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Being out of trim due would produce similar results no?
i have a hard time believing that I was out of c of g with that load.
i have a hard time believing that I was out of c of g with that load.
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Bearhawk 4 Place
IO-540
MGL Odyssey Gen 2 EFIS
ABW 29" Tires
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It wouldn't produce exactly the same result, but it will make it harder to get a sense of how the plane really wants to behave. Once you have it trimmed nicely, you can apply the following rule of thumb.
If it gets really touchy in the pitch axis, to the point of almost being unstable, that is a clear sign that the CG is getting back there. Particularly if it will stay in a medium / steep turn without you holding any back pressure, then you've pushed the limit. If it pulls into the turn for you, that is a real danger sign.
Its impossible to say how far back the CG is from the feel in the stick, but I know I'm near the absolute limit when it gets twitchy.Comment
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I think those are the large tabs? (I don't have mine nearby to verify) Two large tabs combined with the "servo action" of the linkage can get "interesting". If you also happen to have the old thinner trim tab links that can flex under load, then things can get "very interesting"(*).Originally posted by Baloo View Post
When I bought mine and flew it from FL to NH, I had to keep my hand on the stick the entire flight. Any time I removed both hands, the plane would diverge. The plane as a whole was stable, but the elevator was not. All it took was the lightest finger on the stick to make it behave. With Mark's help, I updated the links to the newer stiffer ones and later disabled one of the tabs. The end result is quite reasonable.
* Somewhere (the old yahoo group?) there was a report from someone who had to wrestle a bit of a bucking bronco when their trim tab links flexed in an early test flight causing the elevator to move up and down
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Mr. Kestral is talking about the trim pushrods that were originally 1/4"x.035. They got changed to 5/16" x .035 after a couple guys had problems. Much stiffer. Big radius in the bend is good.
Something builders do not pay enough attention to is how they hook up the trim pushrod. To minimize stick pitch sensitivity - use the closest hole to the elevator bottom on the trim pushrod link with different hole options. And on the aft end where the pushrod attaches to the trim tab - use the lowest hole. How you hook that pushrod up can make a noticeable difference. MarkComment
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I just had a chance to fly it with the new rigging. I am pleased to say that I had ample trim, the airplane flew straighter and get this..... 6kt gain in airspeed. Originally the left and right trim tabs were not symmetrical but they are now. Needless to say I am pleased with the results. You'll see in the photos that the higher TAS was actually recorded 2000ft lower than the lower one so the difference might actually be greater.You do not have permission to view this gallery.
This gallery has 2 photos.Last edited by Baloo; 12-06-2016, 02:10 PM.Bearhawk 4 Place
IO-540
MGL Odyssey Gen 2 EFIS
ABW 29" Tires
Appareo ESG ADSB-Out
Garmin GTR-200 COMMComment
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Glad to hear you've got the trim issue sorted.
I would tend to be dubious about a 6kt airspeed gain, especially from such a minor change in drag profile. My experience has that even tripling the size of the tires doesn't change the airspeed by 6kts, and that is a much larger drag increase. If I had to guess, I would say something else is probably causing the difference - or maybe is was a coincidence.
You would need to do repeditive testing to establish whether any airspeed gain has occurred.Comment
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Way to burst my happy bubble! HahahaBearhawk 4 Place
IO-540
MGL Odyssey Gen 2 EFIS
ABW 29" Tires
Appareo ESG ADSB-Out
Garmin GTR-200 COMMComment
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Hahaha - Yeah I know, sorry about that!
Some days my plane just seems to be fast, other days not so much. Especially when I am loaded up, flying at mid-levels, and in cold weather - it seems to be fast.Comment
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Aft CG will give you more airspeed (sometimes significantly more), because you don't require as much elevator "downforce" to counteract the forward CG. That was one of Roy Lopresti's tricks with speeding up airplanes - fly them at aft CG...
"I'd sell my mother for 3-knots!" - Roy LoprestiJim Parker
Farmersville, TX (NE of Dallas)
RANS S-6ES (E-LSA) with Rotax 912ULS (100 HP)Comment
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I guess that effect would be greater in an aircraft with a fully trimable horizontal stabiliser.Comment
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I thought I would add a couple of data points to this thread at the completion of my own flight testing.Originally posted by whee View Post
The MTOW of the 4-place Bravo model plans now states 2700lbs, and a MLW of 2500 lbs,with an aft CG limit of 22.5".
I've done it twice and found the experience unenjoyable. Loading to above 2500lbs drew my attention initially when the cargo door was difficult to close. The tailwheel didn't track straight either, instead producing a constant requirement to correct the tracking. We've checked the tailwheel to ensure it was installed correctly.
Once airborne, the max weight wasn't an issue as far as handling went. However my experience with flying with the CG at 22.5" mirrors what Whee and Jared experienced. As soon as I was airborne there was an immediate "pitchyness". I was concerned about the approach and landing for the duration of the flight (had to burn the fuel off), although surprisingly it wasn't any more pitch sensitive on approach. However, although it was fully controllable at 22.5", I made a mental note to use 21" as a self imposed aft limit. I then test flew at 2500lbs and 21", and found the aircraft to be much more user friendly.
This doesn't limit my load carrying capacity at all because with the added weight of an IO540 up front, the CG envelope of 10.5" - 21.0" doesn't tend to be the limiting factor.
BTW, don't take this as a reason to chose a heavier engine, because like all things there is a trade off in approach speed and trim range at light weights. There are a number of other considerations and Bob has probably optimized the thrust vector, vertical stab offset, pitch trim range etc for the weight and power of the O360 so as you would expect there are some differences with an additional 200lbs in the nose.
Regarding the experience of those above with limited pitch trim range, I moved my trim range forward so that I was able to trim it on the forward CG at light weights. I then flew at aft CG again to check that I had sufficient trim range to cover the aft limit. It reiterated two things. The trim range is sufficient, and, I still don't like flying on the aft CG limit.
If the trim push rods are carefully bend a small amount more in a pipe bender, the rod ends are left with more adjustment range to move the trim range forward.
Incidentally, for those that have changed their pitch trim to a non-servoed tab, I would be very interested to know if you were able to achieve the full trim range at both forward and aft CG limits, and also how you find re-trimming during a go-around.Last edited by Nev; 01-22-2022, 11:40 PM.Nev Bailey
Christchurch, NZ
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I have no trouble trimming the plane at the aft limit. I can not reach the forward limit so I can't comment on that specifically but I am confident there would be no issue. There is plenty of range on the trim, most of which is in excess of my current needs.Originally posted by Nev View Post
Re-trimming during a go around is not any different than with the factory system and I can't see why it would be. Perhaps I'm not understanding the question? The trim system is powerful but it has never been an issue during a simulated go around.
A couple comments not related to your question:
The fuselage flex at heavy weights is normal and even expected for a truss type structure. I haven't experienced the tailwheel not tracking but I haven't been able to load my plane all the way to 2700lbs, 2600 and some change is as heavy as I've been.
During flight testing I followed the EAA manual to determine the CG limits empirically rather than just going with what the plans specify without verifying them. I hoped the trim change would allow me to fly at the 22.5 inch aft limit comfortably. With a family of 5 my plane is frequently loaded heavy with an aft CG. I tested to 23.5" before the plane started to show neutral stability in pitch. I decided I did not need to go further and set the aft limit at 22.5 and have flown there comfortably.Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
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Everyone should be doing this. Don't assume any of the numbers are correct in the plans for stations or where you'll find the limit.Originally posted by whee View Post
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Cessna's and pipers and Boeings are all controllable an inch or five out of the aft limit. That limit is there for stability, which you hit before you find an unrecoverable stall situation in most airplanes.Originally posted by Nev View Post
Since we're experimental, you don't have to have a stable airplane. But if you want a static/dynamically stable airplane in pitch, like a certified airplane is, you'd not use the 22.5" limit. Flight test and find where the positive stability ends, and put it there.
For me, at 22.5", it has negative static stability.Last edited by zkelley2; 01-25-2022, 10:40 AM.Comment
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Can you define “negative stability” for this newbie?Originally posted by zkelley2 View Post
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Great post Zach. I wish you had made it a regular post rather than a comment, it’s important info and you’ve made it clear.
Do you recall what the EAA flight test manual says should be the max member of oscillations should be before the plane recovers for the aft limit? My manual is at the hangar so I can’t check right now but I think it was 3 oscillations.
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