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Old Oct 25, 2012, 05:33 PM
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Canada, NS, Halifax
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Stepped wings

The use of stepped wings is for low performance easy to build planes. The drag tends to be high and the aft part of the wing produces little lift. Hard to get clean effective air flow to the control surfaces. You may end up using large deflections and therefore large drag to get a response

Clark Y is a high lift high drag wing. The use of various airfoils is dependent on the purpose of the plane and the speeds it must fly. I cannot say if the Clark Y is the most suitable. A Clark Y is many airfoils. Most airfoils are defined by a specific cross section at various percentages of thickness.

So an SD 7080 can be built at various thicknesses. These are expressed as a % of chord.

A very thin wing is hard to build strong and can have low drag.. A very thick wing has a lot of drag but it is easier to build a strong wing.

This link http://soartech-aero.com/ has free downloadable technical journals. which are now here

http://www.ae.illinois.edu/m-selig/uiuc_lsat.html

You might want to download and skim to find the relevant areas you need for your plane. The "Airfoils at Low Speed" (SoarTech 8) and the 3 others below it are VERY valuable. You should have a copy of each of them.

Also see the UIUC links given on that page.That is where you can download them. Even if you do not read or skim through them all, download them as serious reference material for the future.

If I recall correctly there is a summary of the types and their application with each various group tested. That will lead you to a possible more suitable airfoil for your plane.

Also for good aerodynamic info go here.

http://www.mh-aerotools.de/airfoils/

Short informative and technical articles by a leader in aerodynamics.
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Old Oct 26, 2012, 05:40 AM
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Half a kilo for the tail set seems extreme. What's the dimensions of the panels that make the vee?
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Old Oct 26, 2012, 05:48 AM
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Cardiff, UK
Joined Aug 2008
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Tail

Quote:
Originally Posted by awmeade View Post
Half a kilo for the tail set seems extreme. What's the dimensions of the panels that make the vee?
The tail is 360g incl servos and hardware.

The dimensions for each panel are 380mm x 230mm. It is a naca0012 airfoil. Symmetrical.
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Old Oct 26, 2012, 04:21 PM
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Airfoils..

Quote:
Originally Posted by small_rcer View Post
The use of stepped wings is for low performance easy to build planes. The drag tends to be high and the aft part of the wing produces little lift. Hard to get clean effective air flow to the control surfaces. You may end up using large deflections and therefore large drag to get a response

Clark Y is a high lift high drag wing. The use of various airfoils is dependent on the purpose of the plane and the speeds it must fly. I cannot say if the Clark Y is the most suitable. A Clark Y is many airfoils. Most airfoils are defined by a specific cross section at various percentages of thickness.

So an SD 7080 can be built at various thicknesses. These are expressed as a % of chord.

A very thin wing is hard to build strong and can have low drag.. A very thick wing has a lot of drag but it is easier to build a strong wing.

This link http://soartech-aero.com/ has free downloadable technical journals. which are now here

http://www.ae.illinois.edu/m-selig/uiuc_lsat.html

You might want to download and skim to find the relevant areas you need for your plane. The "Airfoils at Low Speed" (SoarTech 8) and the 3 others below it are VERY valuable. You should have a copy of each of them.

Also see the UIUC links given on that page.That is where you can download them. Even if you do not read or skim through them all, download them as serious reference material for the future.

If I recall correctly there is a summary of the types and their application with each various group tested. That will lead you to a possible more suitable airfoil for your plane.

Also for good aerodynamic info go here.

http://www.mh-aerotools.de/airfoils/

Short informative and technical articles by a leader in aerodynamics.
Hi Jim, Cheers for yet again more sound advice, I have looked through some of the resources, and will study in greater detail once i am ready to plan my next build...

I am currently in the process of hacking away at the rear of the fuselage - It is quite apparent that there is a large amount of glue, ply and foam that does not serve any structural purpose, and have removed a substantial amount of material....

I do have a question regarding Wheels, In your last post you mentioned where the main gear needs to sit, 1 inch contact point behind cg when aircraft is at 7-10 degrees nose up... is this going to be necessary definitely?

In pics...
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Old Oct 28, 2012, 10:54 AM
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Canada, NS, Halifax
Joined Dec 2005
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Landing Gear position

If you place the contact point too far aft of the CG, you may find it hard to rotate the aircraft on take off. If it is too far back you need a large elevator deflection which causes drag and can slow you down at the critical take off point. Then once the plane starts to rotate you might over rotate and stall.

You do not want to be in a position where you have to pull a lot of up and then have to push the stick forward once it starts to rotate. You could get into a PIO or Pilot Induced Oscillation. Very unpleasant position to be in at take off.

So on that size plane I "guestimated" about 3/4 inch aft of the CG in a 7 to 10 deg nose up position. The reason I suggested that the LG be mounted further forward and slope aft and down is to get the weight of the mounting point structure and the LG itself as far forward as possible to make it easier to balance the plane.

When you have the plane sitting on the ground with the CG where it should be, push down by hand on the tail to rotate the nose of the plane up to 10 deg nose high. The downforce you use to push the tail plane down is what the elevators will have to generate to rotate the plane. Too much and the elevators may not have enough force to do it or require a large deflection.
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Old Oct 28, 2012, 11:20 AM
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Canada, ON, Milton
Joined Jan 2009
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I wish I had this explanation for the LG last year. I had done what you wrote, put the gear too far back on a home built. When it came to the first flight. Had to get stupid speed before it rotated. Then all of a sudden it popped off the ground. Then I over corrected and slammed into the ground. Took all of 2 seconds. After repairing I couldn`t figure out what went wrong. Then looked at a real plane then noticed where the gear was. Moved mine just behind Cg. Next first flight it lifted nice and slow, Lesson learned Thanks
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Old Oct 28, 2012, 11:25 AM
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Cardiff, UK
Joined Aug 2008
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Quote:
Originally Posted by small_rcer View Post
If you place the contact point too far aft of the CG, you may find it hard to rotate the aircraft on take off. If it is too far back you need a large elevator deflection which causes drag and can slow you down at the critical take off point. Then once the plane starts to rotate you might over rotate and stall.

You do not want to be in a position where you have to pull a lot of up and then have to push the stick forward once it starts to rotate. You could get into a PIO or Pilot Induced Oscillation. Very unpleasant position to be in at take off.

So on that size plane I "guestimated" about 3/4 inch aft of the CG in a 7 to 10 deg nose up position. The reason I suggested that the LG be mounted further forward and slope aft and down is to get the weight of the mounting point structure and the LG itself as far forward as possible to make it easier to balance the plane.

When you have the plane sitting on the ground with the CG where it should be, push down by hand on the tail to rotate the nose of the plane up to 10 deg nose high. The downforce you use to push the tail plane down is what the elevators will have to generate to rotate the plane. Too much and the elevators may not have enough force to do it or require a large deflection.
Cheers for the info... I am going to do this next on my task list. I have measured the distance between contact point and vertical between CG at 7-10 degrees, a i think its currently sitting 2.5 inches behind. so needs to be moved 1.5 inches.

I've worked out i need to make tail 215g total. A friend of mine is up to the challenge. I am going to use DOW Blue foam instead of EPS foam as it gives a smoother finish. the the resin will go on smoother and not fill so many voids causing weight.
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Old Oct 28, 2012, 11:29 AM
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Motor respositioned and secured in place

The prop is about 10mm from the TE of wing... hope its okay, it will be a bit noisy i'm guessing, but has better airflow around it compared to before... so should get almost max power out of it...

in pics...
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Old Oct 28, 2012, 06:15 PM
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Getting there....

She almost balances with a single 5000mah lipo in the nose now...

lighter wing servos and I may be there... AUW is now 4.3kg
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Old Oct 29, 2012, 03:42 AM
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Question about Blue foam... I am looking at rebuilding tail from Blue foam, and have never bought it before....The Local Insulators has varying types of blue foam, and Dow floor-mate... with different numbers next to it, I was just wondering which number I need?

I will probably purchase 50mm thick ..

Here are the options....

Dow Floormate 200-X
Dow Floormate 300-A
Dow Floormate 500-A
Dow Floormate 500-X
Dow Floormate 700-A

Cheers, Rich
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Old Nov 04, 2012, 10:10 AM
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Cardiff, UK
Joined Aug 2008
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Small update...

I've been spending money! Bought the following to try and get this thing in the air...
  1. 14mm x 11mm x 1000mm Pultruded Carbon fiber tube to reploace alloy booms
  2. 1mm x 6mm CF strip to reinforce fuse
  3. 8mm tube for wing joiner
  4. 2500mm x 600mm x 50mm Floormate 300A from Sheffield insulations to cut new wing and tail panels
  5. SP106 resin for wing and tail
  6. 100mm dia wheels to replace the 70mm dia wheels currently on plane.
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Old Nov 05, 2012, 07:36 AM
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See your picture 4854 and Pic 4859

In the first picture you can taper the thickness of the fuselage side foam to improve airflow to the prop.. Also use some 3/32 or 2 mm balsa and sheet in the bottom area under the motor to improve airflow. If you make the profile of this area curved to avoid the sharp change of direction in the airflow it will reduce prop noise. Look at pic 4859 and make the bottom of fuse curve up to the prop.

Also you can shape the trailing edge to provide a cutout for the propeller as done on the RiteWing flying wings. See www.ritewingrc.com

By relocating the LG by 2.5 inches the CG should be very close. Measure from the target CG position of the plane to the approx center of mass of the LG as it is now. Multiply the distance by the weight of the LG. Write the number down.

Then multiply the distance of the new location of the LG to the target CG by the weight of the LG. Write this 2nd number down.

What you now have are a torque arm calculation for the current position and new position of the LG. The difference between these two numbers is the inch oz or gram centimeters of torque change, caused by moving the LG. Knowing this number you can test the effect of the relocation.

To test this without moving the existing LG, you calculate what weigh placed ahead of the planned CG has the same effect. So I will assume you have a number of 100. That is 10 oz at 10 inches or 5 oz at 20 inches.

Next place weights on the nose of the plane to get the plane CG where you want it. Measure the distance from this weight to the target CG position. Multiply by the weight. In my example, if this number is more than 100, moving the LG will not be enough to get the CG correct. If this number is less than 100, then moving the LG will probably get you close to the CG. Then the battery or other equipment can adjust the CG to the final position.

What you have effectively done is the old school yard teeter totter calculation. Heavy person short distance, light person long distance. By calculating the existing versus the target you can get a pretty close approximation.

If this process is old hat to you, I apologize. If not try it out and see what you get. It is the process I use for equipment relocation.
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Old Nov 06, 2012, 12:41 PM
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Cardiff, UK
Joined Aug 2008
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Cheers for the Advice..... again! :-)

In the first picture you can taper the thickness of the fuselage side foam to improve airflow to the prop..
Agreed, I will do this

Also use some 3/32 or 2 mm balsa and sheet in the bottom area under the motor to improve airflow. If you make the profile of this area curved to avoid the sharp change of direction in the airflow it will reduce prop noise. Look at pic 4859 and make the bottom of fuse curve up to the prop.
I'll do my best with this, i could possibly use a foam block

Also you can shape the trailing edge to provide a cutout for the propeller as done on the RiteWing flying wings. See www.ritewingrc.com
I'll cut the TE of the wing out to make more room for the prop, How bigger gap do you recommend from prop to trailing edge?

By relocating the LG by 2.5 inches the CG should be very close. Measure from the target CG position of the plane to the approx center of mass of the LG as it is now. Multiply the distance by the weight of the LG. Write the number down.

Then multiply the distance of the new location of the LG to the target CG by the weight of the LG. Write this 2nd number down.

What you now have are a torque arm calculation for the current position and new position of the LG. The difference between these two numbers is the inch oz or gram centimeters of torque change, caused by moving the LG. Knowing this number you can test the effect of the relocation.

To test this without moving the existing LG, you calculate what weigh placed ahead of the planned CG has the same effect. So I will assume you have a number of 100. That is 10 oz at 10 inches or 5 oz at 20 inches.

Next place weights on the nose of the plane to get the plane CG where you want it. Measure the distance from this weight to the target CG position. Multiply by the weight. In my example, if this number is more than 100, moving the LG will not be enough to get the CG correct. If this number is less than 100, then moving the LG will probably get you close to the CG. Then the battery or other equipment can adjust the CG to the final position.

What you have effectively done is the old school yard teeter totter calculation. Heavy person short distance, light person long distance. By calculating the existing versus the target you can get a pretty close approximation.

If this process is old hat to you, I apologize. If not try it out and see what you get. It is the process I use for equipment relocation.
This process looks good, I am going to need the weight of the LG in order to do it. I do have approximate weights, but dont know how heavy the glue, ply and screws are that the LG bolts onto, i'm guessing total weight of rear LG is around 290g.

Cheers for the advice, Iwill be doing the mods on thursday evening as well as replacing alloy booms with lighter carbon booms.
Had some annoying news today, Sheffield insulations refunded me the money i paid for a single sheet of bluecor styro foam, as apparent they arent able to get single sheets of dow floormate any more. I'm going to try b and q , knauf spaceboard next, failing that, i'll use EPS Polystyrene again, I may reduce temp on hot wire to get a smoother cut, and cut slower.

I'll follow Thursdays work with an update ....

Ciao.
Rich
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Old Nov 09, 2012, 02:03 AM
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Almost there...

The plane is almost balancing at 33% Chord. I am aiming for 25% if i can, so i can do a maiden with a bit nose heavy weight. The calculator says it should balance at 110mm.

Next up, moving the gear forward a few inches.... then it should be ready for a maiden!
Rich
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Old Nov 09, 2012, 04:36 AM
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Good progress, I like it
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