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Old Jan 17, 2013, 09:38 PM
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Australia, ACT, Kambah
Joined Feb 2001
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The cambered aerofoil has a nose down pitching moment, generally countered by down force from the tail. accelerate without trimming and the downforce from the tail increases with the higher airspeed. if as is usual the tail moment is stronger than the wing pitching moment, the acft pitches up. The very existence of the positively cambered airfoil ND moment is a key factor in the normal pitch up at untrimmed higher speed.

And positively cambered airfoils still develop some lift at small -ve aoa
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Old Jan 18, 2013, 05:29 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,765 Posts
Quote:
Originally Posted by scirocco View Post
The cambered aerofoil has a nose down pitching moment, generally countered by down force from the tail. accelerate without trimming and the downforce from the tail increases with the higher airspeed. if as is usual the tail moment is stronger than the wing pitching moment, the acft pitches up. The very existence of the positively cambered airfoil ND moment is a key factor in the normal pitch up at untrimmed higher speed.

And positively cambered airfoils still develop some lift at small -ve aoa
Perhaps that's why we build model planes to fly, not just build a wing and wonder why it pitches down and just rotates to the ground.

Not wishing to be too stupid, but it starts to annoy me when the 'politically correct' aerodynamic answers to simple questions and replies seem to crop up in various threads, (not saying they have in this thread, naturally ).

There are many modelers who basically do not understand the hows and whys of model flight, (and probably don't want or need to), but are still willing to read posts to get some idea of why somethings happen.
One of the reasons why I try to reply in simple terms, is because I feel that a simple answer is an opening for someone to dig deeper is they want to. (Ok, plus the technical side of aerodynamics puts me off spouting formula, and I admit I don't know it ).
I tend more to rely on experience and logic to try and answer questions in relatively simple easy to understand terms, (I hope). .

If anyone reading the various posts in any threads wants to learn a lot more of the technical side, I suggest looking and asking in the - Modeling Science forum - which generally has all the aerodynamic experts.
(But occasionally some seem to escape ).
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Old Jan 18, 2013, 05:53 AM
222 km/hr Parkjet flyer
solentlife's Avatar
Latvia, Ventspils pilsēta, Ventspils
Joined Jan 2010
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eflight ... I also look to answer in simple terms, leaving the science to others.

If a model pitches up or down .. often the other only needs to know how to cure or trim ... not a physics lesson in aerodynamics that soon after reading is put aside.

I also get a little bit annoyed when people quote Wikipedia as written in stone absolute fact. many people do not realise that Wikipedia is READER EDITABLE .. yes if you don't agree or want to add to the text - you can !

I have various books at home ... covering technical aspects of MODEL aerodynamics, scale and Helicopters ... but I don't quote out of them unless pressed ... mainly because I regard it as going too far and unnecessary.

i also chuckle when i see comments 'as a real pilot' ... you can fly a real plane and know jack-s*** about how it works ..... same as my wife can drive a car but knows nought about how it works.

Lighten up people ... it's a hobby !!

Nigel
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Old Jan 18, 2013, 08:45 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
12,123 Posts
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Originally Posted by solentlife View Post
As to flat-bottom wing pitching down ... tell that to millions of RC'rs flying average trainers ... increase throttle and UP she goes ... Only time I see such going DOWN is when downthrust or other physical factor is seriously in error.

Nigel
Nigel, it's the tail that makes it pitch up.. The wing 'on its own' would pitch down.

Try making a 'plank' flying wing with a flat bottom airfoil and find out for yourself
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Last edited by JetPlaneFlyer; Jan 18, 2013 at 10:18 AM.
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Old Jan 18, 2013, 09:33 AM
Registered User
Wasaga Beach, Ontario
Joined Aug 2007
1,198 Posts
Quote:
Originally Posted by eflightray View Post
Could you perhaps define what 'aerodynamic couples', means for those fairly new to model aircraft.

I know this isn't the Beginners forum, but many of the questions asked are by beginners looking for simple answers.

Thanks.
I was trying to find a pic to illustrate it but couldn't, so I'll try and explain it:

Pretend you draw a flat horizontal line front to back through an airplane's CG. In general, this line will pass through neither the engine nor the horizontal stab. The engine will be below it and the horizontal stab will be above it. This creates a torque moment when you increase or decrease throttle.

In ASCII is looks something like this:

Line through CG:

>-----------CG------------>

Couples:

>-----------|
. . . . . . . . |CG|
. . . . . . . . . . . | ------------->

These couples cause the initial tendency of a plane to pitch with throttle changes. Since RC planes have a massive P:W ratio they won't be as pronounced as FS (and likely don't exist on certain types of RC planes). The overall tendency of a plane to pitch will be effected by, as you said, longitudinal stability, trim, and a few other factors.
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Old Jan 18, 2013, 01:23 PM
aka: A.Roger Wilfong
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Novi, Michigan, United States
Joined Jan 2001
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Originally Posted by rcmaverick View Post
Hmm no..

I can leave the elevator alone at about 50% throttle - the plane flies level - If I now want to speed it up to 80% - 90% it starts going vertical.

Perhaps I need to trim the elevator for a higher throttle position.
Me thinks you have thrust line, wing/stab mis-alignment or CG problems. If it nose up to the point where it's going anywhere near vertical, it's either got way too little down thrust, or the CG is way to far forward.

If the CG is too far forward, you have to have a lot of up trim to lift the extra weight - actually puch the tail down. When you speed up, you get more negative lift on the tail causign the nose to pitch up. You can determine if the CG is correct by doing the "dive test":

Dive Test:
- Trim the plane to fly straight and level, hands off, at 50% throttle (sounds like you already have it there).
- Fly past your position at a couple hundred feet altitude and push the nose down into about a 30 degree dive.
- Let the plane dive for about 50 feet and then center the stick (hands-off trim position).
- If the plane keeps diving at the same angle or pulls out of the dive and returns to level flight, the CG is spot on.
- If the plane steepens its dive, it's tail heavy.
- If it pulls up quickly it's nose heavy.
- It it pulls a loop or goes vertical, it's really nose heavy.

You're on your own with the other two posibilities.

- Roger
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Old Jan 19, 2013, 10:24 AM
Cudda-Daddy
hexonxonx's Avatar
United States, MO, Gerald
Joined Jun 2004
93 Posts
i have read about other folks who put a washer or two between the motor and firewall/motor mount on the top two screws . it moves the thrust line down a few degrees for level flight at wide open throttle so you aint scrubbin speed with trim.
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Old Jan 19, 2013, 10:43 AM
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Lnagel's Avatar
Moab, Utah, USA
Joined Apr 2003
6,280 Posts
Quote:
Originally Posted by scirocco View Post
The cambered aerofoil has a nose down pitching moment, generally countered by down force from the tail. accelerate without trimming and the downforce from the tail increases with the higher airspeed. if as is usual the tail moment is stronger than the wing pitching moment, the acft pitches up. The very existence of the positively cambered airfoil ND moment is a key factor in the normal pitch up at untrimmed higher speed.

And positively cambered airfoils still develop some lift at small -ve aoa
Are you saying then that an airplane that has a wing with a symetrical airfoil would not pitch up with increased throttle?

Larry
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Old Jan 19, 2013, 10:53 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,765 Posts
Arrrghhh!

Sometimes I just feel like giving up and looking for a knitting forum.
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Old Jan 19, 2013, 11:49 AM
Cudda-Daddy
hexonxonx's Avatar
United States, MO, Gerald
Joined Jun 2004
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alot of people knit in the nervous hospital.
http://3.bp.blogspot.com/-OajDBTgzUb...s1600/karl.jpg
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Old Jan 19, 2013, 12:53 PM
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Moab, Utah, USA
Joined Apr 2003
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Quote:
Originally Posted by eflightray View Post
Arrrghhh!

Sometimes I just feel like giving up and looking for a knitting forum.
Just because you don't care for the why's wherefore's doesn't mean other people can't discuss them.

Besides, the OP's original premise of maintaining altitude and speed while increasing throttle is fundamentally flawed. With increased throttle one or the other has to change. Finding out which will change and why is the heart of this discussion.

Larry
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Old Jan 19, 2013, 01:07 PM
Cudda-Daddy
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United States, MO, Gerald
Joined Jun 2004
93 Posts
ya`ll are using too many $5 words for me ,, im out.
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Old Jan 19, 2013, 01:57 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,765 Posts
Quote:
Originally Posted by Lnagel View Post
Are you saying then that an airplane that has a wing with a symetrical airfoil would not pitch up with increased throttle?

Larry
Are you saying that an airplane with a symmetrical airfoil would always pitch up with increased throttle ?, not matter how it is set up ?

That it will react the same as a positive cambered wing with increased throttle ?

Perhaps I have just always got it wrong, that the reason for using a symmetrical section as used by pylon racers, aerobatic model etc, was that they don't noticeably change pitch with throttle.
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Old Jan 19, 2013, 03:14 PM
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Palmdale, CA
Joined Oct 2000
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Symmetrical airfoils have no pitching moment. Any trim changes with speed/power are less than those with cambered airfoils, and due to prop effects and downwash.
In full scale flight, the mantra is elevator controls airspeed, power controls altitude.
Push the nose down, go faster.
Add power, go up.
The trim is used to get the flight configuration needed, speed, altitude.... Change any of these, the flight path changes.
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Old Jan 19, 2013, 03:20 PM
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Palmdale, CA
Joined Oct 2000
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Actual center of gravity location on an SIG Seniorita...
Suspended by a looped string around the wing at the leading and trailing edges.
Where the two lines cross is the physical location of the c.g.
Which on the Seniorita is on the thrust line-horizontal line.
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