 did not have a wide or large firewall, so the F-18 would vibrate with a .91 engine.

All in all, the Corsair will be marginal with a .61 engine. The performance will suffer, acceleration will be slower, and the Corsair will "feel" heavier. Also Remember this ... The Corsair has lots of drag created by the very large cowl. So a small engine will not be able to overcome that issue. You will notice much better performance with a .91 vs. a .61 in the Corsair. Your Corsair will also have a better chance of survival with a .91 engine.

Don't always listen to the manufacturer's engine recommendations. Sometimes, they are wrong. Using the smaller end of the engine range is usually NEVER a good idea. A .61 in this Corsair will be sluggish, and you won't enjoy flying it. The .91 will make all the difference in the world and you will really enjoy this plane.

PS
No way this plane will ever weigh only 7 pounds when finished. Not even without fuel. I build light, but strong. My Corsair was 9.5 lb. WITHOUT fuel, had a 14 oz fuel tank, did not have retracts or flaps, was made with real aircraft grade plywood bulkheads, was covered in Monokote, and had Dubro Low-bounce rubber wheels (the best wheels). The alien pilot I used was heavier than normal (5 oz). And granted, a .91 engine is about 14 oz heavier than a .61 engine. But use a .91 engine in this plane. The size of this plane justifies the engine. It will still land a slow speeds, and be stable.

Thank you for your comments and information. I really appreciate the fact you have same model I have. I've been looking at OS engines and I've noticed that they no longer make a 90. They do however make a 95, but with a 95 I will not be able to use the muffler that I bought for the 61. It does appear that I can use it on a .75. It seems that the new OS 75 is the replacement for the 61. It sounds like I could use it without changing my engine mounting and muffler set up. Do you think that would be enough power increase? I also should mention that when I weighed my model, I did try to include the finishings like monokote.

OS AX Engine Series
http://www.towerhobbies.com/products/os_engines/

.75
http://www.towerhobbies.com/products.../osmg0575.html

.95
http://www.towerhobbies.com/products.../osmg0580.html

Top Flite In Cowl Muffler .61-.75 P-51D Mustang
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHY74

Top Flite Exhaust Header O.S. 61SF/61FX
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHY77

Specs for Top Flite Exhaust Header O.S. 61SF/61FX


COMMENTS: This also works for the Magnum .65 engine and the O.S. .60FP engines. DO NOT attempt to use this header/muffler combination js12/8/93 on the 91FX. ir/jl This header will also work with the .75AX (OSMG0575) 9/9/8 GX This will also work with the .65AX ABL (OSMG0558).

Last edited by gregoryshock; 12-23-2013 at 11:22 AM.
Old 12-23-2013, 01:17 PM
  #20  
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I would definitely consider the .75 if you don't believe the .61 will do it. Should be similar in weight

Remember, bigger engines while they do give more power, also increase the weight and wing loading of a plane. Bigger doesn't always equal better and you have to be careful about this. Compare engine weights and try and think about balance of the plane...
Old 12-23-2013, 01:45 PM
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I have always went with the largest engine recommendation for every plane that I have ever built. This is planned before I even order the plane.
Extra horsepower can get you out of trouble and not enough horsepower can get you into it.
I don't think I have ever met an experienced modeler that put together an airplane and went with the smallest engine recommendation.
So this begs the question: Is this your first airplane? Please say no and it is at least your 3rd or 4th.
Old 12-23-2013, 03:49 PM
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Originally Posted by thepamster View Post
I have always went with the largest engine recommendation for every plane that I have ever built. This is planned before I even order the plane.
Extra horsepower can get you out of trouble and not enough horsepower can get you into it.
I don't think I have ever met an experienced modeler that put together an airplane and went with the smallest engine recommendation.
So this begs the question: Is this your first airplane? Please say no and it is at least your 3rd or 4th.
Built the top flight at-6 and put a os 91 four cycle in it,the motor saved my butt many times when it wanted to drop a wing on take off or landing .+1 with thepamster. joe
Old 12-23-2013, 04:42 PM
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Warbird= go large on power... That said, many years ago the discussion of engine size/power vs plane size was highly depated at the field and i recall my dad taking the 60 off of his large telemaster and replacing it with an old enya 10 on about three feet of broomstick to get it out far enough to balance it without added weight. He flew it several times to show the more power junkies what was what....
Old 12-23-2013, 05:02 PM
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Airplanes400
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Greg,
Sorry, but this is not what you want to hear ...
I suggest you go with the .95 engine mostly because of the power it has to swing the large prop that the Corsair needs. Since the Corsair has a large cowl (8.25" diameter) , a large portion of air is blocked by the cowl. So, you need a big prop (15x8) and an engine that can swing it while still maintaining power. The 75 engine will not swing the prop with the torque of a 95 engine when power is needed. The Corsair has lots of wing area, so a slightly heavier engine will not affect wing loading. Besides, as far as warbirds ar concerned, it seemed to me that the Corsair has one of the lightest wing loads.

If you can find a new and unused OS .91, then get it.

My Corsair tended to float a little on landing. It was also easy to land and takeoff, and did not have any ground looping problems ... even in cross-winds. It was the easiest tail dragger I ever had. With the OS.91 in it, I did not have to add any lead weight to the nose (something you will have to do with a lighter engine). All I had to do was position a 4.8v battery in the fuselage, near the CG. So, not only will the .91 (or .95) give you more power, it also eliminates the need for dead weight (lead) that a smaller engine will still have to cope with.

I used a Bisson muffler, this one.
http://www3.towerhobbies.com/cgi-bin...&I=LXNC71&P=ML

Don't use that Top Flite in-cowl muffler. The engine will overheat because the chamber is too small.

Last edited by Airplanes400; 12-23-2013 at 05:30 PM.
Old 12-23-2013, 07:03 PM
  #25  
I_Fly
 
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I fly mine with a 61SF and it flies well. I do fly from a paved runway surface but believe it would have not problem in a manicured grass strip. It was never weighed but I believe that it's in the middle of the range (8.5 or so). The airfoil on this model is fudged to provide more lift and better slow speed characteristics. I don't believe that some additional weight would cause the model to become a dog. In fact I think it would fly fine.

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