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Old Sep 12, 2012, 10:48 AM
Do it Right, the first time!
CoolerByTheLake's Avatar
United States, MN, Hermantown
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AoA and thrust line?

I have got as far as I can go for now, untill I replentish the building fund. Now the wing is the next critical part in the F-86F build. I won't build the wing until I get the mains, struts, and wheels. No guess work here!! I got the incidence ( 0-0) and washout ( 2-3) from a BVM rep.
OK, with that said, I could use some help!

With the nose gear set in final position, the main gear retract units location and strut length will have to determine AoA and incidence. Stab will be set off of this.

So the $64,000 qusetion is, how much AoA do I need, and what is the thrust line relative to this? I should note this, current nose wheel is 2.75", if need be I can squeeze in a 3"

Once I know this, I can start working on bulkheads and formers.
Thanks, Mark
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Old Sep 12, 2012, 12:36 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally Posted by CoolerByTheLake View Post
I have got as far as I can go for now, untill I replentish the building fund. Now the wing is the next critical part in the F-86F build. I won't build the wing until I get the mains, struts, and wheels. No guess work here!! I got the incidence ( 0-0) and washout ( 2-3) from a BVM rep.
OK, with that said, I could use some help!

With the nose gear set in final position, the main gear retract units location and strut length will have to determine AoA and incidence. Stab will be set off of this.

So the $64,000 qusetion is, how much AoA do I need, and what is the thrust line relative to this? I should note this, current nose wheel is 2.75", if need be I can squeeze in a 3"

Once I know this, I can start working on bulkheads and formers.
Thanks, Mark
Hi Mark,

Just to clarify, AoA refers to pitch angle of the wing to the relative wind and is controlled by the elevator while airborne and the landing gear stance while on the ground.

Angle of incidence (AoI) is the setting of the wing relative to the center line of the aircraft. AoI will determine whether the model is stable in the pitch axis while airborne.

Washout is used to force the stall to begin at the wing's root instead of occuring along the entire span.

With the main landing gear set in a position within 15 degrees of the CG, the AoA on the ground is non-critical. For example, my F-4 is set so the center line is level with the ground and my F-106 sits on the ground in a nose-down attitude. Both take-off with equal ease.

Personally, I prefer a positive AoI (2-1/4 to 2-1/2 degrees) on the wing with a stab incidence of zero degrees as this makes the models neutrally stable in the pitch axis while airborne. They go where you point them and don't change direction until you apply control input.

I almost always set the thrust line to zero degrees so that changes in throttle do not make the model climb or descend.

The only time I alter the thrust line is if the fans are set above or below the center line of the model. For example, when designing the flying surfaces for my brother's Beluga, the fans are mounted well below the wings. I set the fans with a negative 2 degree thrust line so that the model would not climb or descend with changes in power. If the fans were mounted above the fuselage, then I would set them with a positive thrust line, and for the same reason.

Remember that in the full-size world, throttle controls altitude and elevator controls attitude. So, when the power setting is changed a corresponding change in elevator trim is always required. The thrust line on models is altered so that elevator trim changes are not required with changes in power settings.
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Old Sep 12, 2012, 02:01 PM
Do it Right, the first time!
CoolerByTheLake's Avatar
United States, MN, Hermantown
Joined Dec 2008
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My apoligies, I wasn't as clear as I should have been.

2 PM's received:
Airfoil refered to--NA64A010
Mark,
I think your airfoil selection is a good one, and would aim for a negative incedence of -2 to -3 at the tip. That would give the desired result, and would be very safe.
You could use a thinner airfoil at the tip, which is common practice, such as the NACA 64A0008 series.
Harley Condra
BVM REP, Skymaster REP, JetCat REP

Mark,
The wing root incedence is not a major thing with a symmetrical airfoil in a model airplane. The simple way is to set it 0-0.
Set everything at 0-0.
Harley Condra
BVM REP, Skymaster REP, JetCat REP

Where I was going with this was, once the incidiece was set, and no adjustment on nose gear, any AoA needed on the ground for take off,would have to be determined by strut length. This thread came to mind: http://www.rcgroups.com/forums/showthread.php?t=1723117

If you look at the PDF in my fisrt post: http://www.rcgroups.com/forums/showthread.php?t=1635049

Essentially this shows what you said, correct?

Personally, I prefer a positive AoI (2-1/4 to 2-1/2 degrees) on the wing with a stab incidence of zero degrees as this makes the models neutrally stable in the pitch axis while airborne. They go where you point them and don't change direction until you apply control input.

I almost always set the thrust line to zero degrees so that changes in throttle do not make the model climb or descend.

A quote from "National treasure"---" Could it really be that simple?"

Thanks, Mark
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Old Sep 12, 2012, 02:24 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally Posted by CoolerByTheLake View Post
I think your airfoil selection is a good one, and would aim for a negative incedence of -2 to -3 at the tip. That would give the desired result, and would be very safe.

...

The wing root incedence is not a major thing with a symmetrical airfoil in a model airplane. The simple way is to set it 0-0.
The only problem with the suggestion to set the root airfoil at zero degrees and the tip at negative incidence is that on a swept-wing design this will cause a negative lift moment aft of the CG that will change with differences in airspeed.

This is the same thing as setting the wing at zero degrees and the stab at negative incidence. (think of how Guillow's balsa gliders are set up)

The faster the model flies the more negative lift will be developed aft of the CG which will cause the model to develop a nose-up trim condition at higher speeds.

On a swept wing design, positive incidence at the root and zero degrees at the tip will be more balanced at all airspeeds.

At first glance, these both seem like the same thing. But, when you compare the CG location to the wing incidence as it twists along the span, you'll see that there is more negative lift aft of the CG if the root is set to zero degrees with negative incidence at the tip than if the root is set to positive with the tip at zero degrees.
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Old Sep 12, 2012, 02:35 PM
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DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally Posted by CoolerByTheLake View Post
Where I was going with this was, once the incidiece was set, and no adjustment on nose gear, any AoA needed on the ground for take off,would have to be determined by strut length. This thread came to mind: http://www.rcgroups.com/forums/showthread.php?t=1723117

If you look at the PDF in my fisrt post: http://www.rcgroups.com/forums/showthread.php?t=1635049

Essentially this shows what you said, correct?
As long as the main landing gear location is within 15 degrees of the CG, the AoA on the ground doesn't really matter because the elevator will have enough force to rotate the nose into take-off position.

The only time you need a positive AoA on the ground is when the scale main landing gear location is outside the 15 degree 'window'.

Examples of this are the full-size F-4, F-16 and Su-27.

In the case of the F-4, the designers placed the MLG location aft of where it should be so that once the jet touched down on the carrier it would 'plant' the nose gear. Because of this, the full-size F-4 has to have the ability to hydraulically raise the nose gear so the airplane sits in a nose-high ground attitude. Models that have the MLG located in the scale location also need to do this or else the model will not take off easily and will instead 'leap' into the air once enough air is passing over the elevator so that it has enough force to rotate the nose. BVM added this feature to his F-4 nose gear so that it would fly itself off the runway instead of leaping into the air.

The full-size F-16 and Su-27 MLG location is aft because the CG is aft due to relaxed static stability for increased maneuverability in the air. When these MLG locations are transferred to the model world, the elevators don't have enough force to get the nose off the ground for take-off. The only solution available is to have a nose-high ground attitude. That's why the BVM F-16 sits in a non-scale nose-high attitude on the ground.

When I designed my Su-27 I intentionally moved the MLG location forward so it would be within 15 degrees of the calculated CG at 25% of the MAC so that it could sit level on the ground in a scale attitude, but still be able to take off easily. I felt that moving the MLG location to a non-scale location was the lesser of two evils between scale looks and model flyability.
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Old Sep 12, 2012, 02:39 PM
turbonut's Avatar
upland CA
Joined Dec 2004
7,195 Posts
I am with Dan all the way on this...100%
BVM guy???? how can saying 0 aoi at the root with a symetric airfoil ever be a good idea on a Scale model...no aoi angle means no lift if sitting level on the ground..and the plane will look like its plowing in flite(nose high)......now depending on airfoil use you can do it that way ..but non symetric air floils will make some lift at 0 aoi and aoa ..I think a clark y makes lift up to a minus angle of 3 or 4 deg
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Old Sep 12, 2012, 03:12 PM
DELTAS RULE
corsair nut's Avatar
tehachapi, CA
Joined Jan 2006
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very interesting topic!!! thanks for all the great info. building my big T-38, i wanted 0 deg incidence....somehow i ended up with about 1.5-2 deg, with 1.5 deg of washout....now im feeling much better.

i also moved the gear way up, because the scale gear location on a T-38 is way after of the models cg. like 4" aft. im only 1.75" aft of the cg on mine.
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Old Sep 12, 2012, 03:17 PM
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DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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What Harley is saying is that with an AoI of zero degrees at the root and -2-3 degrees at the tip will cause a net positive AoI across the whole wing. To a certain extent this is right.

As I noted, the problem is that on a swept wing design it causes the model to be set up like a Guillow's glider that has the wing at zero AoI and the elevator at negative AoL.

It will fly well only at a certain speed and when it is flown faster it will cause a nose-up trim condition and when flown slower it will cause a nose-down trim condition.

But, if the root has a positive AoI and the tip is set at zero AoI, then the wing will be more balanced across the entire span and will not change trim at difference speeds.
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Old Sep 12, 2012, 03:25 PM
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I should add that there are full-size designs that do have the root AoI at zero and the tips at a negative AoI.

Two such designs are the F-16 and Su-27. But these are special cases where the airplanes fly around most of the time at positive AoA which causes the root of the wing to be flown at a positive AoA and the tip at zero AoA.
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Old Sep 12, 2012, 03:27 PM
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United States, MN, Hermantown
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Thanks Dan, I appreciate your expertise. Knowbody likes to look foolish in what they post, but I went by what the BVM rep had said..
One thing I didn't mention was that I do have ( on loan ) a set of BVM F-86 ARF plans. The reason being, they are not really plans, more like drawings for loacation of components. Not very scale to work from. That said, on plans, CG is 10" from L.E. at root. Where the rubber meets the road, is approx. 1 3/8" back from there. Falls with-in your 15%. So by that I will use this:
-------------------------------------------------------------------------------------------------------
Personally, I prefer a positive AoI (2-1/4 to 2-1/2 degrees) on the wing with a stab incidence of zero degrees as this makes the models neutrally stable in the pitch axis while airborne. They go where you point them and don't change direction until you apply control input.

I almost always set the thrust line to zero degrees so that changes in throttle do not make the model climb or descend
-------------------------------------------------------------------------------------------------------
With all the ARF's and RTF's out there, this kind of information just isn't needed any more. Not only did you provide me with the right info, you gave a great explaination as to why.

Thank You. Actually a good lesson for all.
Mark
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Old Sep 12, 2012, 03:29 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally Posted by corsair nut View Post
very interesting topic!!! thanks for all the great info. building my big T-38, i wanted 0 deg incidence....somehow i ended up with about 1.5-2 deg, with 1.5 deg of washout....now im feeling much better.

i also moved the gear way up, because the scale gear location on a T-38 is way after of the models cg. like 4" aft. im only 1.75" aft of the cg on mine.
While researching how to set up my F-111 I found that the full-scale wing has the root rib is set to +1.5 degrees and the tip at -1.5 degrees for a net washout of 3 degrees. I decided to keep this because I wanted it to provide nose-up trim as the wings sweep back to help counteract the slight nose-heavy condition that is a characteristic of the F-111 design.

I also moved the MLG location forward so that it is close to 15 degrees of the CG location. It's actually at about 20 degrees, but the scale location is somewhere about 25-30 degrees, which is, IMO, too far aft for flyability.
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Old Sep 12, 2012, 03:33 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
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Originally Posted by CoolerByTheLake View Post
Thanks Dan, I appreciate your expertise. Knowbody likes to look foolish in what they post, but I made the assumption the BVM rep knew what he was talking about.
It depends on what design you're talking about. With the modern blended wing designs like the F-15, F-16, F-18, Su-27, etc., the wings are set to zero degress AoI at the root and negative AoI at the tip. But, these are special cases. Designs like these with full-flying tails are less susceptible to the tail-down trim effects I was describing.

Mostly, though, it's better to have the root rib set to positvie AoI and the tip at zero AoI.
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Old Sep 12, 2012, 04:09 PM
Do it Right, the first time!
CoolerByTheLake's Avatar
United States, MN, Hermantown
Joined Dec 2008
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Right now my only concen is with my F-86F build. One build at a time!

When I started this build I knew there would be some technical aspects I would need help with. I don't mean to sound presumptuous, but the generosity of people like yourself, who help us out when we push ourselves in something new, is why we do!

Mark
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Old Sep 12, 2012, 07:15 PM
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BillO's Avatar
San Francisco & Santa Cruz
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Great discussion! Now what I wonder is, what technique do you use to set the thrust line? The thrust tube is buried inside the plane, so how to do you tell which way it is pointing exactly?
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Old Sep 12, 2012, 10:10 PM
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DanSavage's Avatar
Trabuco Canyon, CA
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Originally Posted by BillO View Post
Great discussion! Now what I wonder is, what technique do you use to set the thrust line? The thrust tube is buried inside the plane, so how to do you tell which way it is pointing exactly?
If you're designing the model from scratch, you have complete control so you set it up however you want.

If the model is already built, such as an ARF or RTF, it's a lot tougher.
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