What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

RV8A and Anti Splat Cowl Flaps Spin Tendencies

Majorpayne317641

Well Known Member
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:
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
 
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.
 
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.
 
[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.

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
 
Last edited:
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...
 
RVSM runs from 29K-49K', not 18K. And yes, RVSM aircraft do have critical areas around static ports.
 
s
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...
OK. Cool (no pun intended) and surprising it makes that much difference. In general if you are totally coordinated at stall the break should be more or less without massive drop off, unless the plane is out of rig. Not saying your plane is out of rig, but apparently your cowl flap produces yaw? Yet your ball is center? Also re reading your OP you are 50 degree nose up? Really read the Standards for Commercial Pilot. You should not need to be at that pitch. Start at a slow speed and raise nose slowly for a bleed off of 2 kts a second. If you yank it vertically you are doing an aerobatic maneuver not a power on stall. RV's do have lots of power and low stall, and at full power may not stall at all. You have to cheat it into a stall.

If you pitch real slow it will stall at a much lower pitch attitude than 50 degrees, way lower. It may not stall at all!!! CFI's who only have only flown in lower performance planes like Piper Arrow and never flown an RV-8 don't understand the RV's characteristics. It does not make them bad a CFI, but what works in a Piper Arrow is not going to work in a RV-8. A high powered RV doing a power on stall from cruise is going to result in excessive pitch up, unless your rate of pitch change is real real slow. Even then it may not stall but just mush (sink). You have to initate stall recovery when you have zero ROC. If you pitch up too fast you are doing a "wing over" or humpty bump. This is why there is the tendency to hurry this maneuver and pitch up fast to WING OVER land to force a stall that is not coming, because an RV takes too long to slow down and the maneuver will take way too long. That is why you start at slow speed, pitch up quickly at first so speed does not build. Then pitch more slowly at a rate of 1 to 3 kts a seconds. The key is start the power on stall maneuver at slow flight speed and don't let it accelerate when you add full power, This way the "stall" will not take too long. Pitch control is important. Learn what your pitch attitude is when it starts to mush or stall power (at min pitch), but 50 degrees is way too much IMHO. You can finesse this maneuver but ham handedness will result is weird and less than pretty maneuver. That finesse is what COMMERICAL rating is to teach, precise pitch, bank and yaw control though out the total flight envelope. The reason RV's do not stall easily is the airfoil NACA 23013.5, high power weight ratio, effective controls, so the power on stall is power on sink. You have to just break the "stall" when your ROC goes below ZERO. It still may not have "stalled" but going to excess PITCH to get a real stall break (that may not come), is really just low airspeed sink or an impromptu aerobatic maneuver. I don't understand why your plane does the right roll/yaw with cowl flap down and not up up. However your 50 degree pitch seems excessive and your cowl down issue may go away if you follow some of my suggestions. Just my opinion.

I don't know if Van's Aircraft will care, but if you get a chance you should reach out to them, one of the managers, not whomever answers the customer service number. I find over decades Van's can be a little dismissive of criticism or suggestions.

It would be goodness and interesting to repeat this experiment (one of the pillars of science, repeatable experiments) with another RV-8 with same cowl flap. I have very few RV-8 hours, but over 1000 hrs in the RV-4. Two people made the RV-4 a very different plane. Solo I was near forward CG limit. Two people could push it to aft limit. My girlfriend is tiny so we could fly two up and bags, although some baggage was in cockpit for long x-country camping trip. RV-8 is similar but better with the fwd baggage area. If I were doing maneuvers or aerobatics two up I would put ballast in Fwd baggage to drive the CG Fwd. This typically improves yaw stability and spin recovery.

Does it do this solo? Has any one spun the RV-8 with aft most CG and determined spin recovery response?

RVSM runs from 29K-49K', not 18K. And yes, RVSM aircraft do have critical areas around static ports.
Who said RVSM starts at 18,000 feet RVSM. :rolleyes: Anyway my RV goes up to FL410, at Mach 0.82. Oh wrong plane. ha ha. Thanks for the correction. The point was in response to surface finish on the spinner being a factor. My opinion is it will have nil effect, as the prop beating the air to death has the biggest effect on aircraft aerodynamics and control (P-factor, Torque, Slip Stream Spiral and Gyroscopic Precession) or LEFT TURNING tendencies. The fact he is rolling/yawing right and he thinks it is the cowl flap, not sure. I think is in my bold dissertation above.
 
Last edited:
You are right that an RV8 power on stall is hard to do. Easing into the power on stall is what I would do each time. Max power and slowing pulling nose high, it takes almost 50 degrees to get the plane to start to slow to a stall speed. The instructor I have built and flies his own RV8. He has a few hours in those aircraft. He also said you need to cheat a little to do a power on stall or any other maneuver for the commercial as this airplane is over powered to do them according to the ACS. For example the lazy 8s, he demo'd a lazy 8 as it was done in a t37, vastly more aggressive than what the FAA wants, but the rv8 handled it so well that it actually was easier to do. Either way, whatever is going on would require testing, with strings, tape and go pros. I don't have the time to flush it out other than to be aware of it. The plane is not mis rigged, it is straight and it flies straight. All the suggestions you have make sense and was exactly what my instructor already brought up. I just didn't want to bore everyone with those details.
 
Could not duplicate

I have two cowl flaps on my -8, on the bottom, either side of the 4 pipe exhaust.

I tried to duplicate this yesterday beginning from about 75kts, pitch up, smoothly applying full power, wings level, straight ahead departure stall.

The first two times I had a boot full of right rudder to keep the ball centered, perhaps even maxed (felt like it anyway). I had a normal straight ahead stall break around 50kts IAS. I recovered immediately.

The third time I had significant right rudder but probably not ball centered. At the stall break around 48 kts IAS the right wing dropped but not much at all. Again... immediate recovery.

I suppose there are many differences that might affect this (nose gear, louvers, scoop, etc.)

Nonetheless it was a fun exercise.
 
I have two cowl flaps on my -8, on the bottom, either side of the 4 pipe exhaust.

I tried to duplicate this yesterday beginning from about 75kts, pitch up, smoothly applying full power, wings level, straight ahead departure stall.

The first two times I had a boot full of right rudder to keep the ball centered, perhaps even maxed (felt like it anyway). I had a normal straight ahead stall break around 50kts IAS. I recovered immediately.

The third time I had significant right rudder but probably not ball centered. At the stall break around 48 kts IAS the right wing dropped but not much at all. Again... immediate recovery.

I suppose there are many differences that might affect this (nose gear, louvers, scoop, etc.)

Nonetheless it was a fun exercise.

Thanks for that, I am sure each aircraft will be a little different because of placement of the cowl flaps, w&b, and configuration differences of all of our experimental. The only way I think this could be analyzed is with instrumentation.
 
Back
Top