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Old Jan 17, 2013, 05:40 AM
222 km/hr Parkjet flyer
solentlife's Avatar
Latvia, Ventspils pilsēta, Ventspils
Joined Jan 2010
8,566 Posts
Quote:
Originally Posted by rcmaverick View Post
Question is where is WOT? My P51 will go bonkers at full throttle and god knows what will happen to the ESC
WOT = Wide Open Throttle

i bet your P51 is set-up for general warbird style sport flying .... NOT racing.

When we set-up our models ... we have choice .... as sport machines that have cruise settings and tendency to climb if speeded up ... or re-align tail / main wing etc. and remove that so we can go full out with minimum drag and max speed without the climb. he penalty of course is tendency to nose down when speed reduced.

ESC ? what will happen ? It's only doing its job - what difference does it make if model is neutral or not ... and at full throttle ESC is at it's max efficiency ... as long as correctly matched to power train - what's problem ?



Nigel
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Old Jan 17, 2013, 08:23 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,613 Posts
If people want speed from a model, they should consider if the planes airfoil section is really suitable.

A full symmetrical section will be a lot less affected in pitch with changes in speed. That also hold true for smaller changes in speed for aerobatic models.

The cambered airfoil section, (flat bottom section etc), used on may 'trainer/ beginner' type models, is designed for higher lift at lower speeds, generally what is required for trainers.

Adding more power, (so called upgrades), just to try and get more speed can be a waste if it is done to the wrong type of model. The more down-thrust required just shows the model is overpowered for the speed it was designed to fly at.

Want speeeeeeed ?, then get a plane designed for it.
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Old Jan 17, 2013, 08:48 AM
222 km/hr Parkjet flyer
solentlife's Avatar
Latvia, Ventspils pilsēta, Ventspils
Joined Jan 2010
8,566 Posts
Quote:
Originally Posted by eflightray View Post
If people want speed from a model, they should consider if the planes airfoil section is really suitable.

A full symmetrical section will be a lot less affected in pitch with changes in speed. That also hold true for smaller changes in speed for aerobatic models.

The cambered airfoil section, (flat bottom section etc), used on may 'trainer/ beginner' type models, is designed for higher lift at lower speeds, generally what is required for trainers.

Adding more power, (so called upgrades), just to try and get more speed can be a waste if it is done to the wrong type of model. The more down-thrust required just shows the model is overpowered for the speed it was designed to fly at.

Want speeeeeeed ?, then get a plane designed for it.
Very True ... all my racers were thin symet wings ... they NEEDED speed to really fly.

Nigel
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Old Jan 17, 2013, 10:15 AM
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Lnagel's Avatar
Moab, Utah, USA
Joined Apr 2003
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Quote:
Originally Posted by eflightray View Post
A full symmetrical section will be a lot less affected in pitch with changes in speed.
As long as an airplane is longitudinally stable a change in speed will cause the same amount of pitch change regardless of the airfoil.

Larry
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Old Jan 17, 2013, 10:27 AM
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DustBen's Avatar
United States, AL, Gardendale
Joined Dec 2011
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sounds like a flat bottom wing... increase airspeed, lift component increases and the model "balloons".

Flat bottom airfoils are a PITA at different speeds.
Semi-syms are less so but still problematic.
Fully symmetrical wings tend to fly straight as an arrow at most speeds (except slow to very slow).
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Old Jan 17, 2013, 10:51 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
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Quote:
Originally Posted by DustBen View Post
sounds like a flat bottom wing... increase airspeed, lift component increases and the model "balloons".

Flat bottom airfoils are a PITA at different speeds.
Semi-syms are less so but still problematic.
Fully symmetrical wings tend to fly straight as an arrow at most speeds (except slow to very slow).
Nope, airfoil type makes no significant difference to pitching up with speed.

Quote:
Originally Posted by Lnagel View Post
As long as an airplane is longitudinally stable a change in speed will cause the same amount of pitch change regardless of the airfoil.

Larry
Correct
The biggest influences on how much a plane pitches up as it goes faster is how longitudinally stable it is. The more stable it is the more it pitches up. This is why highly stable trainers tend to pitch up a lot whereas aerobatic models with little inherent stability don't pitch up much, if at all. It's nothing to do with the airfoil. In fact if you look at the airfoil in isolation the flat bottom one tries to pitch down, not up.
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Old Jan 17, 2013, 11:08 AM
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Wasaga Beach, Ontario
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Quote:
Originally Posted by DustBen View Post
sounds like a flat bottom wing... increase airspeed, lift component increases and the model "balloons".

Flat bottom airfoils are a PITA at different speeds.
Semi-syms are less so but still problematic.
Fully symmetrical wings tend to fly straight as an arrow at most speeds (except slow to very slow).
Sorry, incorrect.

A plane changing pitch and maintaining airspeed with throttle changes is normal behavior.

Flying is energy management. A plane trimmed to fly at a certain airspeed will always seek that airspeed. Always. Thus any changes in energy must be dealt with somewhere else. Left to it's own devices, a trimmed plane will use the additional energy to gain altitude (pitch up) in order to maintain its trimmed speed, or compensate for the decrease in energy by diving (nose down) in order to maintain energy for its trimmed speed.
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Old Jan 17, 2013, 11:10 AM
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United States, AL, Gardendale
Joined Dec 2011
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Quote:
Originally Posted by kalnaren View Post
Sorry, incorrect.

A plane changing pitch and maintaining airspeed with throttle changes is normal behavior.

Flying is energy management. A plane trimmed to fly at a certain airspeed will always seek that airspeed. Always. Thus any changes in energy must be dealt with somewhere else. Left to it's own devices, a trimmed plane will use the additional energy to gain altitude (pitch up) in order to maintain its trimmed speed, or compensate for the decrease in energy by diving (nose down) in order to maintain energy for its trimmed speed.
After being involved in pattern for years, I reject your false statements as if they are fact when they are not.
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Old Jan 17, 2013, 11:51 AM
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Wasaga Beach, Ontario
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After being a real pilot for years, I stick by my "false statements".
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Old Jan 17, 2013, 12:36 PM
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United States, IN, Bloomington
Joined Sep 2012
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Quote:
Originally Posted by DustBen View Post
After being involved in pattern for years, I reject your false statements as if they are fact when they are not.
Quote:
Originally Posted by kalnaren View Post
After being a real pilot for years, I stick by my "false statements".
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Old Jan 17, 2013, 01:27 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,613 Posts
Quote:
Originally Posted by Lnagel View Post
As long as an airplane is longitudinally stable a change in speed will cause the same amount of pitch change regardless of the airfoil.

Larry
So you could use any airfoil for a pylon racer as long as it's longitudinally stable ???

Or does 'longitudinally stability' need fully defining ?

Addition.
I just saw JPF's explanation, post #21, my deepest apologies for doubting you initial statement.
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Last edited by eflightray; Jan 17, 2013 at 01:32 PM. Reason: Addition.
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Old Jan 17, 2013, 01:29 PM
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Wasaga Beach, Ontario
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Quote:
Originally Posted by JetPlaneFlyer View Post
Correct
The biggest influences on how much a plane pitches up as it goes faster is how longitudinally stable it is. The more stable it is the more it pitches up. This is why highly stable trainers tend to pitch up a lot whereas aerobatic models with little inherent stability don't pitch up much, if at all. It's nothing to do with the airfoil. In fact if you look at the airfoil in isolation the flat bottom one tries to pitch down, not up.
The initial tendency for a plane to pitch with throttle changes is more due to aerodynamic couples. Overall tendency is due to changes in lift (I will agree that longitudinal stability plays a role here).
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Old Jan 17, 2013, 02:02 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,613 Posts
Quote:
Originally Posted by kalnaren View Post
The initial tendency for a plane to pitch with throttle changes is more due to aerodynamic couples. Overall tendency is due to changes in lift (I will agree that longitudinal stability plays a role here).
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.
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Old Jan 17, 2013, 02:13 PM
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Australia, NSW, Sydney
Joined Mar 2012
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The P51 is a real "puppy" when flown with a 2 cell lipo. With a 3 cell it really wants to kick ass but I guess this airframe is not designed for those kind of speeds. JPF suggested a change in CG. I might just try and put a heavier battery.
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Old Jan 17, 2013, 08:27 PM
222 km/hr Parkjet flyer
solentlife's Avatar
Latvia, Ventspils pilsēta, Ventspils
Joined Jan 2010
8,566 Posts
Any aerofoil needs a +ve angle of attack to fly, whether flat-plate or symet ...

If that angle does not change but airspeed does - then climb or descend will occur.... due to change of lift amount.

Racers, such as my Pylon jobs ... had thin symet wings, with near zero difference of alignment of tail / main wings. The main wing was also mounted with very little built in angle of attack. It relied on speed combined with small AoA to fly. Alter either and she would fly significantly differently. General sport and trainer models usually have significant built in difference tail to main wing as well as angle of attack in the main wing. A recipe for exactly the Climb on Throttle scenario.

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
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