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Checking trim before first flight

M

Mich48041

Well Known Member
Friend
For those about to conduct the first flight, the elevator trim tab should NOT be set in the middle. It should be set for nose down, perhaps 1/4 to 1/3 up from the bottom of the EFIS display.
Also, the stall warning horn has not worked at first on some RV-12s until the vane has been adjusted using trial and error after each test flight.
Joe Gores
 
This is a VERY IMPORTANT POST about determining trim setting for takeoff! Unless things have changed, there is no such good, specific advice in the RV-12 plans. http://www.vansairforce.com/community/showpost.php?p=736012&postcount=22
Thanks go to Jetguy and Rdog. I got and used this guidance from them in my transition training, before my own first flight, which was thankfully exciting but uneventful.

A really bad trim setting on the first flight could be a factor in an accident. It would be interesting to hear from Vans about what it is like to do take offs with both full up and full down trim. Of course, on such tests you know to expect abnormal stick forces. Perhaps a new thread would be appropriate for that.
 
My wording was careful; I am aware of the speculation issue and agree with VAF policy on avoiding that. Earlier posts had opened the topic of trim in this thread. In any case, a new thread by RVbuilder2002 on the topic would be most welcome - his postings are always full of useful and knowledgeable advice. Particularly, is it (still) correct that the plans do not have a method for determining the initial first-flight setting for takeoff trim comparable to the photo method posted in http://www.vansairforce.com/community/showpost.php?p=736012&postcount=22. ?
 
For what little it's worth I set my Dynon reference mark at 1/2 travel and found that slightly nose down trim produces 75 KIAS climb with 1 notch of flaps.
 
Y'know, years ago, it was customary to first do "bunny hops" on initial flights. We'd accelerate, lift off and fly down the runway for a bit, throttle back and land. Seems to me that there may well be some merit to doing that. You'd discover pretty quickly where your trim is set at and end the flight.

Opinions?
 
pierre smith said:
Y'know, years ago, it was customary to first do "bunny hops" on initial flights. We'd accelerate, lift off and fly down the runway for a bit, throttle back and land. Seems to me that there may well be some merit to doing that. You'd discover pretty quickly where your trim is set at and end the flight.
Click to expand...

Oh that makes extremely good sense to me Pierre. My dear departed Dad & I would fly to airshows years ago to watch Bob Hoover perform. I have videos about the man and his flying.

If you will note, when he takes off an air plane, he will get airborn, then fly almost the entire length of the runway about 20 to 30 feet high, before climb out. Hoover is very nice and approachable. We were in Harlingen, TX, I believe 1975 and I was getting his autograph and asked about his long takeoff run. He looked me in the eye grinning and said: "I wanna make sure its gonna work" and went back to writing his name. Take a look at him in the P-51 or the Shrike. He will lift off and hug the runway until you, the spectator, is wondering, is he ever going to climb????

To this day, I lift off, get a few feet and do another really abbreviated test of the controls. I've successfully aborted some takeoffs if detecting "something not right."

Of course, I'm not talking about doing that when getting out of a cow pasture.

And for those younger who may not be well acquainted with Bob Hoover. He was a highly regarded test pilot who survived just a butt load of accidents.

Sorry for the long drift; but Pierre is spot on there I'm a thinking.

Happy & safe flying to all,
 
pierre smith said:
Y'know, years ago, it was customary to first do "bunny hops" on initial flights. We'd accelerate, lift off and fly down the runway for a bit, throttle back and land. Seems to me that there may well be some merit to doing that. You'd discover pretty quickly where your trim is set at and end the flight.

Opinions?
Click to expand...

Pierre,
I realize logic may seem to make this a good choice, but statistics have proven that it is not something we should be promoting. There was even a fatal RV accident within the past year or so that occured while doing fast taxi / short hops.

Far too often this results in airplanes running off the end of runways or worse (see above).
A pilot of limited experience (in the airplane model anyway), flying in close proximity to the ground, in an airplane that may be out of trim, is not a good combination.
 
rvbuilder2002 said:
A pilot of limited experience (in the airplane model anyway), flying in close proximity to the ground, in an airplane that may be out of trim, is not a good combination.
Click to expand...

I gotta agree with Scott on this one, I would much rather be at two mistakes high, than at 1/2.
 
I know Vans, Scott, and Mike Seager have writings that advise against Bunny Hops, but I'm very much with you, Pierre. That's how many famous airplanes made their first flights - some even unintentional - and that's how Boeing (sort of) does it today.

On an all new airplane, they do runway runs at increasing speeds, testing the brakes, the flight controls, the thrust reversers. Eventually, they get into quite high speeds and rotate the airplane, first a little, then a lot - rolling along with the nose in the air for a substantial distance, then gently lowering the nose gear back to the runway, and performing a stop. If you look closely at one of the 787 rotations, you might even be able to see daylight under the MLG tires - which generated quite some speculation about whether or not this was indeed the unofficial "First Flight."

Tony LeVier's First Flight of the U-2 was just such an unintentional first flight.

If you're ever around Renton, Washington - you can watch production 737s make their First Flights - they make two/day. The airplanes normally takeoff to the North over the Lake. The crew starts up, taxis to the north end of the runway, apply power and accelerate to take-off speed heading south. At around V1, they reduce thrust to idle, apply reverse thrust and substantial braking, and stop at the South end. Having verified all the critical systems for flight - engines, brakes, reversers - and stab trim - they do a 180 and take-off to the North. (If you watch them do this, BTW, you will see them turn to the right and fly up the East Channel between Mercer Island and the Mainland. After takeoff, you will see the gear come up - and then immediately come back down. Something wrong? No - they make sure the gear will extend so if they have a problem, they will have several hours to troubleshoot the problem, instead of finding out at the end of a three hour flight. It also cools the brakes, heated up during the prior aborted takeoff.)

So - I think your thinking has good company Pierre. An accident that may result from this technique has a better chance of being two-dimensional rather than three-dimensional. You may mess up your airplane, tear the gear off, etc, but that beats diving into the ground. You can learn a lot about your airplane and gain personal confidence in a reasonably controlled environment.

FWIW, I have made about 140 of these high speed tests on the 737, where a lot was explored without ever leaving the ground (I think!) All were over 100 kts and the fastest reached 144 kts - on an airplane where the weight made 107 kts "flying speed." One bump in the runway or big expansion strip and we would have been flying!

Bob Bogash
N737G
 
I think the point Scott was attempting to make was that the technique may be just fine for a professional test pilot, but probably not wise for the typical RV builder making their first flight. Just because Boeing test pilots do it, doesn't necesarrily mean that the average builder should.

So the question then becomes, how do we educate inexperienced pilots attempting to make their first flight in their newly built aircraft, on how to set and measure their trim to better guarantee that they won't get into an out of trim state on takeoff?
 
There is a huge difference between a professional test pilot make crow hops in a new design, and a pilot with a couple of hours of transition training (or less) in make/model and maybe virtually no experience in flying a wide variety of different airplanes with different handling characteristics (specific training that most all Professional test pilots receive.

As I already mentioned... statistics prove that it is a bad plan for novice test pilots to utilize.

You beat me to it Bob!
 
Scott - is the RV12 trim that much more sensitive than the other models? Even gross amounts "out of trim" can be overcome with the other models if your paying attention and fly the airplane.
 
I built 4 home builts and hopped each one of them. In two cases it revealed significant control issues that were safely fixed before first flight. It would be interesting to see what the real numbers are for accidents with and without hops. Until then all we have is our own anecdotal experience. There are some very competent people on the forum arguing against them, but in my experience hopping is a main part of the test program.
 
Well from someone who has not ever done it, I tend to agree with Bob and Pierre, maybe from old school thinking. I sure intend to do mine that way even though it appears to be ill advised. I am at a 5000 ft paved runway, and feel I would be able to stop before the end of it.
 
Take-off trim and landing trim may be much different. I know it's tough for me to make good landings with the trim all of out whack. I do practice this from time to time.

I vote for take-off and fly the airplane. Maybe get some time in a -12 and see what happens with the trim out of whack before the first flight of your aircraft.
 
What I have found to be best for landing - -

After slowing down to below 83 kts, I put 'flaps' on full, then set trim back to take-off position. Seems to work best to me. You are also ready for the next take-off without re-setting it again. If you don't change the trim, the stick pressure is much higher ( relative of course ).
 
So, it seems like the short hop or not thing is basically a matter of personal preference ------------ do it or do not do as you wish.

However, if the plane has been primed inside, well then ................:confused:
 
Mike, a couple of points made me post the post that I did.

I built a Cassutt F-1 racer in 1973 and it was very sensitive...and fast. Several EAAers suggested the bunny hop, so I did and quickly learned it's feel. I had 4 years of Ag work under my belt by then and was a pretty fair "stick."

Just the other day, if you recall, my -10's trim system went up when I asked for down, because the bellcrank pivot bolt had fallen out. I would have known quickly had it been on the ground, I think, and an aborted takeoff would have been easy. As it turned out, it took almost two hands on the stick to hold the nose down 'til I landed.

I also demo to transition students, at altitude, just how much down force a -10 needs after slow flight and trimmed that way...it surprises all of them.

I'm not necessarily recommending the "bunny hop" method of feeling out an airplane but it has some possibility.

Best,
 
JonJay said:
Scott - is the RV12 trim that much more sensitive than the other models?
Click to expand...

No.
With the exception of the RV-10. Its trim forces can be higher than people are used to in other RV's because it has huge flaps that produce a very large pitching moment and trim change. As a result, the airplane has very powerful pitch trim to counter it.

Flight testing has demonstrated that an RV-12 with full deflection of pitch trim (one way or the other) can be countered one handed. Particularly at lower speeds, such as take-off and initial climb.
 
I first flew my -12 in June of 2010. At that time there was no guidance, of any kind, on where to set take off trim so I just used half way between full up and full down (which turned out to be too nose up). On the first take-off roll the nose came up somewhat suddenly before I expected it to, but gentle forward pressure and a little down trim and it was fine. My father was watching from beside the taxiway and he said he didn't notice that it looked like it came off before I expected it to.

After that I fiddled with different positions and ended up with the same positioning as has been mentioned elswhere.
 
I haven't done a lot of RV transition training but what I've suggested is that the crow hop be avoided. With that said, a couple of runs down the runway can tell you a lot about the controls. For example, starting the run with stick full aft, when does the nose start up? How does that look in relation to predicted stall speed? Can you hold it up and put it down when you want? If you start a run with the stick (aileron) full to the left, at what speed does the airplane start to want to go left? Right? Same speed (that would be good)? At what speed can you use just the rudder with no brakes to get results for directional control. When you sift through that data, does it make sense? How does it compare to the aircraft you used for transition training? Should the data be the same? Different? Why?

None of the above involve crow hopping but can tell a lot about what to expect. So, I don't recommend the crow hop.

Dan
 
You know how you can tell from the radio transmission if a pilot is flying Ed Swearigen's venerable Metroliner?

That incessant "beep beep beep" noise in the background from the trim that the FAA made him install since there was no trim wheel spinning that indicated trim position.

Interestingly, the FAA never made Airbus install that beeper even though the trim on those airplanes is completely automatic....but there is a trim position indicator on the ped panel that qualified as a loophole I guess.

My point? I am not sure I have one.
 
NASA515 said:
A little write-up on this stab trim subject here, Cactus:

http://www.rbogash.com/Safety/autopilots.html

Bob Bogash
N737G
Click to expand...

Read it! Interesting stuff!

After 10 years on the airbus, during the troubles in 2008, I got "downgraded" (my choice - free type rating/wanted refresh my flying skills) to the 737 - even flew some -200 stock. Had to learn how to fly again. Had to learn how to trim again! I was pretty spoiled!

Best comment from an offline jumpseater on a -200 hardball flight...how do you guys know where you are going?!

Relevance to trim...On approach in the summer in the airbus - have to anticipate the up and downdrafts and the autotrim can cause a lot of problems unless you are thinking ahead of the french and english engineers!

On the Boeing its all you and your thumb control (the trim control is on the right side of the yoke)!
 
cactuspilot said:
On the Boeing its all you and your thumb control (the trim control is on the right side of the yoke)!
Click to expand...

Left side, for the Four-striper, John.

You also get a nice trim wheel that spins next to your leg - with a nice handle you can pop out to drive those direct drive cables by hand if the ever have to.

Bob
 
I once had my Cherokee trim cable unspool and jam as I was trying to trim for cruise after climb. I ended up slowing to 80 Kts and used flaps in order to get home with reasonable yoke forces.
 
Interesting approach

The plane is experimental, built by an amateur without factory jigs and tooling, with questionable rigging and no second party critical parameter inspection and acceptance and some here propose a crow hop process with full flight transition functions in a completely untested airplane in a flimsy structure compared to a car within inches of the ground in short runway space at greater than car speeds by someone that has probably not the most current test pilot in the world... what could possibly go wrong?


Bob Axsom
 
Check crimp on trim wires in tail cone.

I have had the trim control fail in one of my earlier flights. The problem was an intermittent connection in the fine wire connectors in the tail. Landing the -12 out of vertical trim, you lose some of the "feel" and must concentrate on attitude and the numbers. It is definitely doable, but in my case, required a lot of attention.

After this experience, I twisted and soldered the 6 wires to the trim assembly, making sure to heat shrink insulate the connections and strain relief with the existing cushioned clamp. I know, I know, not per plans, etc....

Rafael
 
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