RV4 max flap deploy speed…aircraft airspeed vs. prop wash speed?

[Kooshball]

I have been reading about folks with a cracked flap torque tube in their -4s on this forum and there seems to be a consensus that over speeding the flaps is a contributing factor.

I may well find a crack in my torque tube when I get back to the hangar but I am religious about getting well into the white arc before dropping the flaps but…I have recently been practicing emergency go-arounds in preparation for short field / tall obstacle operations. In this scenario I made a normal approach (full flaps), flared to land, then went full throttle to go-around while pulling up the electric flaps (which takes time to retract). Is there a chance that the full throttle prop wash on the flaps for those seconds could have put more force on the flaps than they were designed for despite my aircraft airspeed being in the 65kt range?

Thx!

 

[StuBob]

File this under, “Problems; Inventing” or “Things; Overthinking; Way.”

 

[Kooshball]

File this under, “Problems; Inventing” or “Things; Overthinking; Way.”

Really?

 

[Bill Boyd]

Two things I'd like to see: a pitot mounted inside the prop arc to measure the airspeed the flap sees in this scenario, and an engineering reply from the Mothership once you have that data.

If this is a pervasive fleet issue on -4's, I think your question is legit.

 

[swjohnsey]

Really?

How often do you plan to do this? I fly a RV-4 with electric flaps. I think it takes about 6 seconds for my flaps to get from full down to full up, around 4 seconds to get to take-off, around 17 degrees. My DMMS is 84 mph VREF is 70. Will your RV-4 accelerate beyond flap speed in 6 seconds? I usually take-off with 17 degrees of flaps but normally at around 75% power, 25 x 25, no problem with flaps so far at 3,000+ hours.

 

[fixnflyguy]

After flying my -4 I built with electric flaps for 15 years now, I can count on one hand the number of times I've had to do a go around (with full flaps). I don't see it as a cyclic fatigue scenario. In those go around I'm off the full flap stop in seconds as they come up, so it's really only seconds. I have mine set up HOTAS with button on stick, so I need not take my focus off the climb while retracting. I know there have been a couple weldment that have cracked, but we may never know the cyclic based use/abuse they have sustained unless the owners speak up to their flap usage.

 

[rocketman1988]

Two things I'd like to see: a pitot mounted inside the prop arc to measure the airspeed the flap sees in this scenario, and an engineering reply from the Mothership once you have that data.

If this is a pervasive fleet issue on -4's, I think your question is legit.

I think a true analysis would be a bit more involved. You would need to account for the spiraling slipstream and an asymmetric load on the flaps. Maybe someone with Solidworks talent could model it...

While an interesting question, in reality I would be surprised if it was even an issue.

 

[Roadjunkie1]

Love my manual flaps! Probably one second from full landing flaps (40 degrees) to go around configuration which is half flaps, departure setting (20 degrees), then fully retracted. I retract them 'slowly' so as not to change the transfer from landing configuration to departure too quickly. Go around is full throttle so there is a little time (3-4 seconds perhaps) to make the transition. I have overflown the flaps a few times, usually departure and only for a few oh, seconds. All my departures are full throttle, mostly at 5200 MSL or higher. O-320 160hp, wooden prop. Torque tubes pristine after 1200 hours. Is this a common problem with -4s or any RV? I feel a survey coming on.........

 

[Kooshball]

Ok folks, I have not found any cracks on the tube or the welds but have found wear marks on both flaps which indicate where the flaps actually deploy to while in flight (pictured). It seems that the torque tube is twisting when it has load on the flaps which pushes the flaps back several degrees.

Would this be typical of the design and performance of this kind of setup?

 

[fixnflyguy]

Ok folks, I have not found any cracks on the tube or the welds but have found wear marks on both flaps which indicate where the flaps actually deploy to while in flight (pictured). It seems that the torque tube is twisting when it has load on the flaps which pushes the flaps back several degrees.

Would this be typical of the design and performance of this kind of setup?

Mine don't move in flight during loading as you suspect yours are. Are you sure that's not just a high spot in the skin curvature? The leading edge is formed by brake bumping and not always a smooth radius. When built correctly, the leading edge of the flap is always tight against the wing skin and yours may be rubbing a little harder at that point.

 

[rvbuilder2002]

Ok folks, I have not found any cracks on the tube or the welds but have found wear marks on both flaps which indicate where the flaps actually deploy to while in flight (pictured). It seems that the torque tube is twisting when it has load on the flaps which pushes the flaps back several degrees.

Would this be typical of the design and performance of this kind of setup?

Yes
There is some elasticity in the system.
You can stimulate this effect on the ground by pulling aft/up on a flap with them fully deployed
The flaps will move slightly from air loads.
This is even more pronounced on the side-by-side airplanes because the torque tube is much longer.

 

[rag]

Those of us who have manual flaps can feel the high air loads that are on these flaps. We should be very concerned about not exceeding the flap speeds. I worry about the flap linkage failing on one side - which (I think) is not recoverable. I use 85 kts as a maximum for first notch. I don’t add full flaps until I’m under 75 kts. I don’t use flaps on takeoff. Retracting flaps during go-arounds are a concern for me. I actually don’t go to full power until I have the flaps retracted. Other RV4 pilots have their own procedures/ideas - this is just my way to minimize the risk of a single flap failure.

 

[swjohnsey]

Has there ever been a single flap failure on any RV?

 

[gasman]

With manual flaps it's easy enough to set up for one flap operation and find out if the aircraft is controllable
as you add only one flap manually.

Report back your findings.

 

[plaviation]

Ok folks, I have not found any cracks on the tube or the welds but have found wear marks on both flaps which indicate where the flaps actually deploy to while in flight (pictured). It seems that the torque tube is twisting when it has load on the flaps which pushes the flaps back several degrees.

Would this be typical of the design and performance of this kind of setup?

Kooshball, I am having a similar issue on my -4. I did discover a small crack in the torque tube by the old manual flap weldment but I’m still having airload twisting issues even after repairing it. Starting to think the crack was caused by the twisting and not the other way around.

My -4 has the electric flap motor mounted on the left side of the cockpit by the rear passenger. The only thing holding the torque tube are two Delrin(?) blocks. The left side flap is held firmly by the actuator, but there is really nothing to react a twisting load on the right side. On the ground, my left flap is very rigid, but I can lift the right flap by as much as 10-15deg. In the air, the problem is way worse and causes a substantial rolling moment. I’ve attached a couple images of my setup but I’m at a loss as to what to try next.