Super cat video,out of balance.

[flyerme]

OK just finished a static balance,and perfect!! with 1/2 fuel and 70kg pilot it sits level at 14.2" if moved to max aft 16" it balance's nose down slightly,set at 12"max forward and balance's slightly tail down.

 

NOT a c of g prob? any other ideas?

 

 

[Deskpilot]

Do you have a balanced elevator? If so, is the balance correct?

 

Can anything at the arse end move whist under the influence of prop wash?

 

Just thinking out loud.

 

 

[flyerme]

ok found something causing my "running out on back stick",my bad I put a rubber stop because when full back the control arm hit the back wall and has rubbed a dent,any way removed it (a good 3 mill thick) and replaced with tyre tube cut out,but still doesnt explain needing 1/2-3/4 back stick as soon as she pitch's over? thinking about last flight it feels like not enough air flow over tail unless power applied, will test next chance to see if in level fight at 60 knotts with centre stick,if i back off power see if she will nose over,but already notice in level flight at 60 knotts stick is centred but on approach at 60knotts(less revs) she need 1/2=3/4 back pressure and if you throttle up(as the elevator already 3/4 up) she instantly gets more airflow and lifts,,im really thinking airflow?

 

 

[flyerme]

may have found the cause? any one know about trimming tabs? it has 2 small trimming tabs on elevatot trimed down..I guess to keep it hand free in level criuse? but what happens in approach? as these would still be pulling the nose down yeh? and to be clear the stall at landing was all my mistake with the rubber stop on the elevator..as we know the longer the arm the further the travell .. bloody goose.. im ashamed to say without the stop the elevator has a fair bit more travel at the trailing edge..any way back to trimming tabs,,ive never seen these prior the super cat.Note:at some stage it has been set up for long distance flying and has 409hrs on the clock,vhf,gps (arial and wiring),compass ,aviation thermometer,alt/qne/ etc...so it may be set up for long cruise with tabs and not set best for landings ?

 

 

[Guest pookemon]

Is the range of motion of the elevator even (forward stick to back stick)? i.e. Same deflection angle? Do you have a flight manual/builders guide that indicates what the maximum deflection should be? If not, it might be worth e-mailing Wicks Aircraft Supply and asking them what the deflection should be. Needing an extra 3mm to keep the nose up indicates you have full back stick - which I find a little alarming (especially if the fan stops!).

 

 

[flyerme]

ITS EVEN NOW i REMOVED THE RUBBER STOP i PUT IN...it is now back to origanal and the trailing edge of elevator now moves an extra 3inches, so was just running out of stick..but the pitching I now believe is the trimming tabs which more than likely have been bent down more during tranport 3 weeks ago ,as I did have to bend 1 corner of the left elevator tab as it got bent/folded during transport.

 

anyone know anything about trimming tabs? what would happen on approach if a cessna left the trim up or down?

 

 

[Guest pookemon]

I know that on the Gazelle when doing glide approaches you pull the Trim right back - and still have to apply a bit of back stick to keep the nose up. AFAIK the idea is to be able to fly hands off at cruise (with fixed trim tabs). Not being adjustable the nose would dip under power off, and if you've fixed the deflection issue then it sounds like things are where they should be.

 

 

[flyerme]

I think I'll should be fine next flight now I have all the deflection correct,and just to test ive moved trimmimg tabs up 1/2 their travel to see the differenve?should need a little forward in cruise and need less back on approach? we will see? pitty we got wind all week,,,

 

 

[djpacro]

flyerme, looks like you are on the way to sorting it out - I was going to say something similar to pookemon about checking the specs and comparing yours with other examples.

 

facthunter mentioned the incidence setting of the horizontal stabiliser - that would be next on the list of items to consider (I'd recommend some simple measurements of stick position for various flight conditions and a bit of number crunching again prior to changing that). This setting is typically done on Pitts aeroplanes too.

 

Incidentally, I was involved with a certification program some years ago - after a gap of several years in production - first of the batch to fly didn't meet FAR 23 longitudinal stability requirements - some-one decided that it didn't need the small, fixed elevator trim tab (it had spings for trim) but that was enough.

 

 

[flyerme]

just for the record..thank you so much everyone for all you help and advice.... fingures crossed..think were almost their:happy dance:

 

 

[flyerme]

damn.....Just got an email from thr owner of the rightd to the bobcat/supercat plans and here it is.

 

The trim tabs being bent down will give you an up elevator trim condition, the tabs will fly the elevator in the opposite direction of the deflection. Another thing to check is that your ailerons aren't drooped beyond the normal airfoil and acting like flaps.

 

On my Supercat built in 1994 and with over 720 flying hours, I did a couple of things beyond what was shown on the plans. One I added an electric elevator trim tab in the left elevator half, and the second thing that I did was to enlarge the horizontal stabilizer. Both of those modifications worked very well, and when I acquired the rights to the plans the larger stabilizer was incorporated into the plans.

 

I don't remember where your CG is, but I am flying with the CG at 14.8" aft of the leading edge of the wing and have been very happy with it there. Remember that my stabilizer is larger so it will allow a more CG, but I am still well within the recommended range.

 

Cheers,

 

Steve

 

 

[djpacro]

All good. You know more now.

 

The down elevator tab does indeed trim the elevator up but two things to ponder:

 

- the tab reduces elevator effectiveness as well because it produces lift in the opposite direction - your reduced tab deflection means you need to pull harder to get the same elevator deflection but it gives a little bit more elevator authority for the same deflection

 

- if that large tab deflection is needed to trim the elevator for cruise - then where is the elevator in cruise and is that normal?

 

Which leads to his suggestion to check the aileron droop.

 

 

[flyerme]

2 more flights since and not going well... Now thinking back to the 3 hours of circuit flying at wellington when purchaced,I noted to by standers that the weather was rough coming in to land. since while flying here at home the weather seems rough ussaully when I make my turn from base to final which is always a decending turn,the instant I pitch her down the air becomes turbulant and the approach very turbulant(plane bouncing up and down but worse it skids side to side in its yaw axis,when I level back out in round out she is smooth again and I put her down gently.Now I believed this to be turbulance from weather as it feels,but this mourning I did a test..I took of in my thruster,nice smooth DENCE air , approach made hands free (man she flies nice) then I took off in the supercat,on climb out felt good levelled felt reasonable with slight turbulance,(climbed to 600ft made a 180 and headed back to strip) then as I did my 180 decent turn with 10deg bank at 60knotts it became turbulant and air speed dropped to 45 I gave her throttle and pushed her down to get 65knotts levelled the wings continuing my decent at 65knotts the plane bounced and slipped all over the place and once again needing throttle inputs to keep her at desired speed of 55, once leveled out at round out she flies smooth again and flairs beautifully.. so conclusion is pitch instability, so looks like shell be pushed to back of hanger for a few weeks till I can get her to benalla AGAIN for a complete overhoual$$$$$$$$$$ bloody piece of shit, and as has been mention sevral times to me ,NO I am not going to sell it to some other poor bugger,unlike certain people willing to not only take a carrer pensioners money but willing to risk thier lives for a sale,,,,,you can not argue the evidence... still waiting for the log book?

 

 

[flyerme]

RESEARCHING STABILITY AND CAME ACROSS THIS...

 

Even though the horizontal stabilizer may be level when the airplane is in level flight, there is a downwash of air from the wings. This downwash strikes the top of the stabilizer and produces a downward pressure which, at a certain speed, will be just enough to balance the "lever." The faster the airplane is flying, the greater this downwash and the greater the downward force on the horizontal stabilizer (except "T" tails) (Fig. 17-25). In airplanes with fixed position horizontal stabilizers, the airplane manufacturer sets the stabilizer at an angle that will provide the best stability (or balance) during flight at the design cruising speed and power setting (Fig. 17-26).

 

If the airplane's speed decreases, the speed of the airflow over the wing is decreased. As a result of this decreased flow of air over the wing, the downwash is reduced, causing a lesser downward force on the horizontal stabilizer. In turn, the characteristic nose heaviness is accentuated, causing the airplane's nose to pitch down more. This places the airplane in a nose low attitude, lessening the wing's angle of attack and drag and allowing the airspeed to increase. As the airplane continues in the nose low attitude and its speed increases, the downward force on the horizontal stabilizer is once again increased. Consequently, the tail is again pushed downward and the nose rises into a climbing attitude.

 

As this climb continues, the airspeed again decreases, causing the downward force on the tail to decrease until the nose lowers once more. However, because the airplane is dynamically stable, the nose does not lower as far this time as it did before. The airplane will acquire enough speed in this more gradual dive to start it into another climb, but the climb is not so steep as the preceding one.

 

After several of these diminishing oscillations, in which the nose alternately rises and lowers, the airplane will finally settle down to a speed at which the downward force on the tail exactly counteracts the tendency of the airplane to dive. When this condition is attained the airplane will once again be in balanced flight and will continue in stabilized flight as long as this attitude and airspeed are not changed.

 

A similar effect will be noted upon closing the throttle. The downwash of the wings is reduced and the force at T in Fig. 17-24 is not enough to hold the horizontal stabilizer down. It is as if the force at T on the lever were allowing the force of gravity to pull the nose down. This, of course, is a desirable characteristic because the airplane is inherently trying to regain airspeed and reestablish the proper balance.

 

 

[djpacro]

Increase in throttle also increases airspeed in the propeller slipstream resulting in increased airspeed at the horizontal tail so increases downward lift.

 

(actually, I disagree with that note that closing the throttle decreases the downwash but that's a subject for another time)

 

We are assuming that the basic design is ok and there is something different or rigged incorrectly on your aeroplane - the plan is to find out what that is. Therefore that suggestion to check aileron droop for one thing. It would be good to check wing and horizontal tail incidence angles in my opinion too but you are getting advice from the owner of the design.

 

 

[flyerme]

Found out he is the owner of design but NOT the designer.he just bought the rights,and then modified them,mine is based on the old plans the new plan has his modifacation,which is a bigger stablizer and more rearward c.g,,unfortunatly his not replying at present to emails? think he wants me to buy the plans and stop asking him questions? reminds me of the thruster cenareo... well if im going to lay claim to a design I would certainly be willing to help satify owners! ah the power of the dollar,,hmm

 

 

[flyerme]

flight today ,did 20 mins local flying,engines running nice ,plane seemed fine(awesome infact) and now ive received all the info I was after im satified with performance ,seems to be stacticly balaced in flight,as apposed to the dynamicly balanced thruster...so looking like all is starting to look good.will start some cross country flying soon,and will very dissapointingly sell my pride and joy "Thruster t83" to pay for the supercat.