major increase in elevator trim

[boba65$#]

hey guys....still plugging along with my build of DE F95, and think that I want more trim. Experimental Amateur Built means just THAT....and "that being said", I am asking some input to my considered sketch of this modification. If you see some glaringly stupid things here, please advise. This mod would increase surface area from about 65 sq in, to about 125 sq in. Comment and help please. Thanks....bob

[scottiniowa]

hey guys....still plugging along with my build of DE F95, and think that I want more trim. Experimental Amateur Built means just THAT....and "that being said", I am asking some input to my considered sketch of this modification. If you see some glaringly stupid things here, please advise. This mod would increase surface area from about 65 sq in, to about 125 sq in. Comment and help please. Thanks....bob

Has anyone said they have run short of trim?  (not enough surface area) Or felt they were moving the trim control to much?

[boba65$#]

Scottin iowa...actually, as you know,we may have different input/result expectations. Someone here(VERY SIGNIFICANT), made a similar observation last year.. In a sharing moment, I was just asking if there was anything STUPID about this proposed change. Please illustrate the damage that would occur when we trim out using a full-span secondary-surface on an incredibly slow aircraft(provided,of course,that all common caveats such as limit-stops,etc have been taken into acct).AS im sure you know, when an elevator is deflected around19-20 degrees, flow seperation over the tail results. Often the elevator will lose effectiveness. Lots of Eagle Series pilots experience this and assume the problem is elsewhere. I prefer to fly with trim activity to help me.We all have preferences, but using max elevator deflection should not be the preferred method in slow speed(absolutely the EAGLE)/pitch/landing situations. Elevator trim secondary control surfaces tend to help this as smaller amts have greater effectiveness.There ARE exceptions that I have found in my A&P.IA/Inspector career with Continental airlines. Most notably would be the MD-DC10, MD-11,ATR 42, B727, and others have more than 25degree UP elevator. RARELY is  DOWN elevator deflection more than more than UP. I do NOT have an Aeronautical Engineering Degree...but I did graduate from Embry Riddle University with a BS in Aeronautical Science(1973). Moment loads are better evenly distributed over wide airfoils anyway.BUT, what da hell amigo. I was hoping that the collective here could offer constructive criticism vs snarky "prove-it-to-me" comments. sorry to bother you.

[Dave Stroud]

Hi, Boba....are you thinking that an enlarged elevator trim tab will increase the effectiveness of the elevator itself ?

[okdonn]

I my limited knowledge of these things, I've always thought that the function of such a trim tab is to reduce the control stick force required to get the desired elevator deflection, or provide the desired deflection with zero force.

If I'm right, and you want greater elevator authority at a lower angle of deflection, then you need to increase the size of the elevator, not the trim tab (probably both though).

Of course, I could be way off base here . . .

[boba65$#]

dave and OKdon. Thanks for the responses. I do not seek to "increase elevator effectiveness" per se. Only a size change/and/or increased deflection can do that...as you pointed out. (along with a ton of other variables). I guess I got kinda tense when I was basically told to prove that what I am considering was, somehow wrong, and I apologize to all. Most of our Eagles are bastardized and this is just one of the many (relatively ) minor changes I MIGHT do. Full-width trim tabs arent such a rarity, and I think My using cessna 120 wings would just put  some folks over-the-edge  as well. i have built some strange critters, and they all flew well in spite of what I did.i will refrain from posting those pics until after I have some "numbers" to post. This is a cool little airplane, and we all should thank Leonard for its design. I might have miffed one of the design principles(without knowing it). Think I better shut up. Again, I am NOT an Aeronautical Engineer. Thanks again for your input

[Dan_]

Please don't shut up... 


I for one, like to see what builders come up with. 


Just be careful, as you know, you are making yourself a test pilot.

[scottiniowa]

Well this subject run the rail of comments rather quickly. And I certainly wasn't putting anyone's ability, credibility or engineering ways to test.   First and foremost, I was wondering about the "need" for the airplane.  I re-read my question, and am sorry if it sounded like I was questioning your ability to access or build such changes.  Sure sounds like your up to the task.
  
The primary reason I asked, was simply a aircraft I was involved with, had about 1/2" adjustment, (for flying trim)  to a 6" long trim lever and this was built to plans.  This adjustment of 1/2" in the trim lever was from takeoff-to cruise to landing.  It was so small, that it was actually hard to adjust to hands off flying..  A lot of head scratching went into this "amount of movement" by many folks.  Finally was determined that it was not the tab, to big or to small, but rather, the ratio of trim lever movement to tab movement.   i.e.  tiny bit of  trim lever movement moved the tab a great deal.  In the end, the fix was easy,  but we could have fixed in many ways,  some not so easy.   

This was just an example...  certainly not saying anyone's ideas are wrong, without merit or without proper credential background.  The nest of EAGLES have certainly got better over the years- which is always good.

[boba65$#]

scotiniowa My public apologies for "going-off" on this silly issue. My bad,as the kids say. I harbor no ill will towards ANYONE on this site. The one thing I have learned on this forum, is that there are some really sharp people about. We all contribute to the fun,safety,and future of our aviation community. My only saddness is that I wonder if we will leave our discoveries to a diminished crowd. It seems that so many  youngsters just dont care....just too busy "texting.apping," and other pointless activities . During a period of "layoff" with Continental Airlines, I took a teaching position at Houston Community College as an A&P instructor(circa 1993-1994). I was stunned at what our Public high schools produced as "graduates". Of the 42 initial students....20 months later we graduated 11 A&Ps. Most were simply uncaring and without excitement. I remember washing airplanes as a 10 year old kid at a local grass strip near our home. Sometimes for free, just so I could get another period-of-instruction. We owned a cub that my dad bought after ww2($750),and he taught us kids how to fly. He flew Corsairs in the Pacific. BUT, I wanted to fly something besides a cub as well. I guess when we get old and crusty, we get nostalgic, but I hope I am wrong about the next generation and the future of sport aviation, and individual ownership. Time will tell.With student loans looming as a financial threat to thousands of them,so many of these kids wont even be able to buy  a home of their own......let alone afford a 40 year old c150. !!!

[Tom H]

Boba65, I'll share what we did on DE Stubby for elevator trim and the result:

- trim tab was built same size as plans
- rather than using the wires/pulleys per plans, we used Bowden cable, threaded through the horiz stab in such a way that the cable could twist/turn and not get kinked or over-stressed.  The pilot end is attached to an overhead control lever, with the pivot loaded with split washers to add some friction
- we had CG issues, mostly corrected, but still feel that the airplane is a bit nose heavy, requiring a good amount of stick pull to flare
- present condition is that the airplane flies nicely, can be trimmed to be hands off in level flight.  Slight movement of the trim lever produces noticeable change.  We could change the ratio of the trim lever, but this would reduce the resulting amount of trim tab deflection, not desired for our situation
- on landing, full up trim is used, but the stick still requires a good pull to flare.  I'll guess that the trim tab angle, when fully deflected down (which gives elevator up response) is about 40 degrees.
- our 40,000+ hour pilot does not like the fact that he can not trim to get neutral force when landing.  I've found I can live with it, but he does have the experience backing up his opinion

We have considered what we can do to improve:

- adjust the horiz stab cables and the front mount bolt/spacer to give the horiz stab more "down at the front".  Have held off this mod because it requires replacing the cables (they are at limits with the washers at the cable attachments), and there is a very small amount available at the front stab/fuselage bolt connection.  Overall, may not have much effect for the effort? or, may screw up the nice level flight condition
- make some more adjustments to the flying CG situation (turns out that the high hour pilot is significantly lighter than me, and his flying CG is further forward than mine.  His experience could be more pronounced than what I experience)  We feel that we have taken the mods to the max with our airplane - we don't want to add more tail weight (actually, don't want to add any weight)
- we are on hold, but if you get good results, the full tab mod may be a possibility

My comment on your proposal to make the trim tab full width:

- a question: will the full width tab essentially take elevator surface from the elevator, resulting in less effectiveness of the elevator?
- Strength of the "rear spar" that the tab is mounted to should be evaluated, but if it is made from the bent sheet, welded to the elevator ribs, does not seem to be a concern to me

Hope some of this is relative to your questions.

[boba65$#]

tomh...thanks for the input. WOW 40k hrs. Dude must have been born in the cargo hold! My dad was a career Navy pilot, had close to 23,000 hrs(total),but most of it was after his retirement in the civilian sector.Hard to get those many hours on a Carrier! I feel like I just soloed with only 3700some hrs.  I think that your pilot friend talked to my old man....because he was a big believer in "trim-to-neutral force", and I greatly value that kind of experience. So much of what we do is a culmination of "preference"vs design. Even among professional designers, there is much conflict. At any rate, regarding your questions....I could be very incorrect in my assumption/belief that x-square inches of a control surface is x-square inches. Simple physics. Obviously, the same "square inches",say in an early cessna 150 with 40degree flaps will react in the "barn door " way that most of us have experienced.  certainly could be wrong. Concerning the overall strength of this mod....you are correct in that I am planning to use welded,shaped "u" bracing. I am not overly concerned with weight. I am using cessna 120 wings(9.5 lbs lighter than plan wings each), and im only going to use the right side wing tank,with a more efficient powerplant(also about11lbs lighter). The 120 wings have never had flaps,unlike the 140s, although in either case most folks see not much gain with flaps on that particular craft. The 120 wing area is 160sqft vs about 129sqft on the eagle. I am messin with cg on computer models and old fashioned math stuff. Strange hybrid I have going here. anyway, thanks again..bob

[rfeenstra]

similar response to Tom. My tab was built per plans.  Very effective above about 45 mph.  Perfect for hands off flight. In the landing configuration, full trim still required back pressure to hold descent angle and much more in the flare.  It certainly was not bothersome, however.  My CG was at about 29% MAC.  The mods I'm making will give me quire a bit more trim tab area, similar to a Champ.

[Dan_]

Bob,
Have you thought about a Cub style jack screw setup..? 


If the cg is anywhere near right, you have all the trim in the world. 


I have thought about building a light car type scissor jack out of tubing.

[Dan_]

Bob,

Snatched this off another forum...  It sounds plausible.

  July 8th, 2005, 10:27 AM   #4 orion
 
 
 
 

Unfortunately here I really don't have a pat answer since the size and shape of a trim tab will depend partly on the application and partly on the designer's preferences. Basically, a trim tab is nothing more than a plain flap applied to another surface. As a matter of fact, all the loads on the trim tab are calcualted in exactly the same way that you would calculate the loads on an elevator or aileron.
 
 If we use just good ol' proportionality as applied to flap design, the chord of a trim tab should be about fifteen to twenty five percent of the control surface's chord length. For an ultralight, where the low Reynold's Number and gap leakages tend to reduce the effectiveness of any deflected surface, I would tend to look at about the twenty five percent range. You of course don't want to make it too big though since then you decrease the effectiveness of the elevator itself. If you have a flying tail, you'll want to stay toward the low end of that range.
 
 Span-wise, you'll want a good aspect ratio for good effectiveness. Again depending on a designer's preferences and the airplane's requirements, elevator trim tabs seem to extend anywhere from half the surfaces' span to their full span.
 
 The bigger the tab, the less motion it will need and thus the less drag it will create. But, a too large a tab will make the trim such that you may not have enough finnesse to trim to a specific condition (you'll tend to over-trim). In that case you'll need a long lever arm to reduce the amount of deflection per movement of the trim servo, or just a smaller tab.
 
 In short, cook-book, rule of thumb and eyeball methods will get you close but the final size will, as George said, require a bit of trial and error.

[MrG]

well here's my 2 cents, 65 hrs so far,  300nm (4.5hrs) flying last weekend - I don't run trim, i don't have trim, spent a lot of time playing with horizontal
stab - @60/65 knots flies great. would not change a thing and having a ball 
:-)