.25 Fokker Eindecker takeoff twist..HELP!!  
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All Forums >> RC Airplanes >> Scratch Building, Aircraft Design, 3D/CAD >> .25 Fokker Eindecker takeoff twist..HELP!!
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.25 Fokker Eindecker takeoff twist..HELP!! - 11/12/2002 4:30:27 AM   
ZoomZoom-RCU



Posts: 464
Joined: 6/30/2002
From: Louisa, VA, USA
Status: offline 		Hello all. I would appreciate everyone's help and expertise on this one. I've just finished building a Fokker Eindecker powered with an Os.25 LA engine. Its a 46 inch wingspan with a wing loading of 20 to 21 oz per square foot. Anyway, I took her out for her test flights which I do from a grass runway. I fired her up, gave her the gun and she proceeded to turn right, despite full left rudder and run off into the deep grass on the edge of the runway. Ok ok, I know what this is. I shimmed out behind the engine and gave her a smidge of left thrust. Put her back on the strip and fired it up again. She tracked beautifully with a little rudder input, and at about 25-30 feet out she broke ground and climbed out sweetly. I was so enthralled in fact, just seeing her fly that I was a bit slow in noticing her begin to turn axially to the right as she ascended to about eight or nine feet up. Ok, so I snapped out of it and thought "no sweat", I gave her full left aileron, the roll slowed signifigantly, but continued. By this time, "did I mention this thing is faaast", the plane is beyond a 45 degree bank and losing what little altitude it had left. I just then thought to give it some left rudder as well, but it was too late. the tip of the right wing caught the deep grass and she did an ugly cart wheel into the field. Landing gear ripped clean off. Cowl thrown off, wing saddle broken with a little damage to the foam wing. All repairable, but I dont want this to happen again. Can anyone please give me some suggestions as to what they think will fix this and avoid any future damage!?

I have a few suggestions as to what it could be. Your feed back and other ideas are much appreciated.

1. Axial balance left to right is off. I doubt this though because I checked it and it seemed ok.

2. engine thrust angle bad. what do you guys think of it being a wierd torque issue. i'm using a 9x6 prop. seemed to work fine

3. The wing wires on the right side were a little looser than the ones on the left due to the first attempt. Could this produce more drag or enough to cause this!?

4. The prop is a zinger 9x6....what do you think? good bad ugly?

All of your help and or suggestions will be much appreciated. If I get her in the air I'll post some pics. Thanks again!

< Message edited by ZoomZoom-RCU -- Nov 12 2002 12:36AM >
       Post #: 1

.25 Fokker Eindecker takeoff twist..HELP!! - 11/12/2002 6:54:41 AM   
canadagoose



Posts: 651
Joined: 10/27/2002
From: Sparwood , BC, CANADA
Status: offline 		Make sure the vertical stab is straight with the centerline of the airplane and check the wing for warps.

_____________________________

http://www.tjlewis1.rchomepage.com

(in reply to ZoomZoom-RCU)
       Post #: 2

.25 Fokker Eindecker takeoff twist..HELP!! - 11/12/2002 12:05:16 PM   
tad5150


 

Posts: 8
Joined: 6/27/2002
From: WI
Status: offline 		Hi,

Suggest the following;

1. Rebuild the plane
2. Check the lateral balance and confirm that it is nuetral, if not add weight to the wing tip that is the lightest.

One thing though, how are you checking this? I usually place the propshaft on a box and then follow the thrust line all the way through to the rear of the fuselage and at this point I insert a long pin or a piece of push rod. Then place this on a box or something that is the same height as the propshaft from the ground.

If you do not do this and you try to balance at a point at the rear of the fuselage, you will get an erronous reading due to the differences in distribution of wieght accross the horizontal thrust line.

3. Confirm the balance point on the wing and if in find a point which is 30% the of the wing chords width from the leading edge of the wing.

4. Confirm that the wing is true and that there are no warps in the stucture.

5. Confirm that the wing sits true in the wing saddle and that it is at right angles to the thrust line.

6. Confirm that the elavator is square with the Main wing.

If you have dihedral in the main wing, then place the fuselage on something that will hold it off the ground front and rear and then on one side of the elevator measure the distance between the tip of the elevator and the floor and then compare this with a similar measurement on the other tip of the elevator.

Adjust the position of the plane on whatever it is sitting on, until you get an equal measurement.

After this them check the measurements between the wing tip and the floor for both sides on the wing. if there is a differenc then you will need to adjust the wing saddle either by using packing or sanding away part of the saddle until the measurements are equal.

7. Confirm the rudder is square with the elevator by using a simple carpenters square. If it is not you will need to adjust the rudder so it is.

8. Confirm that the Engine's thrust line is square to the fuselage, wing and elevator. I usually do this by removing the engine and placing a straight edge that is about 2 feet long across the mounting surface where the engine mount is bolted to.

After this I then measure from each tip of the wing to the end of the straight edge and compare the measurements if they are different, then I adjust the wing to get them equal.

Then measure from the tip of the elevator where it is hinged to the straight edge and compare the measurements from both sides, if they are differenet then you have to adjust the tailplane.

I also then measure fron the ends of the straight edge to the point at the end of the fuselage where it finishes and confirm the measurements are the same.

9. As a final check I then measure from the both wing tips to the hinge point on the rudder and compare the measurements.

10. In general if all this is equal then you have an aircraft that has been built square and as such should fly true unless there is a wing warp or you have the engine twisted in the engine mount itslef.

11. My experience is that right thrust on the engine helps taildraggers overcome the tourque effect that the prop creates. It is generally more noticable on take off and this usually accounts for the need for some rudder.

Once you get the aircraft in the air and get it up to a good height then you can check the dynamic balance. This is done by using a series of manuevers to confirm that the horizontal and lateral balance are correct and that the thrust offset is correct.

There are many web sites that can help with this one, I have attached a document that outlines how to do this.

Hope this is of some help.

Craig

PS confirm that the control surfaces are moving in the right direction and that they do with the radio turned on. I have been caught with the reverse function on my computer radio, thinking all was ok, but it was not. The aircraft will roll toward the side that the aileron is up, as it will climb if the elevator surface is up, etc....

Attachments
Click to see the file in new window.Attachment.zip

(in reply to ZoomZoom-RCU)
       Post #: 3

.25 Fokker Eindecker takeoff twist..HELP!! - 11/12/2002 6:03:38 PM   
Edwin


 

Posts: 3323
Joined: 2/8/2002
From: Leander, TX, USA
Status: online 		I have no experience with the Eindecker, but when you said it is fast I had to wonder. Maybe you are flying too fast. There where two other Eindeckers in our club at one time and both went down pretty quick, short life. It just seems to me that it should be a slow flying plane and should be proped accordingly and flown on the wing.
Just an opinion.
Edwin

(in reply to ZoomZoom-RCU)
       Post #: 4

excellent - 11/12/2002 10:25:38 PM   
ZoomZoom-RCU



Posts: 464
Joined: 6/30/2002
From: Louisa, VA, USA
Status: offline 		Thanks for all the great help here. Your suggestions have been very insightfull. I'm currently in the process of rebuilding the plane, and am planning on doing the reccomended checks suggested by you all. I'll let you know what I discover as I rebuild. In the meantime, Craig's response made me think of a question. Could too much right engine thrust cause an aircraft to roll to the right as I described, would it have that effect? Please respond with your opinions...Torque and thrust lines are one of those black majic topics, so I'm curious. Or could it be the opposite....is too much left thrust capable of causing a roll/torque to the right....or vice/versa? Thanks all, let me know what you think.

(in reply to ZoomZoom-RCU)
       Post #: 5

.25 Fokker Eindecker takeoff twist..HELP!! - 11/13/2002 12:23:31 AM   
tad5150


 

Posts: 8
Joined: 6/27/2002
From: WI
Status: offline 		Hi,

In general maybe.......

However you would have to have a considerable amount 10 degrees plus.......

There is one thing that you should make sure of, that the horizontal balance of the aircraft is correct. If it is too tail heavy what happens is that as you take off the aircraft starts to create lift accross the wing and the tailplane. Once off the ground if the tail is heavy what happens is that it sinks in relationship to the main wing, this effectively stalls the tail plane (it cannot produce lift).

Once this happens, then a chain of events are unleashed, the tailplane sinks, the angle of attack on the main wing increases and moves closer to the stall angel as such, if it gets too great, then the main wing starts to stall, resulting in a loss of lift.

The aircraft will then usually roll to one side as the angle of attack increases, this is usually caused by the wing tip stalling faster than the root of the wing. Once this starts you are a passenger, as the aircrafts lifting surfaces are not creating enough lift to support its weight, the roll is a natural reaction try to increase the airflow over the wing, which it does, however it is usually heading for the ground at the time.

At this point usually the ground wins and the aircraft looses.

I have had this happen once and it took a few days of thinking to work out what actually occured, as it really looked like there was something wrong with the wing, aileron, thrust lines etc.

But the one clue when I thought about it was that as it took off the tail was dragging a little and I had to use a fair bit of up elevator to get off the ground. This was the clue......bingo...the horizontal balance was bad, the tail was heavy.....

Double check the horizontal balance point and also double check the CoG point.

Also further thinking overnight has also fixed a gap in my thinking, have you checked the incidence angles of the main wing and the tailplane?

Generally too much incidence on the main wing will induce a stall condition quicker. This is however dependant on the airfoil used and its aerodynamic centre of lift.

So what does all that mean?

If you were to look at the airfoil used on a piece of paper and were then to draw a line between the tip of the leading edge and the tip of the trailing edge this would give you an approximate centre for lift (this is not perfect, as each airfoil has a mathimatcial lift centre that will not be the same, but it is close enough for us)

If you were to take this line on the aircraft and relate it to the Fuselage centre line, then this will tell you the number of degrees of incidence from the aitcraft centreline.

This should generally be in the range of 0 to +3 degrees. If you then relate this to the centre line of the tailplane you will get the decalage (the difference in the angle of attack between the two lifting surfaces).

It is normal to see the following;

Tailplane to Fuselage Aerodynamic centreline - 0 Degrees

Tailplane to Wing = +0 to +3 degrees (leading edge higher than trailing edge. (dependant on airfoil)

Vertical Engine Thrustline to Fuselage Centreline - 0 to -5 degrees Down

Horizontal Engine Thrustline to Fuselage Centreline - 0 to 5 degrees right thrust.

If in doubt I always start with 0 all round and then do the flight tests to adjust all.

Also if you want, send me a copy of the plan you used (if possible and I will give it a look over and send back my thoughts)

Thanks

Craig

(in reply to ZoomZoom-RCU)
       Post #: 6

good thoughts - 11/13/2002 12:43:03 AM   
ZoomZoom-RCU



Posts: 464
Joined: 6/30/2002
From: Louisa, VA, USA
Status: offline 		Thanks again for the input. The wing chord is 8 inches, so I have the plane balancing a trifle nose heavy at two inches back from the L.E.. This is the 25% mark. When approaching the 30% mark its even more nose heavy at 2.4 inches, so this info combined with the fact that it taxied and climbed out nicely "ie no heavy tail symptoms" makes me revert to the lateral balance and wing warp issue as being the fundamental potential causes. I scratch built this thing from original photos and some simple three view drawings so there are no "plans" to speak of, but thanks for asking. If the lateral balance and wing warp removal suggestions dont help then I'm gonna be guessing its a thrust or prop issue, but I'm hoping correcting these first issues will solve the problem. Thanks again.

(in reply to ZoomZoom-RCU)
       Post #: 7

update - 11/14/2002 7:30:28 AM   
ZoomZoom-RCU



Posts: 464
Joined: 6/30/2002
From: Louisa, VA, USA
Status: offline 		Well well, we're ready for another try. This is the post rebuild report.:

As it turns out, it looks like there was a bit of a warp in the left wing. The trailing edge on the left side hung lower than on the right. In addition, the right wing was laterally heavier than the left. I did a balance check and it proved my suspicion. So, I've removed the warp and added balance weights to the left wing tip and now she hangs true, prop to tail. So, in my opinion, these things combined were probably enough to cause the roll to the right after takeoff, and the subsequent lack of control and crash.
Both elements would push the plane into a right bank. The lower left side wing warp would act like a down aileron on that side, causing a right roll. In addition, the heavier left wing would only contribute to this phenomenon, magnifying the effect. I'm hoping this will lick it. I'm not changing engine thrust at this point as roll out and takeoff were clean. I'm thinking these are the fundamentaL issues. Soooo guys. The first nice day that comes, I'm a test pilot again. I'll let you all know how it goes. Thanks again for all the help.

(in reply to ZoomZoom-RCU)
       Post #: 8

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