Surgery on an Old Fogey!

by alibopo | August 31, 2013 | (0) Posted in Projects

Modifying an Old Fogey

[UPDATE - 31/08/2016 - Regarding these modifications in this article, I've been exploring the topic of Angle of Incidence, both from research and from experimenting with builds. The conclusion I've come to is that for most model planes, no more than 2 degrees Angle of Incidence is required for the vast majority of more modern designs. This actually led me to reduce the 'real life' Angle of Incidence on model plan versions of biplanes I designed - why? Because with the 'real life' Angle of Incidence, the wings produced too much lift, which needed to be countered by negative inputs like; adding lots of nose weight, excessive down elevator, or a steep downward thrust angle on the motor. Basically the plane is fighting itself - one part trying to lift the plane, the other part trying to keep it level - it is a waste of battery power that could be used to keep the plane flying longer. With this version of the Old Fogey I kept the 4 degrees Angle of Incidence of the original, and relied on a pile of nose weight to keep it flying level. If I had known at the time, I would have re-profiled the wing seat to reduce the A of I and flown it on the correct CG, which would have given me a freer-flying plane and much better slow-flying performance. I discuss this in my recent article 'Grouchy' Old Fogey getting you down?

NOTE - One of the solutions I suggest in this article, adding more nose weight, is actually counter-productive, making the plane very nose heavy and ruining its slow speed performance. The other suggestion, lifting the tail, is a perfectly valid way to eliminate wing-waggle. However, in the 'Grouchy' Old Fogey article I show a more benign way to modify the Old Fogey fuselage. Plus there's links to two alternate scratch build fuselage designs for the Old Fogey wingset, that have a wide flight envelope and will fly with zero wing-waggle. ]

When I first built the Old Fogey it flew great at slower speeds but as soon as I went faster, there was a bit of a wallowing problem. At the time I fell into the trap of thinking the plane was tail heavy and solved some of the issue by adding nose-weight, (that’s the pile of coins under the yellow tape) producing a final CG that was well forward from the one in the design.

However as the plane aged and became heavier, the wallowing gradually got worse and I just couldn’t get it to fly right. It had done it's job though, getting me through a lot of early flying. As soon as I moved-on to other planes I lost interest in the Old Fogey and put it aside. Then my son said he wanted to learn to fly...

I knew the Old Fogey had worked well for me - so would be a great plane for him to start with. I needed my Old Fogey to fly properly again, I needed it to be super-docile and very predictable.

To that end, I did some research on the Internet, looking to see if anyone else had come across this wallowing problem - specifically with the Old Fogey, and then more generally with other planes. From what I gathered, it seemed that a good way to combat the problem was to make the vertical stabiliser bigger to help dampen the oscillation.

I had a look at my Old Fogey and thought maybe there’s another way to do it… maybe if I could get a bit more airflow on the existing stabiliser that would help. After all, it is tucked-in behind a pretty big fuselage.

Now this solution is a bit drastic! Those of a sensitive nature should look away now.

Basically I shifted the tail up about 70mm. I was too quick to get a 'before' image, but you’ll recognise the outline of the original Old Fogey in this mock-up. The method I used involved shortening the fuselage by 20mm - which is why there's a gap there in the photograph.

Here’s the after. The tail has been shifted up and forward, and then reattached to the main fuselage.

I made the first vertical cut about 50mm back from the rear wing mounting skewers…

…I made a second cut parallel to the first and 20mm further back. Here's the cunning bit! I did NOT cut through the fuselage base. After removing the two side strips I worked forward on the fuselage, cutting through the glue join to free the next 120mm of fuselage base from the sides.

I then made two folding scores on the fuselage base – one score on the INSIDE of the fuselage at the end of the cuts I’d just made, and one on the OUTSIDE where the fuselage base projects from the tail. When these two bends are made at the same time, the tail ‘hinges’ up - rising and moving in to meet the main part of the fuselage.

Because the original cuts were straight, parallel and the same width apart - when the tail connects its alignment remains the same. Take care during the re-gluing to get it in the right place. Make this job easier by gluing one side at a time - as you glue the second side you can adjust up and down a little to get things just right.  

Following the new 'sporty' line of the fuselage base, trim away the excess sides. Now re-glue the sides to the base.

For added strength I put a plate on top of the tail. First I removed the curve, marking-on and making a straight cut from the tail to meet the old fuselage line just behind the rear skewers.

I then made-up a strengthening panel - making it long enough to overlap the tail, where I added a little slot in the centre to help keep the tail vertical…

…and long enough at the front to extend under the wing to give a strong connection to the main fuselage. The last part of the pad needs to step-in so that it fits between the sides.  I re-led the control rods, running them over the new panel.

My servos are on a higher mount (long story), yours will be ‘as original’ and you can poke/run the control rods over, through or under the new panel.

I checked my balance – purposely making the plane nose-heavy on the ‘plan’ CG. Neutral CG for me is about 10-15mm forward from the under-camber fold. [This, in retrospect, was a poor decision, a better answer is discussed in the UPDATE at the start of the article.]


Here's my Old Fogey back in the air. For me there's a significant improvement in this plane’s behaviour. The roll is almost completely gone, and when it does happen (due to wind gusts) it settles very quickly. This was a moderately windy day – low down near the ground it was gusty due to the wind 'rolling' over the tops of tall trees near where I was flying. Higher up, where the wind was more uniform, the plane appeared very stable. It could just be a lucky accident, but I now have the ideal plane for my son to start out on.

After the flight I increased the motor power to a 14A motor (roughly 150W). This should give me a better climb rate and more consistent speed. It also allowed me to remove all my coin ballast.

I've limited the motor's output by setting the throttle maximum to 60% - this should contain any boy-racer urges my son has. As with the original Old Fogey, this plane does not need to go fast to fly well.

Onwards and upwards, Alibopo :) 

COMMENTS

Purplezinger on September 25, 2013
I made a new rudder/vertical stab, same general shape but about 25% larger. I also moved the balance point 15 mm forward as you suggested. The difference is amazing. It now flies like you would expect. Nice slow lazy turns, really nice figure 8's, a pleasure to fly. The glide slope for landing is steeper than I like so I'm going to try a longer wing. Easy to build and easy to do AB comparison. Thanks for the suggestions, they made a huge difference ~ ECW purplezinger@gmail.com
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alibopo on September 26, 2013
Good to hear! The increased size vertical stab should be making a significant difference to the stability. You could try shifting the new CG back a little for slower lighter wind flying, that should still work? I push it forward for the stronger winds (nose heavy?). Possibly the 'correct' CG is only about 10 forward. I suggested 15mm so that people would really see the difference. Happy flying!
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Purplezinger on September 14, 2013
My Old Fogey has never flown well. When balanced at the original B.P. it wallows, sways side to side and stalls. The only way I've ever gotten it to fly somewhat normal, is to move everything forwarded as far as possible. The down side is almost no glide. I may try the larger vertical stab.
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alibopo on September 15, 2013
I think CG is important (and part of the answer) - but getting a bit more wind on the tail has proven very helpful. Have a look at my blustery day video of my Old Fogey. Also if you've ever seen the RCDOX utube videos (some of which are a scream due to the girl introducing the series - she's great!) you'll see some interesting plane designs - http://www.youtube.com/watch?v=oilw3Ly7ut8 & http://www.youtube.com/watch?v=gVYBG48_b6M - there's others - many of them feature planes with pretty big tailplanes - and when they eventually get them in the air (you'll see what I mean) they seem to fly pretty well. I've watched a lot of Old Fogey maidens and flights - and many of them seem to fly high-alpha (tail heavy?) which I don't think works terribly well with the dihedral wing. To my mind the Old Fogey tail should be big enough - it just needs to get a bit more air around it, which is why I opted for my solution. I'd like to see how a 'bigtail' Old Fogey would perform. In the second RCDOX video I've listed they modify their plane (1.10 into the video) to have a bigger tail by adding a top panel to the rudder that overlaps the vertical stabiliser - that looked a neat solution. Worth a try, but I'd also adjust CG as well. My Old Fogey can still cruise around at pretty slow speeds, but I do shift the CG back a little from my CG position (5-10 mm forward of the fold) to get that slow speed performance.
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CStence on September 15, 2013
Kinda reminds me of a Cessna 337 :) Cool!
http://en.wikipedia.org/wiki/Cessna_Skymaster
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Gryf on June 24, 2014
I also extended my Fogey's fin/rudder, making it taller to get more surface into the airflow. It made a noticeable difference, and made a fun airplane even more enjoyable. I agree with you on the CG. I glued a large hex nut to the backside of the firewall, and that seemed to level it out nicely. I can fine-tune it via battery placement.

Gryf
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Surgery on an Old Fogey!