Hello all i have a new piper cub 81" wing that has one very bad habit. it likes to climb out of site. under 3/4 and above power. i can get it to fly perfect at 1/4 throttle then you give it gas and it rockets up at about a 60* angle i can trim it but then on take off i have to hold almost full up elevator to get it to lift off.

I'm really enjoying flying it for my first high wing, but this habit is driving me nuts, any help would be greatly appreciated.

if you need more info to help me just ask away .[][] [][][][] [][]

It sounds like you may have up thrust in your engine mounting. Try shimming the top of your engine mount and give her some down thrust.

I'd also check and make sure the rear stab is level and no more the 2 degrees of up incidence in the wing.

Let us know how you make out.

Bob[8D]

Thanks Bob:

the stab is level,, the wing is set as this was an arf with two wing tube mounting. I thought the same thing about the engine. now for the really silly question. with the stab level on plane what shoud the engine be setting at?

and the best way to check for positive engine thrust is ?

thanks again
Don

I use Robart Incidence meters to level the main wing. Then I put a small level on the stab. If the stab is off, I level it and read the meters on the wing. If the wing is within +/- 1 degree, I leave it alone.

The Robart meter is also able to mount to the prop shaft to check the thrust angle.

You could mount some sort of plate over the prop shaft, against the hub, and use a small bubble level on the plate.

A little down is better than up.

This is a really good question and the answer isn't at all obvious. Generally all angles are relative to "center line" that runs through the fuselage from nose to tail. Generally, but not always that center line runs directly through the the center of the engine. On some aircraft this center line corresponds to some feature of the fuselage (for example the top longeron), but that's not always the case.

Anyway, you're first task is to prop the fuselage up so that the this center line is parallel to the floor/table/etc (and may sure to check that the floor/table/etc/ is also level). Once you've done that, you might even be able to eyeball whether or not you've got some upthrust. If you're lucking the firewall on your model will be exactly perpendicular to the center line. In which case, you can use a triangle to check whether the engine is tipped up, down, or is level.

There's also several specialized tools, for example, a laser angle finder. I have one but find it's rather difficult to use or read accurately.

Same as what was said above, but set the stab absolutely level (use that as your reference).. Check the wing incidence as mentioned above... Engine down thrust can be anywheres up to 6 degrees down in some planes....

What kind of power are you using? If it is overpowered, it will climb.....

I stand corrected (though a bit confused).

That's not true. Thrust is determined to the Datum line of the plane. If the stab is zero and engine is zero to the datum line then you have zero incidence and zero thrust.

Sounds like a Cub. With the Clark Y airfoil the bottom of the airfoil is not 0 it is about +2. The real cub trims by moving the stab, and any power change in the real one requires a retrim. About the best you can do without cutting is add a couple or three washers inbetween the top of the motor mount and firewall and tip the motor down. It will always do it but you can take the bite out of it.

Big-Mo,
I feel your pain. I had a trainer that did this when I was trying to learn to fly and it drove me nuts constantly having to retrim the elevator. It's fairly simple to fix and you'll learn a lot about airplanes in the process. Rule 1 is that the stabilizer always seeks 0 degrees angle of attack (aoa) in level flight.

A wing generates lift force in proportion to the aoa and the square of airstream velocity so as you speed up the plane naturally wants to climb. R/C designers normally counter this tendency by adding downthrust to the engine (angle of an imaginary line through the crankshaft). The downthrust angle literally causes downward thrust in proportion to the speed of the propeller This counterbalances the increased lift generated by velocity as the plane speeds up by decreasing the aoa so that it maintains level flight.

Step 1) Establish the datum line. Since the plane is already assembled the stabilizer is likely epoxied in place. I suggest using that as your reference line (called the datum line because everything will be measured with respect to it). As said earlier, find a good flat table surface and use a bubble level and some shims under the legs to get it level. Now put the fuselage on the table with the gear chocked and the bubble level on the horizontal stabilizer. Raise the tail until it is level. Now the stab is parallel with the table surface.

Step 2) Measure what you have. You need to measure wing incidence and engine thrust line. If you have an incidence meter great! Use it to measure the wing incidence and thrust (if it's a Robart).

If you don't have a meter find yourself some masking tape and a draftsmans triangle, builders square or some other rigid device with a right angled (90 deg) surface. Put a piece of tape on the triangle and a piece of tape on the leading edge of the wing near the fuselage. After installing the wing make a mark on the leading edge at the absolute forwardmost point. This mark is important so take your time here. Now slide the triangle up to the leading edge mark on the wing and mark the triangle to indicate the distance up from the table surface. Do the same thing at the trailing edge so that you now have two marks relatively close together on the triangle. The distance between those two marks is the incidence of the wing. To convert to "degrees" you'll need to measure the wing chord at the root where the other measurements were taken and use the following formula: degrees = arctan[ (l.e.- t.e.)/chord].
Example 1: arctan[(11.25"-11.00")/12.00"] = 1.19 degrees (positive incidence)
Example 2: arctan[(11.00"-11.25")/12.00"] = -1.19 degrees (negative incidence)
[link=http://www.1728.com/trigcalc.htm]Here[/link] is a trig calculator if you need it. (Use the button marked 'ATAN')

Next measure the engine thrust angle. The easiest way I can think of to do this is take the engine out of the plane and place your bubble level on the rails of the engine mount and parallel with the centerline of the fuse. Raise (or lower) the tail of the plane until the bubble shows the rails are level. Then measure the incidence of the stabilizer using the method described above and change the sign. Thats your engine thrust angle.

Step 3) Set the correct angles. If you have documentation that came with the plane start with what they suggest. If not you'll have to experiment. I would recommend starting with 2 degrees downthrust for the engine. Most people use washers behind the mount at the upper screws. If the wing is a flat bottom airfoil, start with - 1 degree wing incidence, if semi-symmetrical start at 0 degrees, and if symmetrical + 1 degree. Most people adjust a high wing by shimming under the trailing edge.

EDIT: I don't fly Cubs so maybe someone else has a better suggestion for starting measurements?

Step 4) Adjust incidence and thrust. If the plane flys level at full throttle but sinks at 2/3 power, increase incidence and downthrust slightly. If the plane flys level at 2/3 power but climbs at full, decrease incidence and downthrust slightly etc. Experiment and have fun. Can you tell I really enjoy trimming planes?

Long post, sorry. Let us know what you discover.

First thing you should try is checking the balance.Sounds like it might be tail heavy

Thanks for all the tips and suggestions guys. I'm used to flying mid and bottom wing planes. ive been flying for 11+ years and have never had a plane do anything like it this bad. ( I know been flying for 11 years and don't know how to trim a plane) Believe it or not i did y training with a Walt mocha novi, was the sweetest plane i ever had. (buddy) spun it into the ground when he ripped the vertical stab lose.. sigh oh well.

There is no way for me to adjust the wing incidence i believe as it is a two aluminum tube wing support(plug in wing in a arf), I will check the engine thrust tonight to see what is it . and adjust as directed. the plans show nothing as it was a arf. almost like the hangar9 15 scale cub only a little bigger.

I don't think i mentioned this part but it will take off in about 30 feet IF the elevator is at neutral. I have about all the elevator trim i can put into it with the trim switch.

When you measure the engine thrust do at least measure the wing incidence so you'll know if it is or isn't the problem. Because of an error somewhere the wing could be at 4 or 5 degrees positive making it virtually unflyable. But don't worry, just about anything can be adjusted. Once you know the problem we'll help you fix it or work around it.

I think it is a simple matter of too much power on an airframe that was not designed for it.  What engine and prop are you using, and how much does this airplane weigh?<div>
</div><div>The full scale airplane was designed for very low power.  It's true that downthrust would help, but it will probably look conspicuous.  I think the real problem is that you want to be able to fly this plane at a wide range of speeds, and that's not what Cubs do unless they are modified.</div><div>
</div><div>Jim</div>

Hey Jim.

Well the power plant is a os 70 surpass II running a 14/8 prop. the plane weight in right at 9 lbs. it actually 8.14 lbs.

that was the 4 stroke recomendations on the plans. 70 to 90 4 stroke or 50 to 80 2 stroke.

to much motor?

thanks
Don

That's a good combo. Have you checked the engine thrust line yet?

Don, the Sig 1/5 scale flies with a smaller motor (.50 4S is enough) but you should be OK with that engine.  I'd use less pitch, even a 14x4 which is what I use on my OS 70 FS.  You might like landings better and it probably won't go as fast.<div>
</div><div>I would trim it for a nice Cub-like cruise and stay away from high throttle.  No full scale Cub "rockets up at a 60 degree angle".  You can probably take off at 1/3 throttle, maybe less.  Bottom line is you have more power than you need for scale-like flight so you will do best limiting the high end of the throttle.</div><div>
</div><div>But try a low pitch prop I think you will enjoy it more that way.</div><div>
</div><div>Let us know,</div><div>
</div><div>Jim</div>

OK guys here is all the findings finally with the horizontal stab set at 0 degrees incidence the engine has a -2degrees or 2 degrees downward incidence and the wing has 2.5* positive incidence. the wing has two aluminum tube plug ins so i can not figure out how to get 0* incidence in the wing. i moved the cg forward to 100mm which is the start. cg is 100 to 120 mm . let me know you thought now.
thanks a lot for all the advice.
Don


Oh also got rid of the apc prop and put a old timer style prop on it. didn't have a 4 in anything so i went with the same size just a different prop .

one other thing you can do is to raise both ailerons about 1/16 inch for starters and see if this helps at all as it will reduce the lift generated by the wing.this is a try some and adjust more or less as you go.

Don, it's best just to use some down trim.  The way downthrust works is to create pressure on the bottom of the stab from the prop wash.  The vector component of a couple degrees of downthrust is negligible; it is the impact of the prop wash on the stab that counts.  <div>
</div><div>There are two effects: trim and downthrust.  You can change the trim by decreasing the angle of attack of the wing, or by down trim on the elevator.  Both will work, but changing the wing incidence will also reduce the downthrust.  What matters is the prop blast hitting the underside of the stab/elevator.  If you use down trim then prop wash will hit the underside of the elevator.  That's good.  The more power, the more the effect.  Reduce the wing incidence and you can trim the plane just fine, but the more power, the more out of trim it will become.  Obviously, you don't have enough down thrust for the way you want to fly, so don't make it worse by reducing the incidence.</div><div>
</div><div>The old 65 hp Cub had a rate of climb of 450 ft/min.  So your 1/5 Cub, to look realistic in flight, should take a minute to reach an altitude of 90 feet.  That's not very high, and a minute feels like a long time when you're climbing out.  If you trim out the plane for a realistic power level you will not have problems.  Our sport models that we are used to fly very differently from full scale Cubs.  They have far more power proportionally.  That's the real problem, and the solution is scale-like flight.</div><div>
</div><div>Jim</div>

OK, now you know it's an incidenceproblem as we suspected. If you'd like me to help you adjust itsend me a PM.

<span style="font-family: Verdana; font-size: 13px; -webkit-border-horizontal-spacing: 1px; -webkit-border-vertical-spacing: 1px; "><p class="MsoNormal"><span style="font-family: ArialMT; ">I'll try an explanation I just gave in another forum.  The angle of attack of the wing in flight depends on the speed and weight of the plane, the wing characteristics, air density, etc.<span>  </span>So changing the “angle of incidence” by shimming the wing has no effect on the AOA in flight.<span>  </span>All it changes is the angles of the other components going along for the ride.</span></p><p class="MsoNormal"><span style="font-family: ArialMT; ">Suppose you decrease the angle of incidence of the wing.<span> </span>Now the whole plane has to fly at a more positive angle to keep the wing at the same AOA. That includes the engine.<span>  </span>So now, by decreasing the angle of incidence, you have raised the angle of the thrust line.<span>  </span>If you started with 2 degrees of downthrust and you decreased the angle of incidence of the wing by 2 degrees, you’ve just wiped out your downthrust.<span> </span></span></p><p class="MsoNormal"><span style="font-family: ArialMT;">If you needed downtrim before, then reducing the angle of incidence of the wing will allow you to reduce or eliminate the downtrim.  It will "work" in that respect.  However, it will not improve the problem you raised, which is the difference in trim between high throttle and low throttle.  To solve that problem, you need more downthrust.  </span></p></span>

I would agree that he needs more down thrust. But.. doesn't down trim create more LIFT on the stab there by changing the attitude about the aerodynamic center and hence the nose down attitude?

OK, I think Iknowthe source of controversy now. A wing doesnot hunt for anangle of attack with noregard to the other forces acting on the body of the airplane. All the forces acting on the plane must be in equilibrium.I often draw asketch called a free body diagram which displays every force and acceleration acting on an object. That way nothing gets missplaced or forgotten.</p>

If we consider a typical airplane in profile view there would bea weight vector (mg), the lift vector (L), the engine thrust vector (T), a drag vector (D), and possibly a stabilizer negative lift vector (S). All these forces (actually it's their moments) will be in equilibrium when the plane is flying or an acceleration must occur (a pitch rotation).</p>

Rapid gain in altitude with power increase indicates that when all the other forces are in equilibrium the wing is at an AOA that generates lift in excess of mg. When incidence is reduced, the planes wing will fly at a slightly reduced AOA at the same speed as before. With less AOA there is less Cl and less lift.</p>

For a moment lets suspend physical reality and assume a wing "decides" itsAOA. A trainer with 2 degrees downthrust and +2.5 wing incidence ballons in altitude with increase in airspeed.So wereducewing incidence (<u>relative to our datum</u>) from +2.5 down to 0. According to Buzzard the wing now "causes"the plane to fly at a +2.5 higher AOAat every speed. This implies that now the stab is flying 2" lowerbut not generating any positive liftfor some reason and the engine is higherwhich "somehow" appliesa +0.5 upthrust to the airplane even though it's direction of thrust and momenthave not changed relative tothe center of mass of the airplane. Wow, weird science indeed.

OK back to reality....

The wing AOAis a committee decision between stab lift, engine thrust and to a small extent cg because its usually slightly ahead of the neutral point. All these players cause torque reactions that find equilibrium and determine the wing AOA. The wing has hardly any power in deciding it's own AOA because it's lift and drag vectors are so close to the cg (center of mass to be more accurate)that their moment arms are very short.

Another example. To trim an airplane that gains altitude what do we do? Put in a click of down elevator right? This changes the camber of the stab creating slightly more lift, creating a slight pitch rotation to it's new equilibrium, which changes the AOAof the wing, which causes it to generate slightly less lift.

Buzzard, your understanding of flight mechanics (and even simple physics) is completely wrong. Sorry to be harshbut the notion of a wingdeterminingit's own AOAand that idea of downthrust causing propblast on the underside of the stabilizer ishomespun mythology and has simply confused everyone in the middle of helping a guyfix a simpleincidence problem.If Isay something incorrect I'd like to know.But next time you feel the urge to "contradict"me at leastmake sure you havesome grasp of the subjectfirst ok?</p>

Crate.. thanks for confirming my understanding of aerodynamics.</p>