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Titanium has to be hardened or it will bend quickly. Mark
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I spent a lot of time dealing with metals and their properties, but I have to admit near ignorance of titanium. I know it's really light and really strong. If Jim, or other, comes up with a titanium replacement (in an appropriate condition) for the rod type tail spring, please let me know. I'm interested!
BillComment
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I used grade 5, 6AL-4V which is can be machined in the treated condition as is about as strong as can be used for a spring.
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I heard Sonex offered a titanium tail spring so I emailed and asked what alloy, they said it was 6AL-4V (grade 5) as well. This titanium alloy has superior mechanical properties to the 6150 spring steel called for in the plans but is also "springier". I'm not sure what practical effect that has for us, but it's used on other aircraft for this purpose.Comment
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Is being tail heavy a common problem with these aircraft?Comment
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There is not a consensus (hence all the discussion) but in my experience, no. -
My situation (Patrol, not 4 place)., is that I built complete interior aluminum panels, not fabric interior. My battery is under the pilot seat, not on the firewall. My pitch servo and associated installation is aft of the baggage bulkhead. I have the tundra tailwheel. The tundra tailwheel is a really great looking unit, but it's HEAVY. Finally, I have a 2 blade MT prop, which is quite light.
My empty CG is 14.45", which is probably more aft than most. The plane flies great and CG isn't a concern. All that being said, If I could lose some tail weight, without extraordinary measures, I'm interested.
Bill
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Not something I have heard. Almost all turn out comfortably within the CG range.Originally posted by Cameron Ramsey View Post
But it does depend on the builder's choices. If you use a heavy three blade metal prop you can create a heavy nose, I understand.
Mine is not tail heavy, best I be totally clear on that. It's CG near the front of the window, just not as near as it was originally. The plane flies very well at all CG locations, better than most.
I know a person who has flown the 4 place with a very aft CG outside the range. They said the plane was still easily controllable provided you keep your hands on the stick. I would not recommend that approach to aft CG, it should be treated with respect.Comment
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Thank you everyone for the replies and clarification.Comment
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During this thread we had some discussion about possible causes of my CG change, given the numbers did not stack up. I have spent a long time chasing this down...
Likely Cause:
I think it's several factors combined. I cannot find a single factor which totally explains the change.
- lighter prop installed
- extra weight in the new wingtips, pulling the CG back towards the mid-point a little.
- true arm length to the baggage area. I have a newfound understanding of where my 12kg (26.5lbs) of "sundries" are located, thanks to Jared's discussions in the CG arms thread. "Sundries" means charts, manuals, survival gear, pickets, misc stuff you just end up carrying.
- small differences between calculated and actual CG when other mods were added (bushwheels, relocated oil cooler, etc)
- weight-gain in the cabin. As we know, planes experience weight gain as they age - the struggle is real. Minor changes no accounted for, detritus build up, paint touch-ups, it happens...
I think that is what was screwing up my numbers, those factors combined.
Pursuit of a Solution:
I've moved my battery to the firewall in an effort to improve things. Per my post in that thread about CG, it doesn't help much unless you had your battery in the tail originally - about 10mm forward movement on CG location, or 3% of the total CG range. I do like the firewall battery change because it gets rid of weighty cables, shortens the cable runs to the starter motor, and gets the battery + solenoid it out of the cabin. Also the install looks a lot better and makes the whole firewall forward area pop, visually.
I have pulled out my baggage tube, not to be replaced, and replaced my aluminium after bulkhead with a fibreglass aft bulkhead. The baggage tube was large and removed almost 10lbs where I needed it most. The fibreglass aft bulkhead is much lighter than aluminium, until you paint it... Then they are almost the same.
I am being a lot more proactive about relocating the heavy "sundries" to more forward locations when I have a load on board. I am looking at a permanent move to under the copilots seat for the heaviest items like pickets bag. I just need to shield the control cables first.
Results
When I started this thread, the empty weight CG location had moved from 15.8% TOMAC to 26.1%.
With the changes discussed here, I have successfully moved it back to 19%.
The limits are 16% to 34% TOMAC.
In absolute terms, I have been able to recover 38% of the total CG range available. I am pretty happy about this, though still working to get closer to the original location.Comment
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I did not read the whole tread , but by now most will start to recognize the importance of planing for a desired C of G
Any weight out back is hard to deal with, on a nose heavy plane it is easy to throw the survival gear in the back
I have fought this W&B battle on all our Aircraft, careful planing and execution makes for a good flying aircraft.
If in Doubt less is better. Less weight - less distractions -and less to go wrong.
The best thing to add to an Aircraft is gas -- go flying that is what it is all about.
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"On a nose heavy plane it is easy to throw the survival gear in the back."
Well, maybe not always... I owned a plane for several years that was quite a bit nose heavy. The plane had a heavy IO-540 with a McCauley aluminum constant-speed prop, and an aftermarket turbo-normalizer. The fuel tanks were slightly forward of the CG mid-point, so with full fuel, it moved further forward. Ditto for the front row of seats. On this plane, the CG range became narrower as the gross weight increased, moving aft almost 2 inches at max GW. As a result, two heavy guys in the front seats with a full fuel load put the CG well beyond the forward limit.
Flying by myself, I routinely kept several large jugs of water in the baggage area (tied down, of course) and my 60+ lb tool box – maybe 100 lbs of extra weight, just to be able to fill the tanks. When flying with a friend, I could only fuel to the tabs, unless I carried at least 200 lbs in the baggage area. The battery and all the avionics were already located on an avionics shelf behind the baggage area – literally in the tail cone.
The "book solution" was to replace the heavy prop with a lightweight MT prop (the only lightweight prop STC available), but the cost was truly staggering – out of the question. My A&P/IA suggested we could put some lead weight at the very back of the avionics shelf to solve the issue. We ran the numbers, and found we would have needed to add 60 lbs of lead back there to really make a difference. (Further back in the tail cone was impossible, because it would have required DAR approval for some serious structural modification.) The problem was that with a much lighter pilot, the airplane could easily have been loaded beyond the rear CG limit...
Bob did an amazing job in selecting the airfoil and designing the Bearhawk airplanes to have a truly awesome CG range...
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After messing with my cg a good bit as well the last month, if I were to do it again there is no way I'd do a 2 blade composite prop. 3 blade composite is probably ok, but if you're 2 blade, I strongly suggest going aluminum. It's cheaper anyways. Adding 20lbs to my prop would make my empty CG in a pretty good spot. As it sits, I can't really do 4 adults and much in the baggage. Most of that is I cannot figure out how anyone has a 77" baggage arm. Unless I was stacking gold bars up against the back of the rear seat, that's just not an accurate representation of weight in the baggage area.
To do full fuel, 4 FAA adults and 250lbs of bags, I'd have to add 120lbs to the prop. That or pretend the baggage area only consists of the first forward inch of the actual space and just fly wildly aft of the CG limit. I actually can't for the life of me figure out how to put 250lbs in the baggage area. Your empty CG would have to be 1.5"
On my list of things to do eventually as an improvement is to remake all the doors out of carbon. Especially the rear. I think getting rid of all that steel back there could be of great help.Last edited by zkelley2; 05-18-2020, 10:40 AM.Comment
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O-540. I even have 2 alternators, battery on the firewall. Everything I could possibly put forward has been.
Where the CG is now is still going to work for my purposes. 4 seats on occasion for camping trips and the like, but 2 seats on hunting trips, which allows me to get a lot more weight into it. Which makes it like every 4 seater out there. A 2 seat airplane when serious weight is involved. (I'm at 11.32" empty).Last edited by zkelley2; 05-18-2020, 10:17 AM.
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Hi Jim your comment above just proofs the point
Careful planing and execution makes for a good flying aircraft.
If in doubt less is better your engine choice clearly is outside the reasonable range for the aircraft.
Sometimes what we think we need is not realistic or good for us been there done that too.
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Zach, Where did your empty CG end up?Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
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Hmm, I wouldn’t have expected that. You fill the tailwheel with lead?Originally posted by zkelley2 View PostScratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
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No, I think it's my lightweight 2 blade carbon prop. Like I said, another 20lbs on the nose would be a good thing.Originally posted by whee View PostComment
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Heavy alternator, lightweight starter. I hate adding weight for no reason. I think with the floats this will become a non-issue and then some point in the future I'll go 3 blade composite.
But I cannot state enough that you need as heavy of a nose as possible to actually be able to put a lot in the baggage area... if you plan to use all the useful load the bearhawk has.
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