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Old 09-21-2011, 03:26 AM   #1
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Default More downthrust needed?

I rebuilt the motor mount on my C-172 and I was careful to measure the drawing to get the thrust angles correct. However, before I rebuilt it there was a similar problem.

It climbs quite a lot above half throttle and even cruising at 1/3 requires quite a bit of down trim. When I cut the throttle or idle down to land the nose drops quite quickly. Once I setup my landing and level out for the last bit it tends to balloon a little and just keep on flying.

It is balanced according to the plans and I tried some extra nose weight and it didn't really help and it seemed want to cruise somewhat faster and have a higher stall speed even with flaps deployed.

So, I am pretty sure it needs more down thrust. My question is do the experts agree and if so how much down thrust would be a good to add as a test.

Secondarily, it wants to turn left on climb out and cruise and requires some right aileron trim to fly straight. It isn't a lot but I do notice it.

Thanks,

Dave
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Old 09-21-2011, 07:32 AM   #2
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Sounds like your CG could go back a bit. That, once you re-trim the elevator to achieve level cruise flight, would actually reduce the climb under power and also reduce the tendency for the nose to drop in the glide. You do need to be careful with moving the CG back and do it in very small steps. The CG position in the manual is usually quite conservative so it's not uncommon to be able to improve a plane's handling by carefully edging the CG back a little. Also when you do balance the model it should balance perfectly level when supported at the desired CG location. Any slight nose down tilt indicates that you are nose heavy.

You are likely to also need to add some right and downthrust. It's impossible to say how much as it varies from model to model. About 3 degrees of each is a reasonable starting point but only flight testing will indicate exactly how much is needed for your model. High wing planes do tend to need quite a lot of downthrust.

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Old 09-21-2011, 04:15 PM   #3
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Dave don't forget - we are "odd" in the RC world in that we don't want our planes to climb at higher power settings. In the full scale game that is then entire intent of more power (to climb, well and to go a bit faster ).

In fact I have a really funny story when I was taking full scale lessons about how to climb an airplane. I answered wrong (I said you use the elevator to climb) and the instructor taught me a valuable lesson when he shut the engine off and said "now climb"!

So scale models regularly do just that - climb with power and drop without it.

Down-thrust helps but will NEVER completely compensate for that on high lift models.

It is easy to battle that tendency with thrust lines and if you have a mount that allows it a good way to play with it is add washers to the "top" side of the motor so that it tilts down more. Make sense?

Mike
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Old 09-21-2011, 04:59 PM   #4
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Steve,

I think you are correct about the CG, I moved the battery aft just a little.

Mike,

I would agree, it is normal for there to be some climb at higher power levels but in the case of this plane it is pretty exaggerated. I also agree that the nose should drop a little when power is pulled back and I want that to happen because in the landing pattern I want to be descending and on approach I want a controlled but sometimes rather steep descent (small fields).

I added a couple of washers to the top two motor mounts so I will see today on my lunch break what happens.

I learned to fly the full scale 172 and I understand what you are saying about RC vs. full scale. I have never been one to worry about some climb at full throttle. I don't usually fly there for very long. If I was flying a CAP 21 or Extra it would be more appropriate to be concerned about climbing at full throttle but those have wings that are right on the thrust line which is certainly an advantage when it comes to trimming at various power levels.

I miss the days of full scale flying but I don't miss the cost!

Thanks,

Dave
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Old 09-21-2011, 05:21 PM   #5
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I do miss full scale but agree the cost was insane. I never finished because of that . What I want to do now is ultralight stuff.

Only problem wife thinks not .

O well....
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Old 09-21-2011, 05:46 PM   #6
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Another matter that can cause extreme pitch change as you describe is that elevator vs wing incidence is not correct. If wing is at greater angle of attack than design calls for ... you have to trim down with elevator to cure climb at speed. Close throttle and elevator down as speed decreases causes nose to pitch down.

I repaired my ME109 and ended up with exactly as you described - fuselage had an unintended banana bend in it ! .. cure ? I cut the fuselage so I could open up a gap ... filled with balsa and glue to set better incidence..... result ? good flight path over wider speed range.

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Old 09-21-2011, 09:01 PM   #7
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Took it out today with the CG just slightly back and a few washers under the top two engine mount points and it is really close. I think it may need some minor down thrust adjustment but now I will try a small amount of right thrust (I hat to change 3 things at once!).

I believe the incidence angles are fine but recently I built a micro size Art Chester Jeep and I had some issues with correct wing incidence and that problem is really tricky to correct.

Thanks for the opinions!

Dave
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Old 09-21-2011, 09:09 PM   #8
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I have the same problem with an old Great Planes J-3 Cub. I bought it one day and flew it the next. I did not do much research as to the correct motor size, I just picked one that put the prop about where I thought it should be in relation to the cabin and the picture of a J-3 in my mind.

What I did was put a motor on it that is at least twice as powerful as it should have been. At anything above 1/3 throttle it climbs, at WOT it will go vertical if you hold down elev.

When I was learning RC in the early 70's the Goldberg Falcon 56 was almost the standard trainer. The first generation kits were designed for 15 to 19 size glo motors (maybe as small as a good 09) and single channel radio.

We all put at least a SuperTigre 23 or more on them, the larger motors gave more speed of course, and it flew better. Some people installed a 40 and I saw one with a 60 ( 1 flight then he blew his wing apart )

But the bottom line thing we all had to learn was that, if you drive a lifting airfoil wing faster than the intended flying speed of the plane, we had to change the angle of the wing,

We would put shims under the trailing edge until we liked the way the plane handled, then we would cut the front of the wing down into the fuse to match the angle.

Later versions of the Falcon 56 had the wing about where we ended up with it.

I am going to start shimming my J-3 to reduce but not totally eliminate the climbing under power.

Maybe this would help your plane, but it can make a scale plane look a little wrong.

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Old 09-21-2011, 09:57 PM   #9
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Wildflyer,

Shimming the wing will work only if you do it in conjunction with moving the CG back, and there are limits to haw far back the CG will go before the model gets 'squirrelly'.

If you shim the wing and leave the CG as it is then all that will happen is that you have to add up trim to stop the model diving and basically you are just back where you started.

Steve
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Old 09-21-2011, 11:30 PM   #10
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Originally Posted by payne9999 View Post
I rebuilt the motor mount on my C-172 and I was careful to measure the drawing to get the thrust angles correct. However, before I rebuilt it there was a similar problem.

It climbs quite a lot above half throttle and even cruising at 1/3 requires quite a bit of down trim. When I cut the throttle or idle down to land the nose drops quite quickly. Once I setup my landing and level out for the last bit it tends to balloon a little and just keep on flying.

It is balanced according to the plans and I tried some extra nose weight and it didn't really help and it seemed want to cruise somewhat faster and have a higher stall speed even with flaps deployed.

So, I am pretty sure it needs more down thrust. My question is do the experts agree and if so how much down thrust would be a good to add as a test.

Secondarily, it wants to turn left on climb out and cruise and requires some right aileron trim to fly straight. It isn't a lot but I do notice it.

Thanks,

Dave
Hi Dave You have to Force your plane to fly Right If you ask 10 different RC flyers on how to set up a RC Plane to make it fly Stable, you will get 10 Different answers LOL, Here is how i set up my planes, it works for me, but not everyone, I like my Planes to fly like if on rails, from 1/2 to full throttle, I dont want my planes to lift, and have to add down elevator, I like my planes to fly like pattern ships, but not all plane designs will do that, that being said, here is what i do.
1. make sure that I have a 0-0 wing incidence.
2. give my planes 3 to 5 degrees of down thrust to prevent climbing when throttle is added.
3. CG a tad nose heavy, when power is cut, the nose will fall down to prevent a stall.
4. add a tad of weight to the right wing tip, to counter for motor torque.
Thats the way i set up my planes, and know what, everyone that flies my planes really enjoy the way that they fly, very responsive and solid, and the little extra wing weight does not hamper landing like some people might think. I have my 103 mph funjet set up with the extra wing weight too, and it flies like a dream.
5. this is the way I set up my planes after over 40 years of Rc flying, it works for me, it may not work for everyones style of flying, but it works great for me

I may be getting Older, But I Refuse to grow Up I am Having to much Fun to Grow Up LOL
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Old 09-22-2011, 01:43 AM   #11
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Originally Posted by CHELLIE View Post
Hi Dave You have to Force your plane to fly Right If you ask 10 different RC flyers on how to set up a RC Plane to make it fly Stable, you will get 10 Different answers LOL, Here is how i set up my planes, it works for me, but not everyone, I like my Planes to fly like if on rails, from 1/2 to full throttle, I dont want my planes to lift, and have to add down elevator, I like my planes to fly like pattern ships, but not all plane designs will do that, that being said, here is what i do.
1. make sure that I have a 0-0 wing incidence.
2. give my planes 3 to 5 degrees of down thrust to prevent climbing when throttle is added.
3. CG a tad nose heavy, when power is cut, the nose will fall down to prevent a stall.
4. add a tad of weight to the right wing tip, to counter for motor torque.
Thats the way i set up my planes, and know what, everyone that flies my planes really enjoy the way that they fly, very responsive and solid, and the little extra wing weight does not hamper landing like some people might think. I have my 103 mph funjet set up with the extra wing weight too, and it flies like a dream.
5. this is the way I set up my planes after over 40 years of Rc flying, it works for me, it may not work for everyones style of flying, but it works great for me
Chellie,

You don't mention right thrust. Does your item 4 take care of the side thrust?

Sorry for doing some hijacking but my curiousity could not help but ask.

Roger
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Old 09-22-2011, 01:55 AM   #12
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Originally Posted by RightFly View Post
Chellie,

You don't mention right thrust. Does your item 4 take care of the side thrust?

Sorry for doing some hijacking but my curiousity could not help but ask.

Roger
Hi Rodger the right wing weight takes care of the left motor thrust in most cases, it balances it out, I dont add in any right motor thrust on my planes, they dont need it,as long as the plane is built straight, you might need some right motor thrust if you have a warped/bent wing, but other than that, i never add right motor thrust.

I may be getting Older, But I Refuse to grow Up I am Having to much Fun to Grow Up LOL
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Old 09-22-2011, 04:01 AM   #13
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Originally Posted by CHELLIE View Post
Hi Rodger the right wing weight takes care of the left motor thrust in most cases, it balances it out, I dont add in any right motor thrust on my planes, they dont need it,as long as the plane is built straight, you might need some right motor thrust if you have a warped/bent wing, but other than that, i never add right motor thrust.
Question : Do you build or fly a model as designed ? What I mean is do you REMOVE any offset designed into the motor mount before you do your wing weight / other mods ?

Reason I ask is that 99.99999999% of models out of box whether kit or RTF / ARF will have right thrust already designed in.

Dare I say that for many years ... that right and down thrust has been standard even on many pattern ships. It is a beast of our using a torque and thrust propellor. Use EDF and most of that magically dissapears ! apart from making sure thrust is suitably aligned.

To OP ... if design thrust is having to be altered to get designed flying style - then something is defnitely wrong ...
If it's to gain flying style to personal taste ... then that's a different ball game.

Final comment ... any lift surface that is not fully symmetrical and at zero incidence will increase it's lift factor as speed increases ... Second even a fully symmet wing that needs to be at +ve incidence to fly will also increase lift factor as speed increases.
To set a non-symmet wing at zero incidence in fact is still setting a +ve virtual angle of attack by virtue of wing shape. To get a zero would need setting a non-symmet wing at -ve physical angle.
Theoretically - the only wing that will not climb or dive with change of speed within a flight capable envelope is a fully symmet wing at zero angle of attack. But then it wouldn't fly as there is no differential top and bottom of wing to create lift.

NOW watch the wolves come out and tear above apart !!

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Old 09-22-2011, 07:53 AM   #14
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Originally Posted by solentlife View Post
NOW watch the wolves come out and tear above apart !!
No tearing apart from me, I agree with every word! ANY wing that makes any lift at all will make more lift when it flies faster (if it's angle of attack remains the same). The only wing that wont produced increased lift at speed is a wing that was never making any lift in the first place, in which case it wont be flying. What downthrust tries to do is push the nose down and so cause the angle of attack to decrease when you add throttle, which cancels out the additional lift due to increased speed and prevents the model climbing so much.


Originally Posted by CHELLIE
add a tad of weight to the right wing tip, to counter for motor torque.
Adding wing weight will counter motor torque only at one throttle setting. When you close the throttle for landing and there is no motor torque then the wing weight must cause the plane to roll to the right; maybe it's only a little but if it must be happening. Also weight wont help at all with the yaw you get due to the swirling prop wash, which causes among other things, the plane to yaw left on it's take off run. Plus with wing weight when you fly inverted or do any aerobatics that have inverted flight in them (e.g. loops) then the weight ends up on the wrong tip and the plane will roll even worse than ever!
In the world of real planes all sorts of tricks have been tried to counter the effects of engine torque/prop was. Italian Macchi WWII fighters had the left wing longer than the right (similar idea to your wing weight). Several planes have the entire vertical stabiliser offset a few degrees to the right. Personally I find right thrust does a good job of countering motor asymmetry effects, it's not perfect but it works pretty well.

Steve
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Old 09-22-2011, 08:57 AM   #15
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I have to admit that I stayed of saying that wing weight is not a good idea. But having had another open that matter - I would say that unless done with extreme care - inbalanced wing weight is undesirable fopr the very reasons Jetflyer gives ... landing, inverted, change of speed etc.

The effects of prop torque on a plane are proportional to a) prop speed and b) pitch creating thrust. These have an equal and opposing force ... change any one of these and you have differing result. The wing weight cannot be altered. It would have to be a compromise similar to tail feathers offset, anti-yaw strips, side thrust etc.

The advantage of using side-thrust via engine is that shut engine down and model / plane is still balanced weight wise and should glide level. Giving a landing or glide without induced problems. Add weight to one tip and unbalance laterally the plane and whoops !! tip digging in on landing may be result.

Now the funny thing is that as speed increases the tendency of trhe plane to roll as a result of torque is less and less ... but yaw may not decrease as much. The yaw is not as some believe due to engine prop pulling to one side by virtue of rotation direction .... but by the air driven back along fuselage spiralling and hitting tail vertical on one side more than another. This then shows that prop speed has a direct influence and that side thrust is the better way to combat it.

Various aircraft over years have tried to cure tendencies and very few produced desired results. Bit like Tiger Moth having the anti-stall strips added to rear fuselage ... in fact they made little difference apart from in Pilot confidence in believing they did !

Personally ... I want my wings to be balanced so I don't have tips digging in / cartwheels or other effects from heavier wing side.

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Old 09-22-2011, 10:00 AM   #16
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Hey everyone thanks for all the knowledge and again I apologize for hijacking this thread. Perhaps I could push my luck a little more. I recently repowered a Flyzone Flylite with a 2712-12 motor from HeadsUpRc. It is over 20 mm longer and I am using a collet style prop adapter that extnds the overall length a little more. The firewall is placed in the plane with the down and side thrust preset so sliding the motor anywhere on the firewall surface maintains the thrust angles. I mounted the motor centered on the firewall as the oriinal Rimfire 250 had been mounted.

I did not like the results as the center of the prop hub was clearly off center about 3/16 of an inch. In my inexperienced opinion the center of the prop hub should be as close to the centerline of the fuse as possible. Is this correct?

Here is a picture that I have overlayed some lines to show what I mean. The yellow cross is approxiametly the prop hub center and the green line is the fuse C/L.

Roger


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Old 09-22-2011, 11:43 AM   #17
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All model kits that I have built over the years have the prop centre on the fuselage centreline when viewed vertically.. The motor obviously is then not mounted at centre but offset by the req'd amount to get prop boss at centre.
Note that when viewed horizontally - it is not necessary to centre the prop boss unless specific to wing incidence.

My Skymaster biplane has motor near 1/3" offset to one side to get req'd side thrust but prop boss centre of cowl. It's a 15cc petrol engine so a larger offset than you'd need .. but a good example.

When I swap a motor in a plane ... I mark centre as per cowl on the firewall ... then measure out to offset for req'd number of degrees ... that then becomes centre of mount. Shims then placed under mount to bring prop boss centre accordingly.

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Old 09-22-2011, 12:23 PM   #18
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Originally Posted by solentlife View Post
Question : Do you build or fly a model as designed ? What I mean is do you REMOVE any offset designed into the motor mount before you do your wing weight / other mods ?

Reason I ask is that 99.99999999% of models out of box whether kit or RTF / ARF will have right thrust already designed in.

Dare I say that for many years ... that right and down thrust has been standard even on many pattern ships. It is a beast of our using a torque and thrust propellor. Use EDF and most of that magically dissapears ! apart from making sure thrust is suitably aligned.

To OP ... if design thrust is having to be altered to get designed flying style - then something is defnitely wrong ...
If it's to gain flying style to personal taste ... then that's a different ball game.

Final comment ... any lift surface that is not fully symmetrical and at zero incidence will increase it's lift factor as speed increases ... Second even a fully symmet wing that needs to be at +ve incidence to fly will also increase lift factor as speed increases.
To set a non-symmet wing at zero incidence in fact is still setting a +ve virtual angle of attack by virtue of wing shape. To get a zero would need setting a non-symmet wing at -ve physical angle.
Theoretically - the only wing that will not climb or dive with change of speed within a flight capable envelope is a fully symmet wing at zero angle of attack. But then it wouldn't fly as there is no differential top and bottom of wing to create lift.

NOW watch the wolves come out and tear above apart !!
Half of my planes are arf and the other half are Scratch built,i set them up all the same, even if they have a flat wing with down thrust and right side wing weight, and no right thrust, if the arf kit came with right thrust, i take it out, reason being, is that i want them to track straight, removing the right motor thrust and adding right side wing weight allows my planes to do that, over 40+ years of flying has taught me some tricks through trial and error, people that have flown my planes, cant belive how well they fly, I dont use very much right side weight, a quarter is a lot of weight when its hanging at the end of a planes wing, thats what i use for larger planes and a 6 to 8 penny size finish nail on the smaller planes, half of my planes have flat wings and the other half has fully symmetrical wings, they get set up the same way, what your are doing is putting a load on the wing, the down thrust will pull the plane down a tad and a little up elevator trim puts the load on the wing, kind of like Isometrics, the down thrust, wing and the elevator are putting a little load on each other, that stabilizes the plane at just about all speeds, also with the wing weight, the torque and the wing weight put a load on each other, again like isometrics, and load the wing, and that is what makes my planes fly very stable and straight, you just have to try it and see, Like i said, this setup is not for everyone, I fly pattern and 3D and this works great for me. Take care and have fun, Chellie

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Old 09-22-2011, 01:21 PM   #19
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Originally Posted by RightFly View Post
...I did not like the results as the center of the prop hub was clearly off center about 3/16 of an inch.
Roger,
Adding to what Solentlife has already said: the prop should be central on the fuselage; this will mean that the motor mounting must be offset to the left (port) side.

You can work out exactly how much you need to move the motor over. the formula is:

offset = sin A x L

Where:
A = Thrust offset angle in degrees
L = Length between motor mount and prop hub

For example; if you had 3degrees right thust and the length between motor mount and prop was 80mm:

Offset = sin 3 x 80 = 4.2mm

Steve
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Old 09-22-2011, 03:00 PM   #20
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Yikes, Right thrust





Hi Roger Try this and see if it works for you, 0 - right thrust, 5 degrees of down thrust, tape a penny to the right wing tip, set the CG at 25% of the wing cord, from the leading edge, I bet it flies like a dream for you, Take care, Chellie


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Old 09-22-2011, 06:25 PM   #21
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Hmmm,

IMHO CG at 25% chord would be very nose heavy on this model. The spec calls for
CG (center of gravity): 2-1/8" (54mm) - 2-7/8" (73mm) from the leading edge of the wing
On a 7.5" chord wing (which I approximately calculated from area and span) that translates to 28 - 37%. So if it were me I'd be putting the CG in the middle of that range (say 33%) and working from there, rather than more nose heavy than the most nose heavy recommended location.. Each to his (or her) own though.

And as for right thrust.. It works for every world champion aerobatic flyer, pretty much every high end ARF 3D or aerobatic model comes with it built in and it has worked for me with every model I've had; but we are all free to set up our models whatever way we wish and whatever technique works for each of us is good.
I leaned my trimming on freeflight models which have no radio control to cover up for poor trim set up. In freeflight if you get the trim wrong the model crashes, simple as that. Right thrust works on a freeflight model where wing weighting does not work so well because with wing weighting the model will tend to spiral right when the motor stops... been there done that.

Steve
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Old 09-23-2011, 12:22 AM   #22
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Thank you Jetplaneflyer, Chellie and solentlife.

Your information was very informative and helpfull. The level of expertize on Watts continues to amaze me. I have moved the mounting over nearly 4 mm and the balanced the plane at 2 1/2 " from the leading edge of the wing. The center of the prop hub now lines up with the fuse centerline and it only required a Penny on the tail to level it on the balancer.

Now all I need is the wind to fall off - not in the forcast though.

Thanks so much for your willingness to share your knowledge.

Roger
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Old 09-23-2011, 04:25 AM   #23
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Originally Posted by RightFly View Post
Thank you Jetplaneflyer, Chellie and solentlife.

Your information was very informative and helpfull. The level of expertize on Watts continues to amaze me. I have moved the mounting over nearly 4 mm and the balanced the plane at 2 1/2 " from the leading edge of the wing. The center of the prop hub now lines up with the fuse centerline and it only required a Penny on the tail to level it on the balancer.

Now all I need is the wind to fall off - not in the forcast though.

Thanks so much for your willingness to share your knowledge.

Roger
As another says ... we all have our ways. Not all the same and that makes it more interesting.
We all learn every day as well .. if we knew all the answers - we wouldn't have forums to discuss would we !

What I like about many of the topics aired on forums, whether you agree or not with a post - they make you think more about your own beliefs / methods. I know it certainly does for me. I've been modelling since middle 1960's ... control line through water / land based stuff and air. With change of materials, new radio systems, electrics etc. - the model world is like computers ... evolving at a pace that is astounding.
We are now flying electric that would have had us screaming with joy back in the 70's 80's ... flying models that only scratch builders even contemplated.

I'm glad that whether it's my posting or another that helps another ... help is given and we all benefit ... Long may it live on !

Now get out there dude and FLY !!

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Old 09-23-2011, 06:27 AM   #24
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Originally Posted by Wildflyer View Post
What I did was put a motor on it that is at least twice as powerful as it should have been. At anything above 1/3 throttle it climbs, at WOT it will go vertical if you hold down elev.
Brings back memorys of one of my old 10 foot sailplanes that was electrified to get it up in the air. Power was a 18 cell RC2400 pack, Astroflight geared 40 brush motor and a 15 inch folding prop.

You had to just about hold full down elevator during power on to keep the model from looping. Did a lot of flying with it, had a number of two hour flights on it. Had two of them, finally sold them 10 years ago.

DennyV
Retired and the days are just too short, busier than ever!
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