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RE: flight trim on full stabs - 11/22/2012 2:06 PM   
tp777fo



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Status: offline 		You trim an airplane for a "speed", i.e. trim allows you to remove all control pressure from the yoke/stick at a specific airspeed. Reason is the lift generated at that speed is equal to the lift required to maintain a equilibrium in flight. Change the speed (slower for example), your airplane stills requires X amount of lift to fly level but the airspeed doesnt allow the lift to be generated. You then pull back on the stick and increase the angle of attack thus generating more lift. Increasing the angle of attack also generates more drag so you must increase power to maintain a given airspeed. To keep you from having to hold back pressure on the stick/yoke you trim up elevator to remove the pressure. Accelerate and you must remove the trim to keep from climbing. So your airplane must be retrimmed every time you change the speed.

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RE: flight trim on full stabs - 11/22/2012 3:39 PM   
FalconWings



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Status: offline 		FWIW, I always keep a good touch of down trim on all my models. I find that a slight positive pressure on the sticks increases precision and makes for some really good low passes. I also hate it when a model sits on its tail during final. I'd rather hold some up elevator pressure rather than pushing the nose down. This has worked for me since my Trainer 60 days.

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RE: flight trim on full stabs - 11/22/2012 4:12 PM   
turnnburn


 

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Status: offline 		Had the same problem with my JL L-39. In my case it was a lateral balance issue. I was able to fix it with the addition of about an ounce of weight inside the tip tank on the lighter side. You might try to search for my old post on this from about a year or so ago.

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RE: flight trim on full stabs - 11/23/2012 8:02 AM   
darryltarr



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Status: offline 		David,

A fixed wing air-frame (once airborne) does NOT recognize a moving air mass. In other words when you fly your jet it as no idea there is any wind - period.

Obviously sudden changes in wind direction and or speed (like gusts, eddies, thermals, wind sheer) do have an effect as these are instant and the air-frame will experience turbulence, but your theory about upwind versus downwind is absolute rubbish.

When you fly a circuit (or a holding pattern), the airspeed remains constant for that power setting. If you are trying to slow down when you are flying downwind it is because YOU have recognized the higher GROUND SPEED, and confused this with airspeed.

The only aircraft that recognizes a moving air mass is a helicopter - for obvious reasons.

Darryl

< Message edited by darryltarr -- 11/23/2012 2:24 PM >

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RE: flight trim on full stabs - 11/23/2012 8:19 AM   
HarryC


 

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quote:

ORIGINAL: David
Using a throttle mix to correct an airspeed issue just makes the issue worse. Example: Upwind airspeed at a certain throttle setting is higher that downwind setting at the same throttle setting. Now you have one trim for upwind and a different one

David S

WHAT! You really think airspeed at constant throttle is higher when flying into wind than when downwind???

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RE: flight trim on full stabs - 11/23/2012 8:25 AM   
HarryC


 

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RE: flight trim on full stabs - 11/23/2012 12:40 PM   
HarryC


 

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quote:

ORIGINAL: rctech2k7


quote:

ORIGINAL: JohnMac


quote:

ORIGINAL: rgburrill

So what you all are saying is taht jet engines do not have torque.  Interesting.

Really!? Did they?


IMO, the torque of our jet engine is a approximately the product of mass of the rotating parts, average distance to the center and its acceleration. Not sure about the mass of rotor but radius I know is small. At constant rpm acceleration is zero therefore it's only exist as transient...


A jet engine has zero torque reaction. The shaft is generating torque but unlike piston engines and electric motors there is no torque reaction, the engine case will not try to spin in opposition to the shaft.


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RE: flight trim on full stabs - 11/23/2012 4:13 PM   
rctech2k7



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quote:

ORIGINAL: HarryC

quote:

ORIGINAL: rctech2k7


quote:

ORIGINAL: JohnMac


quote:

ORIGINAL: rgburrill

So what you all are saying is taht jet engines do not have torque.  Interesting.

Really!? Did they?


IMO, the torque of our jet engine is a approximately the product of mass of the rotating parts, average distance to the center and its acceleration. Not sure about the mass of rotor but radius I know is small. At constant rpm acceleration is zero therefore it's only exist as transient...


A jet engine has zero torque reaction. The shaft is generating torque but unlike piston engines and electric motors there is no torque reaction, the engine case will not try to spin in opposition to the shaft.



Thanks for bringing up because I don't want to create confusion with regards to the torque of jet engine. That's my opinion to show them that the torque exist during change in rpm. Any rotor with mass when start to spin create opposite reaction because of inertia and energy is being transfer to produce work...

I agree that torque reaction is zero because of the following:
-acceleration is not that fast
-radius is too small
-resistance (drag and friction) is negligible
-mass is too smal
-aircraft has considerably high mass
-aircraft has bigger wing span and surface area compare to the engine

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RE: flight trim on full stabs - 11/23/2012 4:29 PM   
rctech2k7



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ORIGINAL: tp777fo

You trim an airplane for a ''speed'', i.e. trim allows you to remove all control pressure from the yoke/stick at a specific airspeed. Reason is the lift generated at that speed is equal to the lift required to maintain a equilibrium in flight. Change the speed (slower for example), your airplane stills requires X amount of lift to fly level but the airspeed doesnt allow the lift to be generated. You then pull back on the stick and increase the angle of attack thus generating more lift. Increasing the angle of attack also generates more drag so you must increase power to maintain a given airspeed. To keep you from having to hold back pressure on the stick/yoke you trim up elevator to remove the pressure. Accelerate and you must remove the trim to keep from climbing. So your airplane must be retrimmed every time you change the speed.


This is a practical explanation of one I'm thinking about... I believe this could also happen to an aircraft with slight side slip caused by rudder that during certain speed and trimmed (pitch and roll) the aircraft maintain straight flight but when airspeed drops it rolls specially at higher angle...

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RE: flight trim on full stabs - 11/23/2012 6:19 PM   
HarryC


 

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quote:

ORIGINAL: rctech2k7


quote:

ORIGINAL: HarryC

quote:

ORIGINAL: rctech2k7


quote:

ORIGINAL: JohnMac


quote:

ORIGINAL: rgburrill

So what you all are saying is taht jet engines do not have torque.  Interesting.

Really!? Did they?


IMO, the torque of our jet engine is a approximately the product of mass of the rotating parts, average distance to the center and its acceleration. Not sure about the mass of rotor but radius I know is small. At constant rpm acceleration is zero therefore it's only exist as transient...


A jet engine has zero torque reaction. The shaft is generating torque but unlike piston engines and electric motors there is no torque reaction, the engine case will not try to spin in opposition to the shaft.



Thanks for bringing up because I don't want to create confusion with regards to the torque of jet engine. That's my opinion to show them that the torque exist during change in rpm. Any rotor with mass when start to spin create opposite reaction because of inertia and energy is being transfer to produce work...

I agree that torque reaction is zero because of the following:
-acceleration is not that fast
-radius is too small
-resistance (drag and friction) is negligible
-mass is too smal
-aircraft has considerably high mass
-aircraft has bigger wing span and surface area compare to the engine

Sorry but you are still completely wrong. There is no torque reaction, period. There is no mechanism for any force to be exerted as a reaction. You fundamentally misunderstand what is driving the turbine shaft.


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RE: flight trim on full stabs - 11/23/2012 6:27 PM   
FenderBean



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Status: offline 		Its funny where these post end up going sometimes.


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RE: flight trim on full stabs - 11/23/2012 6:55 PM   
HarryC


 

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quote:

ORIGINAL: rctech2k7
Any rotor with mass when start to spin create opposite reaction


That's where your understanding fails. A windmill shaft has torque but the building or structure sees no torque reaction because it is not driving the rotation. A turbine is just a windmill spinning in the wind, what the turbine is attached to is not creating the turning force snd therefore a reaction is physically impossible because there is no action in the first place from the structure. The reaction is in the changed path of the airflow

< Message edited by HarryC -- 11/23/2012 8:13 PM >

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RE: flight trim on full stabs - 11/23/2012 9:35 PM   
rctech2k7



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Status: offline 		Ok I accept your discussion about this matter. What if you fixed the mounting shaft of a turbine wind mill and you reverse the air flow? Also if you sudden stop the turbine wheel.. Where is the energy of compressor fan coming from? Acceleration and deceleration on a rotating parts matters a lot when time approaches to zero because the rate that the energy transfer per unit time rises to critical that when exceed the stress limit of materials can cause damage such as when a compressor blade explode and jam the sudden stop produce the engine housing to slip from the bracket and damage to the mounting block.

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RE: flight trim on full stabs - 11/23/2012 9:54 PM   
HarryC


 

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quote:

ORIGINAL: rctech2k7

Ok I accept your discussion about this matter. What if you fixed the mounting shaft of a turbine wind mill and you reverse the air flow? Also if you sudden stop the turbine wheel.. Where is the energy of compressor fan coming from? Acceleration and deceleration on a rotating parts matters a lot when time approaches to zero because the rate that the energy transfer per unit time rises to critical that when exceed the stress limit of materials can cause damage such as when a compressor blade explode and jam the sudden stop produce the engine housing to slip from the bracket and damage to the mounting block.

You still fundamentally misunderstand it. If you lock the structure to the shaft it simply becomes part of the shaft and rotates with it. A torque reaction would make it rotate the opposite way. In a a piston or electric motor a force is created between case and shaft and both try to rotate away from each other with equal force. In a jet there is no force pushing case and shaft apart, no action therefore no reaction. Airflow over the turbine causes the shaft rotation and the reaction is a change in airflow direction the opposite way. The jet engine feels a force along the same axis as the shaft as thrust, other motors feel a force perpendicular to the shaft as torque. Jets have no torque reaction, it's nothing to do with acceleration, relative masses of shaft/engine/plane etc.

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RE: flight trim on full stabs - 11/24/2012 12:23 AM   
rctech2k7



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ORIGINAL: FenderBean

Its funny where these post end up going sometimes.


That why I tried to disregard that after my post and leave the latest from tp777fo which I believe his post will lead to the solution. Maybe there 's a purpose...

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RE: flight trim on full stabs - 11/24/2012 12:36 AM   
rctech2k7



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Status: offline 		HarryC, I got what you mean and appreciate for highlighting, You're on opposite reaction and I'm pertaining to a force that acted on a wheel where a torque is develop caused by starting or when a turbine shaft is loaded...

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RE: flight trim on full stabs - 11/24/2012 12:38 AM   
Rob2160



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quote:

ORIGINAL: David Searles

quote:

ORIGINAL: sailing1

You could just build a thortte to aileron (or stab) mix to adjust the trim between 3/4 to full throttle when you get your roll.


Guys,

Using a throttle mix to correct an airspeed issue just makes the issue worse. Example: Upwind airspeed at a certain throttle setting is higher that downwind setting at the same throttle setting. Now you have one trim for upwind and a different one for downwind. Another example: Pulling vertical, you will immediately start to bleed airspeed, even though the throttle setting is the same, the airspeed isn't, and will continue to change the longer the vertical line. Doesn't work.

Falconwings has the right idea. You have to fix the problem at it's source, which seems to be the stabs being offset per his experience.

David S


Sorry, I have to step in here..

This statement about Upwind vs Downwind is incorrect.. your upwind or downwing AIRSPEED will be exactly the same for any throttle setting..

You are thinking about GROUNDSPEED, which will make the aircraft look to be flying faster when flying downwind (same direction as the wind)

The aerodynamic effect will be the same at any throttle setting regardless of wind direction vs aircraft direction. (in level flight)  Plenty to read about it here..

You are completely correct about airspeed changing when pulling vertical with full throttle, I agree with that comment entirely.

I also agree it could be caused by slight trim differences between Stab and ailerons.  Due to the ailerons being further out on the wing they will be more effective, aerodynamic force increases as function of airspeed (V - squared actually) so I would start by trimming ailerons left slightly and stab right at the lower speed and see what happens at higher speeds..



< Message edited by Rob2160 -- 11/24/2012 1:06 AM >

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RE: flight trim on full stabs - 11/24/2012 12:44 AM   
Rob2160



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quote:

ORIGINAL: FalconWings

FWIW, I always keep a good touch of down trim on all my models. I find that a slight positive pressure on the sticks increases precision and makes for some really good low passes. I also hate it when a model sits on its tail during final. I'd rather hold some up elevator pressure rather than pushing the nose down. This has worked for me since my Trainer 60 days.


Same technique as me.. I agree.. Makes for super low passes.. http://www.youtube.com/watch?v=GYkCNCakP4s

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RE: flight trim on full stabs - 11/24/2012 12:48 AM   
Rob2160



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ORIGINAL: sskianpour



Hehehe, did somebody say "Downwind"? I wonder how everything is affected in a downwind turn?


Shaz


Heh Heh.. yep.. you get it!.

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RE: flight trim on full stabs - 11/24/2012 12:53 AM   
YellowAircraft



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Status: offline 		Hi,

I don't know if anyone has already addressed this because I didn't read the entire thread, but another cause of this problem is an inconsistency with the layup/construction of the wings themselves. If there is even a slight difference between the thickness or shape of the airfoil between the left and right wings, you'll get a situation where your trim is inconsistent for speed. I helped a guy with an F-16 of make I shall not name, and I suspected this problem. I used a push-pin through the wings at an exactly-measured location, and one wing was almost 5/32" thicker in the same spot than the other wing. All it took was a badly set up top skin to do the trick. There was space between the ribs and the top skin, with a glue bead that wasn't compressed. Just a thought...

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RE: flight trim on full stabs - 11/24/2012 12:54 AM   
Rob2160



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quote:

ORIGINAL: rctech2k7


quote:

ORIGINAL: JohnMac


quote:

ORIGINAL: rgburrill

So what you all are saying is taht jet engines do not have torque.  Interesting.

Really!? Did they?


IMO, the torque of our jet engine is a approximately the product of mass of the rotating parts, average distance to the center and its acceleration. Not sure about the mass of rotor but radius I know is small. At constant rpm acceleration is zero therefore it's only exist as transient...


Incorrect comment withdrawn, (was thinking of an Electric Ducted Fan which does have a torque reaction) I'll think before engaging keyboard next time..

< Message edited by Rob2160 -- 11/24/2012 1:16 AM >

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RE: flight trim on full stabs - 11/24/2012 12:59 AM   
Rob2160



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quote:

ORIGINAL: FenderBean

My f-16 with no trim has slight right roll at full throttle, but under 3/4 throttle it flies level. So if i trim it level at full throttle it will roll left when i back off the throttle. Is this the full stabs not being set or some kind of airfoil problem?


If it is any consolation.. most of the real jets I have flown do something similar.. trim roll neutral at one airspeed and it won't always be perfect at other airspeeds.  The Autopilot hide this most of the time.. But always noticable when hand flying..



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