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Brian R

Dorado C of G and flight speed

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Brian R

I was recently flying the Dorado in good lift and with Wing Ballast. The wind was about 25-30mph and the hill at Edenfield provides good lift. Other planes were able to fly at speed but I was not able to get the plane to fly at speeds which I think it should. Loops, rolls etc could be completed with relative ease but the plane did not seem to have good retention of energy and seemed to cruise about. To try to get the plane flying faster I fed in down trim but this did not seem to make much difference. When flying inverted I noticed that the plane actually climbed and this led me to take it home and check everything. 
1. The C of G recommended is 96 but mine measured 94.5 with the ballast in place. Therefore I was flying it nose heavy. 
2. The C of G without ballast is 93 tieing in with the fact that when the ballast is added  the C of G moves back because the ballast tube is at recommended C of G of 96. 
3. The incidence measured of the tailplane, as I was flying on the slope, measured at home was zero relative to the wing. 
4. The plane climbed when inverted with no elevator stick input. 
 

Could somebody please explain the Physics behind this problem. My thinking is that presumably I was flying with down trim and continually correcting its normal flight such that when I inverted the plane it actually climbed,  but why if the incidence is zero of the tailplane to the fully symmetrical wing did it climb. 
 

How can I get the plane to fly faster, it has an all moving tail and fully symmetrical wing so the only drag is from the pilot making correction on the control surfaces and the drag of the plane.
Should I move the C of G back to 96 and/or add more ballast, on days with conditions such as the one I was flying. I have had the plane moving very quickly in strong winds and high lift on Parlick, with the same wing ballast but the days when it flies like that seem to be few and far between. 
 

Any comments appreciated. 
 

Kind Regards, Brian

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satinet

Fully symmetrical wings are worse than normal wings in general flight. Not saying it is that but you won't get the equivalent performance of a normal wing. It won't be as fast as the equivalent size racer.  Also the fuse is larger and the wing fairly low aspect ratio. 

Could be that you were resting your thumb on the stick. If you fly with down trim inverted will be like you are describing. Your cg measuring device could also be wrong and you could be flying reward but generally i think cg doesn't make too much difference to straight line speed unless it miles to far one way or the other. 

It Might also be that it would actually fly better without ballast. Too much weight can make planes fly like a pudding. E.g stalling out the top of loops etc.  Planes turn tighter with less weight in. 

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oipigface
1 hour ago, Brian R said:

Therefore I was flying it nose heavy. 

This is a non sequitur. Recommended Initial CG positions are not necessarily the ‘best’ to make the plane behave in any particular way. They tend, in fact, to err on the side of stability. You need to test the CG position in flight. There are two ways I know of to do this. The first is called the ‘dive test’, the second the ‘inverted test’.


For the dive test get some height on, and then put the plane into a 45degree dive and observe the plane’s behaviour with the elevator at neutral. There are three things that can happen: i) Nose pulls up. This means the CG is too far forward. Take some noseweight out. Ii) Dive steepens. The CG is too far back. Add some noseweight. Iii) Dive continues at 45degrees. Do the ‘inverted test’

The inverted test requires that you get some speed on, enter level flight and do half a roll. If the CG is correct, the plane should continue in level flight. If it dives, the CG needs to be moved forward. If it climbs, the CG needs to be moved back. Only make very small adjustments. I carry some of those 5g sticky backed wheel balance weights with me, so I can stick them on the outside of the fuselage, and move them about until I get things the way like. Then the nose weight can be adjusted permanently.

All of these tests assume that the control surfaces are at neutral when your sticks are at neutral. I think your guess as to the reason for the tendency to climb when inverted is probably right. You had put some down trim in to get it to fly fast. Down trim in inverted flight is uptrim.

What you are trying to do in all these cases is figure out whether the relationship between the wing’s centre of the lift, and the plane’s CG is optimised. Here’s a diagram from Martin Simons’ Model Aircraft Aerodynamics, showing how for stable and controllable flight the centre of lift (which Simons calls the ‘neutral point’) needs to be behind the CG. You can use this to try and understand both tests.  For instance: Going along in level flight, if a quick inversion of the plane causes the nose to pitch down it must be because the centre of lift has moved back, (assuming nothing has fallen off the plane!), so move it forward a bit to compensate. 

661D8AB9-0E54-4087-9B49-BEA13ED8257E.thumb.jpeg.dff7b2bb4f545b29a3ef5a4b41fd1853.jpeg

 

 

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mikef

oipigface,

What type of glider would you set up to continue in a 45 Deg. dive, hands off, in your dive test?  I'm assuming it's not a duration type model.

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Brian R

No its a aerobatic glider 2.36m just over 3 kg with wing ballast. 

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Phil.Taylor

Its an aerobat with a symmetrical section - it will never be as fast as an F3F plane. Enjoy it for what it is - having fun with aerobatics.

CofG - check it next flight with the dive test. 

Phil.

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satinet

If you can do loops and rolls etc well i don't think you have a problem at all. My aeromod voltij was like that. Quite slow but good energy retention.

Thick and symetrical wings are much shower than thin normal wings mated to thin fuselages.  

As i say cg doesn't make that much difference to straight line speed. If you are a few mm in front of optimal it isn't really slowing you down in a straight line.

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Brian R
14 hours ago, oipigface said:

This is a non sequitur. Recommended Initial CG positions are not necessarily the ‘best’ to make the plane behave in any particular way. They tend, in fact, to err on the side of stability. You need to test the CG position in flight. There are two ways I know of to do this. The first is called the ‘dive test’, the second the ‘inverted test’.


For the dive test get some height on, and then put the plane into a 45degree dive and observe the plane’s behaviour with the elevator at neutral. There are three things that can happen: i) Nose pulls up. This means the CG is too far forward. Take some noseweight out. Ii) Dive steepens. The CG is too far back. Add some noseweight. Iii) Dive continues at 45degrees. Do the ‘inverted test’

The inverted test requires that you get some speed on, enter level flight and do half a roll. If the CG is correct, the plane should continue in level flight. If it dives, the CG needs to be moved forward. If it climbs, the CG needs to be moved back. Only make very small adjustments. I carry some of those 5g sticky backed wheel balance weights with me, so I can stick them on the outside of the fuselage, and move them about until I get things the way like. Then the nose weight can be adjusted permanently.

All of these tests assume that the control surfaces are at neutral when your sticks are at neutral. I think your guess as to the reason for the tendency to climb when inverted is probably right. You had put some down trim in to get it to fly fast. Down trim in inverted flight is uptrim.

What you are trying to do in all these cases is figure out whether the relationship between the wing’s centre of the lift, and the plane’s CG is optimised. Here’s a diagram from Martin Simons’ Model Aircraft Aerodynamics, showing how for stable and controllable flight the centre of lift (which Simons calls the ‘neutral point’) needs to be behind the CG. You can use this to try and understand both tests.  For instance: Going along in level flight, if a quick inversion of the plane causes the nose to pitch down it must be because the centre of lift has moved back, (assuming nothing has fallen off the plane!), so move it forward a bit to compensate. 

661D8AB9-0E54-4087-9B49-BEA13ED8257E.thumb.jpeg.dff7b2bb4f545b29a3ef5a4b41fd1853.jpeg

 

 

Thanks for reply, we normally do the dive test and note how it performs at the top of a loop with hands off but I will now try the inverted speed test. 
I however do not follow the last sentence re “the centre of lift has moved back so move it forward a bit to compensate” how do you do that?
 

Cheers Brian

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isoaritfirst

Are you familiar with flying models fast on that hill in that wind speed. ?
 

Some hills get less lifty  in high winds and take significant experience to fly fast in such conditions. 
 

It is  sometimes easy to see with an F3f model, where launching and allowing the model to simply fly, even pushing the nose down won’t generate lots of speed. But an experienced pilot will use the momentum and the wind gradients to make the model fly very fast. 
lots of inputs and large surface deflections like you may expect from an aerobatic model will slow it significantly , being gentle and progressive on the sticks will maintain momentum and result in faster flight and faster rolls etc . 

From a model prospective, check the ailerons are correctly aligned.  Ballast is useful but not as simple as saying add more. Better to add  ballast to change flight character rather than  speed.  A light model will accelerate faster off lift. A heavy model will hold onto its momentum. But a heavy model can be harder to get that speed in the first place 
Find the thermal ,/ find the powerful bit of the hill to gain speed , fly through the bad bits 

 

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oipigface
1 hour ago, Brian R said:

we normally do the dive test and note how it performs at the top of a loop with hands off but I will now try the inverted speed test. 

Well that’s not too different from doing half a roll. I suspect that most planes will lose less energy in half a roll than half a loop.

1 hour ago, Brian R said:

I however do not follow the last sentence re “the centre of lift has moved back so move it forward a bit to compensate” how do you do that?

End of quite a long essay, and poorly proof-read. Sorry. It’s the CG needs moving, and it needs moving back! (I think.)

13 hours ago, mikef said:

oipigface,

What type of glider would you set up to continue in a 45 Deg. dive, hands off, in your dive test?  I'm assuming it's not a duration type model.

I don’t suggest that this is something you would often do when out flying. What the dive test does is give information about the relative positions of CG and centre of lift.
Neither, of course, do I suggest that you never pull out.
It’s a great virtue in an aerobatic model that it goes (and continues to go) where it is pointed until the pilot chooses to point it somewhere else.
Duration models need to be set up efficiently as well. The first time I used the dive test on a glider was almost certainly on the first one I built with RC, which was a Phoenix in the early 80’s, I think.


Despite all that, when the wind is not good for racing, it’s a common recreation for F3F pilots at Alpine sites to use thermals until their plane is specked out, then put it into a long dive to eye level followed by a high-speed aerobatic sequence. People do this in this country too, but it’s not so common because thermals are generally not so reliable. It’s another way of enjoying yourself!

 

11 hours ago, satinet said:

As i say cg doesn't make that much difference to straight line speed. If you are a few mm in front of optimal it isn't really slowing you down in a straight line.

But the thing about aerobatics is that it’s not flying in a straight line. You want the plane to be fairly close to as efficient as it can be given its design and construction.

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Brian R

Thanks everybody for your input, I will experiment more with both C of G and ballast to find the best the model can do and more importantly that I can do in an effort to improve and enjoy. 
 

Brian

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oipigface

Have fun, Brian!

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