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  #1  
Old 04-13-2021, 07:46 AM
BenNabors BenNabors is offline
 
Join Date: Mar 2014
Location: Alabama
Posts: 129
Default RV-4 Flight characteristics near rear CG

Considering transition training in a RV-4. With instructor weight in the rear seat the CG calculates as 66.94 with an allowable range of 58.7 to 67.4 with the initial fuel load. After burning off 12 gallons, it shifts to 67.29.

It is within limits CG limit, but near the aft limit. Should I expect or be aware of any unusual flight characteristic(s)?
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  #2  
Old 04-13-2021, 08:29 AM
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AX-O AX-O is offline
 
Join Date: Jun 2005
Location: SoCal
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Default

You will be pushing forward on landing to keep on glide slope at the correct speed. landing at a faster speed helps the situation as does a wheel landing.

I tell folks often that flying an RV-4 with an "instructor" in the back seat is way more difficult to do than flying solo. Handling qualities get worst on the ground and in flight. I never understood why people jump into the worst case scenario first vice flying a side by side RV, then learning the RV-4 and build confidence.

Good luck. it is doable but please discuss pitch control reversal with your instructor if you have never experience it.
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  #3  
Old 04-13-2021, 10:03 AM
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Jpm757 Jpm757 is offline
 
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As the CG moves aft, you will notice an increase in pitch sensitivity.
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  #4  
Old 04-13-2021, 10:11 AM
Norman CYYJ Norman CYYJ is offline
 
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You will probably run out of trim control also.
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  #5  
Old 04-13-2021, 10:17 AM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta
Posts: 115
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As the others have said, stick forces get quite light when at the aft CG limit. Be very careful when maneuvering to avoid inadvertent excessive Gs.

Decreased stall and spin recovery characteristics present in most light aircraft at aft CG are present, but up to the aft limit should still be positive. Same with longitudinal stability, it will be decreased but should still be positive. EDIT: spin recovery characteristics at the aft CG limit beyond the aerobatic CG limit may be just marginally positive. Don’t spin the plane with a back seater.

Three point landings will be preferred by the plane and you’ll have to keep trim mostly forward in all regimes of flight. Wheel landings are doable but require positive forward stick - be cognizant of prop clearance, particularly with the short geared planes (which you’re probably flying given the use of the old 50” datum).

If you go beyond the aft CG limit, inadvertently or otherwise, you have very little margin before longitudinal stability becomes negative and pitch controls enter a region of reversed command. If this occurs terminate the flight and check your W&B - if your math still checks the plane needs to be weighed as the W&B is proven to be inaccurate.

As someone who also jumped right in the deep end on a -4 with a heavy instructor in the back, don’t let any of this scare you. If you have no RV experience the instructor should keep you in shape as you don’t have any habits that you can get away with at forward CG but not aft.

Last edited by 00Dan : 04-13-2021 at 10:24 AM.
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  #6  
Old 04-13-2021, 11:38 AM
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Roadjunkie1 Roadjunkie1 is offline
 
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Location: Erie, Colorado
Posts: 162
Default Aft CG and trim....

Quote:
Originally Posted by Norman CYYJ View Post
You will probably run out of trim control also.
It depends on how the airplane was built and how the weight is distributed. The heaviest person in my back seat was 195 and I was instantly very aware of the control differences! I had nothing in the baggage compartment (nor should you: tiedowns etc) but never ran out of trim. I've seen -4s that the trim is way forward with just the pilot and baggage. I know of one that would run out of trim regardless of who was in the back seat. As said before, landings will be your more interesting activity. RV's fly with SMALL movements of the stick anyway. Made more evident with aft CG. With a lot of weight in the back, I am a fan of wheel landings to have better control over when the tail is going to quit flying. It's a tail-low landing but not three point. SuzieQ is a fan of wheel landings anyway..... She has the short gear and I have never had problems with prop clearance. The tail has to go WAY up to get the prop..... Pick up the tail sometime with the prop vertical and you'll see how much there is in the real world.....with gear flexing in mind, of course....

Then, when the instructor gets out: WHAT a difference in performance!
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Last edited by Roadjunkie1 : 04-13-2021 at 11:40 AM.
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  #7  
Old 04-14-2021, 07:37 AM
BenNabors BenNabors is offline
 
Join Date: Mar 2014
Location: Alabama
Posts: 129
Default Results of flight

I want to thank everyone for their insight as it was nice to be sensitized to what to be aware of.

The plane flew beautifully. We climbed out and practiced some turns typical of the pattern turns, deployed flaps to learned handling characteristics and again worked maneuvers characteristic of flying the pattern. The one activity I thought was particularly helpful was I flew about 15 minutes back to the airport with full flaps at 80 kts. Flying in that configuration allowed me to dial in my control inputs so that I was not over controlling. Just made everything more natural and intuitive.

Flap speeds: The prior owner said he deployed first notch of flaps at 100 kts, second at 90kts, third at 80kts. I found the forces to be too high. The flaps are really easy to deploy on the ground but not with the flight forces applied at the higher speeds. Much better deploying at 80kts. I also found a crack on the control level on the condition inspection. I think I know why. At what speeds do people generally deploy flaps and what speeds on downwind, base, and final?

It is also impossible to deploy flaps with the back seat passenger's foot against the flap handle. That is a clunky design. Seems like a good opportunity to install an electric flap motor. Is there a standard design out there for electric flaps?

I had a new weight and balance performed on the annual inspection and that gave me confidence of truth in the calculations in making the decision to fly near the aft limit of the CG. I would suggest the same if you are new to the RV-4 and in the same situation of having a first flight with a new instructor near the aft CG. My old weight and balance were nearly identical to the new values, but it was nice to validate.

I did not have any issues with trim control. It was forward of neutral but not at the limit.

I want to thank everyone who provided and answer as to what to be sensitive to. This is a wonderful forum!
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  #8  
Old 04-14-2021, 10:16 AM
00Dan 00Dan is offline
 
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Location: Atlanta
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Quote:
Originally Posted by BenNabors View Post
Flap speeds: The prior owner said he deployed first notch of flaps at 100 kts, second at 90kts, third at 80kts. I found the forces to be too high. The flaps are really easy to deploy on the ground but not with the flight forces applied at the higher speeds. Much better deploying at 80kts. I also found a crack on the control level on the condition inspection. I think I know why. At what speeds do people generally deploy flaps and what speeds on downwind, base, and final?
Most -4s with manual flaps have three settings: 0, 20, and 40 degrees. Per the builders manual, Vfe is 110 MPH for 20 degrees, and 100 MPH for 40 degrees (speeds are indicated).

It sounds like you have a different configuration than standard. I’d figure out what angle of flaps each setting corresponds to. It’s also possible that the previous owner was over speeding the flaps regularly if he was using knots rather than MPH.

All that said, my very early -4 with the standard configuration has a log entry from the builder fairly early in the planes life describing welding a doubler onto the flap handle to address a crack, so perhaps it’s just a problem area.
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  #9  
Old 04-14-2021, 10:49 AM
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Roadjunkie1 Roadjunkie1 is offline
 
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Location: Erie, Colorado
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Default Flaps..........

Quote:
Originally Posted by BenNabors View Post
Flap speeds: The prior owner said he deployed first notch of flaps at 100 kts, second at 90kts, third at 80kts. I found the forces to be too high. The flaps are really easy to deploy on the ground but not with the flight forces applied at the higher speeds.

It is also impossible to deploy flaps with the back seat passenger's foot against the flap handle. That is a clunky design. Seems like a good opportunity to install an electric flap motor. Is there a standard design out there for electric flaps?
I found the strength to pull flaps at 100 to be somewhat difficult/annoying as well and I'm not skinny. I have a doubler on the flap handle but also installed a sliding steel tube that fits around that, that I can slide out when I need that extra leverage and retract to clear the spar when retracting the flaps. It is the length of the "handle" part and doubles the length of that handle. VERY handy (no pun intended!). That extra leverage makes a huge difference. I probably have a picture of that somewhere but it has been years...... I can take a photo of it but my phone doesn't like to download photographs....

Check the attach point of the flap handle with the steel flap mechanism as that can be an area of stress as well. You may have to remove the flap handle to inspect it well. And to put the doubler and extension on!

Electric flaps are OK but is added complexity, weight and a potential failure point. Manual are simple. I have foot wells for the rear seat passenger so feet against the handle is not an issue. Or I say: could you move your foot just a little.....

Happy to hear your flight went well. No trim issues is great news and likely is another indication of a well-built airplane. Now: GO FLY!!!
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Last edited by Roadjunkie1 : 04-14-2021 at 12:00 PM. Reason: Photos
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  #10  
Old 04-14-2021, 02:29 PM
FinnFlyer FinnFlyer is offline
 
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Location: Bell, FL
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Default Electric flap kit plans

https://drive.google.com/file/d/1HTL...ew?usp=sharing
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