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Building F16 from scratch

Old 11-03-2022, 08:07 PM
  #1  
aaindthu
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Default Building F16 from scratch

Hi!
I am building an F16 EDF from scratch.
I am not trying to make it 100 % scale, it will be hand launched, so no landing gears and no all-moving stabs.
I just want to do some aggressive flying with an EDF and F16 has the perfect shape in my opinion.
I have made the basic F16 profile cut out from 5 mm depron sheets. 4 sheets glued on top of each other. 
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The wings and the stabilizers have only two sheets of depron whereas the fuselage has 4 sheets.

The negative dihedral stabs were simply glued with epoxy and has a thin metal sheet running between the glued joints.
The control surfaces have bevels on both sides and has hinges made of old X-Ray sheet glued with epoxy as well.

The EDF I am using is a QX motor 64mm five - blade EDF. I have not decided on the battery yet, but I might use 3S or 4S along with it. With a 3s battery, I may not get a decent amount of flight time. With a 4s, I cannot afford a lot of hard landings. 😅 I'll think about it later on.


Last edited by aaindthu; 11-03-2022 at 09:13 PM.
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Old 11-04-2022, 06:06 AM
  #2  
CHELLIE
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Cool, Good Luck To You, and keep it under 100 MPH
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Old 11-04-2022, 02:02 PM
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That looks rather nice.
The very sort tail moment and the substantial fuselage area ahead of the CofG could make it a bit twitchy in pitch. Remember the full size has all sorts of electronic stability aids.
Even on 3s it should have plenty of power provided you keep the overall weight down but the QX EDFs are power hungry so it will work a smaller lighter battery pretty hard..
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Old 11-05-2022, 05:06 AM
  #4  
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Originally Posted by quorneng View Post
Even on 3s it should have plenty of power provided you keep the overall weight down but the QX EDFs are power hungry so it will work a smaller lighter battery pretty hard..
Thank you for the info on QX motors. This prompted me to search for the specifications of this EDF motor and I found out that Freewing (RC model maker) uses this very same motor in two of their small EDF jets, for P.15 Lippisch and the Thunderchief fighter. Lucky for me, now all I have to do is to make sure that my model's weight, wingspan are comparable to the specifications of the Freewing models. And they also recommend 3S 2200 mah battery for the models. So I am going to do the same.

And I have done some work on the F16.
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This is the underside of the F16. I have glued the control horns and cut out some gap in the fuselage for the EDF. I made some "side-rails" for the EDF to be seated on, out of balsa sheet. I am going to glue the EDF on to the "side-rails" with epoxy permanently. I have also carved out some "slope" in front of the EDF in the fuselage for easy airflow to the EDF. The next step is to glue the EDF to the fuselage. The EDF fits inside the gap in the fuselage perfectly. I will update soon.

Last edited by aaindthu; 11-05-2022 at 05:23 AM.
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Old 11-06-2022, 09:47 PM
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Cut out the servo pockets and fixed the servos into them. It is tight fit and I also poured some white glue around the edges. I also glued the EDF motor with epoxy onto the balsa sheets. Most of the EDF unit is toward the bottom side of the airplane and just a little bit of the EDF unit peeks out from the top side of the airplane. Here are some pictures.
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Next step is to bury the servo cables and connect the motor to the ESC and to tag all the cables neatly to the fuselage. I will also install the receiver next.
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Old 11-08-2022, 04:47 PM
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buried the servo cables into the depron, neatly. And finished making the intake nacelle and it looks just like an F16. I haven't glued it yet as I have to fix ESC first.

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Old 11-08-2022, 04:51 PM
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Originally Posted by quorneng View Post
The very sort tail moment and the substantial fuselage area ahead of the CofG could make it a bit twitchy in pitch. Remember the full size has all sorts of electronic stability aids..
I am hoping to get around this by making my plane a little nose heavy.
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Old 11-08-2022, 07:15 PM
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Originally Posted by CHELLIE View Post
Cool, Good Luck To You, and keep it under 100 MPH
Thanks.. I am just trying to keep it under 600 grams. 🤣🤣
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Old 11-08-2022, 10:24 PM
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What is the inlet area compared to the FSA?
The FSA of a 64 mm fan is about 2600 sqmm.
Ideally all the inlet duct should have a constant area of 1.2 * FSA so for you that is 3120 sqmm.
To achieve that area the half circle inlet would need a radius of 44 mm.
From the pictures it looks to be about that. .
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Old 11-09-2022, 03:52 AM
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Originally Posted by quorneng View Post
What is the inlet area compared to the FSA?
The FSA of a 64 mm fan is about 2600 sqmm.
Ideally all the inlet duct should have a constant area of 1.2 * FSA so for you that is 3120 sqmm.
To achieve that area the half circle inlet would need a radius of 44 mm.
From the pictures it looks to be about that. .
Honestly, I didn't measure any part of the nacelle while making it.
Since I've noticed that almost all the scale rc edf have one or many "cheat" inlets underneath them, I thought it was not very important and just made sure the inlet area and the exit area are bigger than the edf area itself.

I tested the thrust from the edf right away, with and without the inlet nacelle. I can feel the difference, but it's not very big and anyways I can still cut open extra inlets from underneath the inlet pipe.

I also calculated the radius of the inlet from its circumference length that I know is 22cm. The radius is 70mm.
I am definitely not the expert here. Should I be concerned??


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Old 11-09-2022, 06:11 PM
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aaindthu
Don't worry I am sure your inlet is big enough.
As people discover the highest static thrust tends to generated by an EDF that has no inlet at all but just the moulded bell mouth, which is why the manufacturers put it on in the first place! .
Even a simple inlet duct can cost as much as 20% of the 'bare' EDF static thrust..

For a true scale turbo jet the problem is the inlet is never really big enough for an EDF. A gas turbine burns fuel to greatly raise the temperature of the air going in so its volume is increased many times and hence its exit velocity.
On the other hand an EDF is a 'cold' jet so the inlet has to be at least as big as the exhaust. Cheat holes are aerodynamically a rather poor way of getting more air in if the true scale inlet is too small for the EDF.
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Old 11-09-2022, 06:30 PM
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Originally Posted by quorneng View Post
aaindthu
Don't worry I am sure your inlet is big enough.
As people discover the highest static thrust tends to generated by an EDF that has no inlet at all but just the moulded bell mouth, which is why the manufacturers put it on in the first place! .
Even a simple inlet duct can cost as much as 20% of the 'bare' EDF static thrust..

For a true scale turbo jet the problem is the inlet is never really big enough for an EDF. A gas turbine burns fuel to greatly raise the temperature of the air going in so its volume is increased many times and hence its exit velocity.
On the other hand an EDF is a 'cold' jet so the inlet has to be at least as big as the exhaust. Cheat holes are aerodynamically a rather poor way of getting more air in if the true scale inlet is too small for the EDF.
Phew!! Thank you. That was a relief 😅
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Old 11-12-2022, 10:59 AM
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Made the fuselage for the top side of the airplane. Did the Rudder and glued it to the Fuselage. Completed making the Battery bay. Now all that remains is the removable hatch with canopy for the battery bay.
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I may not glue the intake nacelle to the fuselage for the maiden flight to improve performance and also to avoid any serious damage during maiden flight.
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Old 11-12-2022, 12:53 PM
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Looking good! Hope it flies as well!
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Old 11-14-2022, 03:14 PM
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For the canopy, I cut out sheets of depron with gap for the battery and stacked them on top of each other and glued them. Then I sanded this piece of depron block into the canopy shape. There were some gaps, so I have to cover them with newspaper and glue.
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For the latch / locking mechanism, I cut out the clip from the Highlighter sketch pen and made a lock from popsicle stick as shown in the pictures. The lock mechanism is very rigid. To remove the canopy, just have to slide it out.
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The plane looks like this now.
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It is raining heavily for the past few days and the unfinished roads are inundated here. I just have to wait for the water to clear from the roads so that I can carry my EDF jet on bicycle. It has 660 mm wingspan, so it shouldn't be difficult.

The final Specifications - AUW - 620 grams. Wingspan - 660 mm It has Expo on ailerons and elevator. It has flaperon function. No inlet pipe under the belly.

Last edited by aaindthu; 11-14-2022 at 03:30 PM.
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Old 11-14-2022, 03:29 PM
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Originally Posted by Don Sims View Post
Looking good! Hope it flies as well!
Thank you !! I compared my model with a lot of 64mm EDF jets online and I am pretty confident this will fly. Thanks again.
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Old 11-14-2022, 08:59 PM
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aaindthu
It is possible that your F16 will actually be more controllable with the underbelly duct in place.
Your EDF is at the rear of the plane and is positioned so it only draws in air from the underside. This could well create a reduced pressure area under the rear of the plane causing it to pitch up under when under power.
Installing the under belly duct may cost a bit of thrust but it will at least move any intake low pressure area to ahead of the CofG so eliminating any tendency to power on pitch up.
I have no idea how significant any power on pitch up might be with your layout but I do have a couple of 'jets' that have their EDFs mounted literally right at the back in the exhaust nozzle but they do have long inlets too. There is thus no question of the EDF's air being drawn in under the rear of the plane. .
Just an observation.
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Old 11-15-2022, 07:49 AM
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Originally Posted by quorneng View Post
aaindthu
It is possible that your F16 will actually be more controllable with the underbelly duct in place.
Your EDF is at the rear of the plane and is positioned so it only draws in air from the underside. This could well create a reduced pressure area under the rear of the plane causing it to pitch up under when under power.
Installing the under belly duct may cost a bit of thrust but it will at least move any intake low pressure area to ahead of the CofG so eliminating any tendency to power on pitch up.
I have no idea how significant any power on pitch up might be with your layout but I do have a couple of 'jets' that have their EDFs mounted literally right at the back in the exhaust nozzle but they do have long inlets too. There is thus no question of the EDF's air being drawn in under the rear of the plane. .
Just an observation.
I couldn't thank you enough!! Gluing the inlet duct right away. 😊
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Old 11-16-2022, 07:31 AM
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So I glued the inlet pipe underneath the fuselage and I was not very happy with it. The thrust was so diminished even at full throttle. The motor seemed to have low thrust from the beginning even without the inlet duct, the thrust was just enough. So I removed all the inlet pipe.

I figured that I could just cut open cheat holes but on top side of the EDF/fuselage. And it worked. The pitch up moment got reduced to very negligible amount. This is how I sense the pitch up moment - I suspend the airplane nose down holding the leading edges of the wings on both my index fingers. The airplane will sway freely like a pendulum but forward and backward. Using my toes, I give throttle and the airplane did have pitching moment before I cut out the cheat holes, so much that it could pivot the airplane into a horizontal and level position. But after the cheat holes, it doesn't pivot that much. Just 10 or 15 degrees, just guessing, from the nose-down hanging position. Here is a picture of the cheat holes, it's on both sides of the fuselage.
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I am all set for maiden flight. Thanks a lot to Quorneng for pointing it out in the first place !!!
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Old 11-16-2022, 12:06 PM
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aaindthu
Removing the inlet duct and adding top side cheat holes looks like a good way to maintain thrust and achieve a better balanced inlet.
I do wonder if the cheat holes are big enough.
It can be useful to actually measure the thrust by carefully holding the F16 nose down on a set of scales and applying full power.
It doesn't have to be super accurate but it can give an idea of the ratio of thrust to the panes weight.
If the thrust is much below 50% of the planes weight as an F16 it will struggle to fly. 70% or more thrust to weight would be better.
Hope the maiden goes well.
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