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  #1  
Old 07-04-2022, 06:57 PM
Majorpayne317641 Majorpayne317641 is offline
 
Join Date: Sep 2021
Location: Goldsboro, NC
Posts: 89
Default RV8A and Anti Splat Cowl Flaps Spin Tendencies

For anyone who uses cowl flaps like me on my RV8A, be aware of an unknown induced spin tendency my instructor and I discovered in my plane.

I am practicing for my commercial today, the instructor began to demo a power on stall from the back seat, being a hot day I elected to leave my cowl flaps open for the air work. At about 60kts 50ish degrees nose high the aircraft rapidly breaks for a right spin. We tried it again this time me in the front seat, it did the same thing. He at first was puzzled because his 8 does not do this. I then realized I had my cowl flaps open and decided to try with them closed. The following power on stall maneuver was a predictable straight ahead break into a stall.

This is not uncommon in supersonic aircraft, as the first few inches of the nose is checked for any defects greater than a particular size. This affects maneuvers at high AOA. I suppose the RV8A has this same tendency if you have laminar airflow along the bottom of the cowl disturbed by open cowl flaps.

I have a IO360M1B fixed pitch sensenich prop set to a cruise pitch. Everything else is pretty much a standard RV8A aside from the cowl flaps.

I will change my immediate action items to reflect checking cowl flaps closed in a spin recovery. I will also be mindful of open cowl flaps on takeoff, 60kts is way below Vx/y but it's much higher than the normal power on stall. I wanted the rest of the community to be aware of our discovery in my aircraft today.

Last edited by Majorpayne317641 : 07-04-2022 at 08:42 PM.
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  #2  
Old 07-05-2022, 07:04 AM
maxmirot maxmirot is offline
 
Join Date: Feb 2022
Location: Hereford
Posts: 58
Default

Quote:
Originally Posted by Majorpayne317641 View Post
For anyone who uses cowl flaps like me on my RV8A, be aware of an unknown induced spin tendency my instructor and I discovered in my plane.

I am practicing for my commercial today, the instructor began to demo a power on stall from the back seat, being a hot day I elected to leave my cowl flaps open for the air work. At about 60kts 50ish degrees nose high the aircraft rapidly breaks for a right spin. We tried it again this time me in the front seat, it did the same thing. He at first was puzzled because his 8 does not do this. I then realized I had my cowl flaps open and decided to try with them closed. The following power on stall maneuver was a predictable straight ahead break into a stall.

This is not uncommon in supersonic aircraft, as the first few inches of the nose is checked for any defects greater than a particular size. This affects maneuvers at high AOA. I suppose the RV8A has this same tendency if you have laminar airflow along the bottom of the cowl disturbed by open cowl flaps.

I have a IO360M1B fixed pitch sensenich prop set to a cruise pitch. Everything else is pretty much a standard RV8A aside from the cowl flaps.

I will change my immediate action items to reflect checking cowl flaps closed in a spin recovery. I will also be mindful of open cowl flaps on takeoff, 60kts is way below Vx/y but it's much higher than the normal power on stall. I wanted the rest of the community to be aware of our discovery in my aircraft today.
Thanks. I just put this in yesterday on my RV7A.

I imagine this will cause some yaw since the flap placement is off the center line.

I think the effect will likely vary depending on the model and the flap position.

I will test this soon and report my findings

Max
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  #3  
Old 07-05-2022, 11:59 AM
Majorpayne317641 Majorpayne317641 is offline
 
Join Date: Sep 2021
Location: Goldsboro, NC
Posts: 89
Default

I have two and both were open. I never tried opening each one separately and testing. I wondered if that would give a different response. Mine are symmetrically installed on either side of the main exhaust cowl.
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  #4  
Old 07-05-2022, 08:21 PM
Maverick972 Maverick972 is offline
 
Join Date: Sep 2017
Location: Plano, TX
Posts: 134
Default

Thank you for your post, I also have the Antisplat cowl flaps on my 7 and have experienced similar results.
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  #5  
Old 07-06-2022, 06:53 AM
maxmirot maxmirot is offline
 
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Location: Hereford
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I am surprised that 2 symetric flaps would yaw
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  #6  
Old 07-06-2022, 08:50 AM
fixnflyguy fixnflyguy is online now
 
Join Date: Jan 2006
Location: Winston-Salem, N.C.
Posts: 1,566
Default Another thing..-8's/buffet from gear legs

I don't have an -8 or 8A, however, there have been many conversations about the gear legs and wing root area turbulence on -8's, such that some have been fitted with a curved strake at the fuselage just forward of the leading edge to prevent it. I don't recall spin tendencies, but do recall stall buffeting being a side-effect. Is it possible the cowl flap is creating the same phenomenon ? . Since the 8A has a different main gear position, I don't think the 8A ever had the gear/L/E buffet issue experienced by some -8 owners.
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  #7  
Old 07-07-2022, 04:21 AM
Majorpayne317641 Majorpayne317641 is offline
 
Join Date: Sep 2021
Location: Goldsboro, NC
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It's not so much that they cause a yaw, the propeller does that all on its own with the P factor. You now make that worse by being at full power, right rudder input, nose high and slow flight.


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I am surprised that 2 symetric flaps would yaw
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  #8  
Old 07-13-2022, 06:48 AM
gmcjetpilot's Avatar
gmcjetpilot gmcjetpilot is offline
 
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Quote:
Originally Posted by Majorpayne317641 View Post
[SNIP]

"....practicing for my commercial today,...." ; "power on stall...cowl flaps open for the air..... worabout 60kts 50ish degrees nose high the aircraft rapidly breaks for a right spin." "We tried it again this time me in the front seat, it did the same thing......decided to try with them closed. The following power on stall maneuver was a predictable straight ahead break into a stall."
The only thing I can think of is the cowl flap is producing some kind of YAW to the right when open. Where is the SLIP SKID ball when you stall. TRY it with more left rudder and cowl flap open. It should not break into an incepted spin.

How many turns in spin did you do? Or did you just break and recover from an incept spin before developed? I assume you did not allow it to develop into a fully developed spin which takes a turn or two or three. My thought is if the cowl flap has this much effect then what does it do to recovery from fully developed SPIN? It is easy to recover before the spin is developed, less so wonce developed. I say easy to mean quickly recover. Once fully developed the inertial and aero forces can make recovery slower and in some planes near impossible. RV8 spin characteristics I don't know. RV7 was a bit of challenge and why VAN put the larger rudder on it.

There is no rating that requires spins except CFI. I am a CFI. I don't know if you plan on doing your commercial in your RV but your Idea of making sure cowl flap is closed is a good one. However the reason it is causing the plane to start to autorotate (spin) in a full stall is odd, but my guess is there is YAW force imparted by the cowl flap. LOOK AT YOUR SLIP SKID BALL with and without cowl flap both low AOA and high AOA. The COWL flap will REDUCE the effectiveness of the rudder possibly but the rudders on RV's are large and more deflection with your feet should compensate.

Quote:
This is not uncommon in supersonic aircraft, as the first few inches of the nose is checked for any defects greater than a particular size. This affects maneuvers at high AOA. I suppose the RV8A has this same tendency if you have laminar airflow along the bottom of the cowl disturbed by open cowl flaps.
I fly jets and the nose (radome) are beat to death. In a another life I was a Boeing Engineer and worked for Airlines on Maintenance side. Chips and damage on nose has zero, no affect on performance or high AOA aerodynamics. The only issue is the composite repairs must be transparent to radar. Metal repair tape (speed tape) is a no-no on radome. Small damage on outer plies may be allowed in places, but non metallic tape to keep moisture out is required. The only area needing to be flat and without defect is around the Static ports or the pitot static system. Surface around static ports were not critical in past. Now with RVSM requirements where planes are separated by only 1000 ft (pre RVSM)) vs 2000 ft (now) vertically above FL290 (edit not FL180) this area has very critical standards. RVSM stands for Reduced Vertical Separation Minimum.

THEY DON'T CALL THEM EXPERIMENTAL AIRCRAFT FOR NOTHING
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Last edited by gmcjetpilot : 07-24-2022 at 07:07 AM.
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  #9  
Old 07-23-2022, 02:37 AM
Majorpayne317641 Majorpayne317641 is offline
 
Join Date: Sep 2021
Location: Goldsboro, NC
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Quote:
Originally Posted by gmcjetpilot View Post
The only thing I can think of is the cowl flap is producing some kind of YAW to the right when open. Where is the SLIP SKID ball when you stall. TRY it with more left rudder and cowl flap open. It should not break into an incepted spin.

How many turns in spin did you do? Or did you just break and recover from an incept spin before developed? I assume you did not allow it to develop into a fully developed spin which takes a turn or two or three. My thought is if the cowl flap has this much effect then what does it do to recovery from fully developed SPIN? It is easy to recover before the spin is developed, less so wonce developed. I say easy to mean quickly recover. Once fully developed the inertial and aero forces can make recovery slower and in some planes near impossible. RV8 spin characteristics I don't know. RV7 was a bit of challenge and why VAN put the larger rudder on it.

There is no rating that requires spins except CFI. I am a CFI. I don't know if you plan on doing your commercial in your RV but your Idea of making sure cowl flap is closed is a good one. However the reason it is causing the plane to start to autorotate (spin) in a full stall is odd, but my guess is there is YAW force imparted by the cowl flap. LOOK AT YOUR SLIP SKID BALL with and without cowl flap both low AOA and high AOA. The COWL flap will REDUCE the effectiveness of the rudder possibly but the rudders on RV's are large and more deflection with your feet should compensate.

I fly jets and the nose (radome) are beat to death. In a another life I was a Boeing Engineer and worked for Airlines on Maintenance side. Chips and damage on nose has zero, no affect on performance or high AOA aerodynamics. The only issue is the composite repairs must be transparent to radar. Metal repair tape (speed tape) is a no-no on radome. Small damage on outer plies may be allowed in places, but non metallic tape to keep moisture out is required. The only area needing to be flat and without defect is around the Static ports or the pitot static system. Surface around static ports were not critical in past. Now with RVSM requirements where planes are separated by only 1000 ft (now) vs 2000 ft (pre RVSM) vertically above FL180 this area has very critical standards. RVSM stands for Reduced Vertical Separation Minimum.

THEY DON'T CALL THEM EXPERIMENTAL AIRCRAFT FOR NOTHING
I did not go into a full spin. The ball was dead center each time including when my instructor tried.

Sorry the second comment is not correct for fighter jets...
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  #10  
Old 07-23-2022, 07:10 PM
smt6 smt6 is offline
 
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Location: SoCal
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Quote:
Originally Posted by gmcjetpilot View Post
I am a CFI.
Quote:
Originally Posted by gmcjetpilot View Post
Now with RVSM requirements where planes are separated by only 1000 ft (now) vs 2000 ft (pre RVSM) vertically above FL180 this area has very critical standards. RVSM stands for Reduced Vertical Separation Minimum.
Oh really Mr. Jet Pilot? Do educate us.
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