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Freewing F-16 Cheater Hole

Old 05-26-2020, 08:42 AM
  #1  
Suprawill1
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Default Freewing F-16 Cheater Hole

Hi All,
Couldn't find a current thread on this.

I have the FW 70mm F-16 V1. It has a cheater hole right between the main gear on the bottom. It's just a rectangular 3 x 2.25 opening. It would appear that it would disrupt the natural tubular airflow by creating turbulence right in front of the EDF. It seems that the front intake and airflow routing is large enough wherein a cheater isn't needed but not sure.
Has anyone had experience in doing an A/B comparison on an F-16 and concluded which scenario is better?
If using the cheater is better, would it be advantageous to create an air scoop over the hole to direct the air into the fuse rather than have it flow across the hole? It doesn't seem like a well planned air entrance once the F-16 is airborne.
Your thoughts?

Will
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Old 05-26-2020, 10:12 PM
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solentlife
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Photo ?

My Lanzian F16 70mm has no cheater holes .... just relies on a slightly oversized front intake.

If a cheater hole was to be added to avoid a scoop being obvious - I might consider cutting it in the form of a NACA Duct ... same as race cars had in the bonnets ...
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Old 05-27-2020, 12:00 AM
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Suprawill1
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Thanks for the hit solentlife!
Unfortunately, this hole is right on the duct work so I'll have to keep the scoop external, much like the pic I attached.
I'm going to do an A/B thrust check to see if I even need a cheater. It's a 4S to 6S upgrade so it has plenty of power.
Other pics show its position and the inlet and outlet of the EDF duct.
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Old 05-27-2020, 01:00 AM
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Ok, I just had the opportunity to do an A/B check with and without the cheater hole. It's a close race. Without the cheater hole, it peaked out at 3.85lbs of thrust.
With it, it peaked at 4.2lbs. After this upgrade, the F-16 gained almost a pound. Went from 2.24lbs stock to 3.16lbs. Still a good thrust to weight ratio.
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Old 05-27-2020, 03:25 PM
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quorneng
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Whilst a static test gives an indication it does not necessarily reflect what is happening when in flight.
If the inlet duct is 'adequate' the forward velocity of the plane (faster the better) will tend to negate the losses from the inlet duct so a cheat hole may actually make little difference to performance except at the critical launch/climb out phase.
In flight an external scoop will increase the frontal area and thus the drag so tending to negate any EDF airflow advantage. The overall benefit of a scoop will depend on the efficiency (or inefficiency!) of the inlet ducting.

I am not in favour of cheat holes so I tend to size the EDF to match the inlet area, concentrate of the form of the inlet duct to minimise inlet losses and build to achieve a plane weight to suite the chosen EDF.
It works for me!
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Old 05-27-2020, 09:10 PM
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Suprawill1
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Originally Posted by quorneng View Post
Whilst a static test gives an indication it does not necessarily reflect what is happening when in flight.
If the inlet duct is 'adequate' the forward velocity of the plane (faster the better) will tend to negate the losses from the inlet duct so a cheat hole may actually make little difference to performance except at the critical launch/climb out phase.
In flight an external scoop will increase the frontal area and thus the drag so tending to negate any EDF airflow advantage. The overall benefit of a scoop will depend on the efficiency (or inefficiency!) of the inlet ducting.

I am not in favour of cheat holes so I tend to size the EDF to match the inlet area, concentrate of the form of the inlet duct to minimise inlet losses and build to achieve a plane weight to suite the chosen EDF.
It works for me!
Yes, I kind of though that it would unload and get a forced air induction at speed. I'd rather have the cover on the cheater hole for 2 reasons.
Being where it is located, I believe it is creating unneeded turbulence right in front of the EDF fan.
Secondly, I think it breaks up the continuity of the aesthetics.
In the A/B testing, I can make a case that there was a marginal difference. I will still do an A/B testing in flight. Probably won't experience a seat of the pants difference.

If I was involved in the design of this model, I could adhere to EDF matching to inlet sizing but unfortunately, this was already done.
I did modify the power and therefore the weight specs but chose to have power over efficiency. Once I finally find the time to get this thing in the air, I'm sure I'll enjoy what it's got to say. Bought it 3 1/2 years ago and still hadn't flown it. :-o
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Old 05-27-2020, 09:35 PM
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solentlife
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Originally Posted by Suprawill1 View Post
Ok, I just had the opportunity to do an A/B check with and without the cheater hole. It's a close race. Without the cheater hole, it peaked out at 3.85lbs of thrust.
With it, it peaked at 4.2lbs. After this upgrade, the F-16 gained almost a pound. Went from 2.24lbs stock to 3.16lbs. Still a good thrust to weight ratio.
With those figures ... personally I would dump the cheater hole or at least forget about scoop.

Adding that extra pound of weight will affect the flight speed envelope, even though the overall weight is below thrust levels. You will have a higher stall speed ... higher landing speed .... higher speed to obtain lift off ....

IMHO - get that extra weight off and fly at lower weight ...............
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Old 05-27-2020, 09:59 PM
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Yes, I will be dealing with a higher stall speed. Unless I go back to the stock setup, the weight will have to remain.
An RC Lander 70mm aluminum 10 blade was installed. That effectively chanced the power requirement from a 4S 2500mah
to a 6S 3200mah. I also had to exchange the stock ESC from a 45A to 100A.
I've always flown fast and can't get over it.
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