Squeezing the aileron, or Kutta Rules!

In the post "The first hundred days. Part II", see below, or click here, I indicated I had a heavy left wing and planned to squeeze the RH aileron to fix it. I knew the theory, but was very concerned as to just how much adjustment would be required since I had never actually done it in practice.

It has taken a long time to pluck up courage to do it, but as a first step I decided to very gently squeeze two bays of the aileron with my hand seemer as you see here. I protected the paint with some old soft hardboard, and squeezed it very gently so I could just see some change in the reflection when I took the pressure off.

I was quite sure that I had not done enough but decided to add another bay each time I landed until the problem was fixed. So, today I flew, and with this gentlest of adjustment it is perfect. So, if you have to do this, go gently!

Lean carb? Enlarge that jet!

Many carburetted RV report running too lean.

From very early in my experience with my RV4, it became clear to me that while I had no problem climbing at a high rate with reduced power, a full power climb for more than about 3000', resulted in the CHT heading rapidly over 400F. Since much of my early power flying, involved towing heavy gliders on hot days, with the throttle wide open for extended periods, without overheating, and at considerably slower speeds than my RV4, I found this less than satisfactory.

I have spent quite a bit of time understanding how the mixture is managed in the MA-4SPA carburettor, and  thought it might be worth capturing what I have learned in one place, since I had  found the relevant information scattered and hard to find. I can give no guarantees to its accuracy, but it is working well for me.

The standard test for the richness of the mixture is as follows.

Climb to 8,000-8,500 feet density altitude. The purpose of this is to get to a height where the engine can not develop more than 75% power and be damaged by leaning.
With wide open throttle (WOT), and mixture full rich, note all EGT's and start to lean very slowly. The EGTs will rise at first. Note the cylinder which is "first to peak" and note that peak EGT temp. This is relatively simple if you have a recording EFIS, but otherwise you will need one person to fly, and another to manage the engine, and take notes. I would continue leaning and gain an understanding of the behaviour of the successive cylinders.

Take the "first to peak". and calculate the delta between full rich, and peak EGT. 150F min difference at 75% power, rich EGT to peak EGT I understand to be ideal. 80-100F is too little.

Flight testing is always easier in theory than practice, at least for me! In the diagram to the right (open in a new tab if you want more detail) after the 8000' climb, where I am at slightly reduced RPM and higher than normal airspeed, to control CHT comfortably below 400F, I start leaning. Without a vernier control, the first time I do this, the fuel flow reduces so much, that the #1(red) ceases to ignite, and the EGT drops. You can see that I then richen it up and we establish a base EGT of about 1400F for #1. I then lean and it peaks at 1443F, for a total rise of only 43F. Far too lean! #3 rises about 130F and the left hand side of the engine does much better. The bottom line is the engine needs more fuel coming through, for longevity. The disparity in the behaviour of the cylinders is I understand a feature of carburetted Lycoming engines.

While there is a big screw on the back of the carb that is used for adjusting the idle mixture, it has no effect on the full power mixture. Don't touch it for this purpose.The full rich mixture is controlled by the pressure differential between the two ends of the main jet, and the diameter of the bore of the jet. This differential causes fuel to be sucked through the jet. The bigger it is, the more that comes through. (At this point I should confirm that yes, you can make it leaner of course with the mixture control, but you cant make it richer. The leaning mechanism on this model works by controlling the flow of fuel, from the fuel bowl, to the base of the jet.)

It turns out that the appropriate sized jet is somewhat installation dependent, resulting in different certified aircraft having different jets on the same engine. In our home built world, no one has done this testing for, in my case, an RV4. The 160HP O-320 I have, was supplied by Aerosport Power with an MA-4SPA carb and a jet of .096" diameter (#41).

As soon as I mentioned this issue to Aerosport Power they indicated they would increase the jet size if I returned the carb, or I could run a drill through myself. They had done this many times before and it was a 'routine' solution to a familiar problem. (If I were ordering the engine again I would raise this issue before hand, and I am sure it would have arrived with a bigger jet. Indeed I wonder why they do not routinely do this?)

Since I am in the UK, and returning the carb. was a little impractical, they in fact sent me a replacement jet of .104" dia. (#37) and the associated lock washers and gaskets etc.. I was a little concerned about the jump in size, a 17% increase, but they said 'This will work. We have done it many times.' 

I will come back to the process of changing the jet, but right now you are probably wondering what was the result. Well, here is the very first flight with the new jet.

The first change I noticed was that on pushing the throttle in for takeoff the historic flow of 50 to 51 lph (13.5USG) had jumped to about 59 lph. It was reassuring to see  an approximately  17% increase in flow, just about the same as the change in area of the jet bore. At the time, as the pilot, it was also reassuring to see that I could climb from the end of the runway to 8000' with the throttle wide open and the prop just short of 2700 rpm with no tendency for anything to start to bump into 'the yellow'. I flew the climb badly. I was trying to talk to ATC, monitor the engine and hunt for what felt like a good climb speed at what was a significantly steeper angle. However, with all that aside, the climb was at least 130fpm faster. As I said before flight testing is easy on the ground , but in the air I find it more difficult to get everything right. I had done the whole climb with carb heat on! (One day I will get some accurate climb figures but that was not today's mission.)

The result of this is that all of the data to the left of the 16:25:25 mark can be ignored for comparison purposes. You can see the second, dark blue line from the bottom drops, and that is where the carb heat is set to off.

I need more data yet for confirmation but if you look carefully at the peaks now, just to the right of the 16:25:25 mark, you will see rises of:

#1 334F red first to peak

#2 343F black third

#3 185F green second

#4 333F blue last

So now I am running rich, though its a much nicer situation than it was. In fact #3, with a rise of 196F, while not the first to peak, is only a bit richer than commonly recommended. I will do nothing in the short term, but it would be nice to try a jet that was drilled .1015" dia (#38). I would like to live with it for a while and collect more data, and also fly in colder air, and repeat the test. Even at 8000' the air on the day of the test was 31F.

It cant be so rich, because I have tried hard to make it rich cut, but its not hesitating. The power comes through smoothly.

Some readers might think that my overal fuel consumption will increase significantly. This is not true, since the mixture is normally controlled by easing back on the red knob, except in high power operations which are a very brief part of my flights.

One thing has improved which I was not expecting. The oil has always run rather cool. For the first time ever the oil temp was sitting just above 175F for a significant portion of the flight. This is certainly a welcome improvement.

So that is the result to date. I will add more if/when I have significant extra data. It does however leave the question of 'How do you change the jet?', which bothered me significantly before I had done it.

First of all you will need some parts. They are the gaskets you see in the picture, note the tiny one at the base of the jet, lock washers to hold the screws that hold the two halves of the carb together, a new lock washer to stop the jet from rotating (part 12 on the diagram), and an important and small split pin (part number 5o). You will then need either a new jet, or be prepared to drill the one you have. If you do it this way I suggest you go in small increments. I was lucky to be supplied a spare by my engine supplier, though I think it is perhaps one size too large.

I was tempted to split the carb on the aircraft, though was talked out of it. I am glad I was, because it would have been a mistake.

I removed the 4 bolts that hold the two halves together, but they were unwilling to separate at the gasket. If you have to take a knife, as I did, to ease the gasket away, try to leave it initially with the upper half of the carb., since it will be trapped behind the float assembly otherwise. 

As it separates, there are four items you have to be careful of:

1) the piston of the accelerator pump, almost out of sight in this first picture.

2)the brass fitting that responds to pulling the 'red knob'. It sits on a spring and has to be gently coaxed out.

3) the little clip on the centre of the float shaft and the needle of the needle valve which it grips and controls. If you double click on the picture, you can just about see these two part, just below centre.

4) the float assembly itself.

The carb is a precision piece of equipment so take it slow and gentle. Because the gasket stayed stuck to the lower half of the carb., with the blue float trapped by it, I had to gently clip the split pin on the float shaft. 

Either way, you will probably have to remove that pin sooner or later in order to install a new gasket, though it may be possible to do it without doing that.

With the carb in two halves you finally have access to the main jet, but make sure you spot the minute gasket on the lower end of the main jet when you remove it.

In this picture it is worth noting the copper tabbed washer that locks the jet in place. Just to the right is the jet that operates when you function the accelerator pump.

Once you have the jet out you can drill it or replace it etc., and then very carefully reverse the process and put it all back in place, remembering to do all the lock washers as you go. It is the end in this picture that is out of sight that has to be altered.

Make sure you hook the throttle cable through the same hole as before. 

On a certified aircraft I once found the carb coming loose despite the lock washers, so I have taken this rather unconventional step of putting a wire lock across the threads of the pairs of bolts as you see in this picture. I defy anyone to get a nut off without first clipping the lock wire.

With it all back together get it checked over and signed off, before you flight test it.

Information for this came from many sources and conversations. A particularly useful post I found on the Vansairforce web site was this one. I would like to thank all the others who have helped me. They know who they are.

Postscript 8th October '10 - I subsequently reduced the jet very slightly to a #39. This reduced the fuel flow very slightly at full power but still considerably richer than it was. Flow now peaks at 55 lph (14.5USG).

North Coates Icicle Fly-in, 3rd January 2009.

I took myself to the North Coates icicle fly-in at New Year. If they are brave enough to arrange it they need support. A mainly bright but cold and frosty day brought a good turnout.

The airfield traces its origins back to WWI, though its not until WWII that it became really busy bombing German shipping in the N Sea.

I met a very friendly serious amateur photographer there who gave me these excellent pictures, so I thought I would share them since he had no objection.

G-IKON is approaching completion of her first 50 hours, so we are getting to know each other. Still much of my flying is testing and calibration, but she is performing well with only the most minor snags. Dirt in a drain valve has been my most frustrating recent problem.

One issue I am close to giving up on, is that there is no doubt the statics under record the pressure, leading to a 7mph over reporting of speed. And no, they are not leaking to cabin pressure!

Flight Analysis

The AFS3400 is very useful for analysing flight data. During a flight it records around 50 parameters every 5 seconds. I have written a macro that takes this information into Excel and charts it. It then performs some additional features. I want to know in particular where my -4 operates most efficiently in the cruise at a reasonable speed. The grey line at the top of the graph (You will have to double click it to get a large enough display I think) shows where the miles per litre is exceeding 6 and the aircraft is not changing height. This has lead me to cruise, at least at 3000' at 22.5" and 2050rpm. I can lean out to 22lph or a little less and still make good progress. I found these charts useful also for adjusting the baffeling to even out the CHT. Its a carburated engine so only so much can be achieved!

The first 100 days. Part II

In Part I, I covered my reactions to flying the –4. Here I thought I would summarise my thoughts about the results of the decisions I made along the way, and the problems I ran into during the first 25 hours of flight test.

Many of the decisions related to weight. I have covered my thoughts and the result elsewhere in the blog, so I wont repeat that here. Needless to say, keep it light. Whatever it is, if you think long enough, you probably don’t really need it. Without it you will be lighter, have more money in your pocket and what you dont have can't go wrong.

The first issue I had was with the 4-pipe exhaust from Larry Vetterman. Pipe #1 just touched the sump. Larry first suggested I bend it, but as soon as that was not going well he said “…best I make you a new pipe.” It was here in the UK in no time. Great customer care! The 4-pipe is undoubtedly slightly noisier, but friends say not a problem. They also comment it has a very distinctive and pleasant tone.

Oil Pressure. I am delighted with my engine from Aerosport Power, and I cant imagine better service, but it did come with the oil pressure set too high. I had to reduce the pressure by about 10 psi, which was about 3 turns of the adjuster, to get it to stay inside the upper limit (90psi). The problem isn’t so much adjusting it though, as deciding if the problem is the pressure, or the gauge. In the end I managed to borrow a test gauge, plumbed it in, and it confirmed my ACS 3400 was right on the money. It was disconcerting on a new aeroplane though, until I got is sorted.

Oil temperature. The oil runs cool. I am not sure I have yet seen more than 179F. I have blocked off much of the entry to the 3” SCAT that takes air to the cooler. This appears to have improved things slightly but I need to do some more flying before I am sure.

I decided to take the exit air from the cooler into the cabin for cabin heat. This is working quite well, though its still only autumn in the UK.

With an electric heating pad in the seat, I think this will be quite adequate. It might yet cause me to put a variable restrictor on the air to the cooler.

Oil Leak. I had a significant leak on the cooler, for no other reason than I had failed to tighten the fitting adequately.. It did not show up after the first brief ground run, but it sure did after I landed from the very first flight. I probably only lost a quarter of a pint of oil if that, but a little goes a long way!

For the first flight I had a very heavy left wing, but a temporary trim tab on the rudder reduced this considerably. I was left with a slightly heavy left wing so I put a small coffee stirring stick on the underside of the right aileron.

This has balanced things up for now.

Sometime I will squeeze the RH aileron which will get rid of it due to the ‘Kutta Condition’. If you want to understand why that works there is an adequate explanation here.

VANS 'heavy wing' article which is worth reading if you have the problem is here.

Rubber Wing Root fairing. It was slightly disconcerting when these came loose in flight. A dab of RTV appears to now be holding them securely to the metal fairing.

I found the AoA in the AFS3400 hard to set. Just me I think, being ham fisted. In the end Rob Hickman gave me the file of settings for his –4, and that has worked well for me ever since.

A small leak in the static side of the ASI had a remarkably large impact on the indicated speed. It was quite frustrating to track down, but I got there in the end. Since the leak was to cabin pressure I was surprised by just how much this must differ from the ambient pressure. Even now, the system over reads slightly, but I put this down to the less than satisfactory rivet heads that VANS use.

The most frustrating thing of all was the intercom. Having indicated my interest in mono, with stereo would be “nice to have”, I got no suggestion from PS Engineering that I needed to allow for this in the wiring. I would strongly recommend people to go to Flightcom, since their documentation and understanding is far superior. They even supply the little switches and labels you need. The story is in fact much longer, uglier and expensive than that. Never an apology from PS for all the grief and expense they caused me, though I am pleased to see they have updated their FAQ to reflect the issue.

The Skytec LS starter bent its bracket after just a few hours of use. Aerosport Power had a new starter shipped to me in no time, and chose to replace the LS with the NL model at no extra cost. This is a much more serious looking piece of equipment. The LS normally has a problem when the engine kicks back, but since the P-mags retard when starting, Skytec guarantee the LS unit against this. My own view is the bracket is just too weak for a new engine with good compression, and a fully charged battery with a low resistance starter circuit. Who knows?

The dual P-mags have run faultlessly to date. I have little paper tell tales, that change colour if they overheat. Overheating has been a problem on some installations. They are still white. In the interest of simplicity I decided not to install a fuel primer, and rely on the carburettor pump to prime the engine. 3 strokes of the throttle, both P-mags on, and she starts really easily. Lets see what happens come the winter.

My oversize tyres work well. Near my hanger I have some gravel which I have to pull the aeroplane over. The –9a with the standard tyres I had, used to sink in and get stuck. With these, its not a problem. Despite their size they fit well inside the wheel pants, and to date they are not getting clogged with either the output from these sheep, who mow the runway, or mud.

The 2-blade MT prop is a delight. Really smooth in operation. The governor had to be very slightly adjusted slightly initially to stop it over speeding. Now it maxis out just below 2690. My only reservation is that it would be easy to scratch target="_blank"in the hangar. I am quite clear a 3-blade would have been a big mistake for the reasons I gave before.

The current outstanding issues are:

1. I would like to get a little more cooling on #3 and a little less on # target="_blank", so I am playing with a baffle to push more air over the top of #1.
2. #2 CHT can run a little hot, and the EGT cooler than the others, so I am wondering if I have a tiny induction leak, or if this is the variability of a carburetted system. Next time the lower cowl is off I will have a good look around at #2 induction.
3. Taxiing I have a very slight rattle in the gear. My thought was that it must be the gear leg fairing moving wrt the wheel pant, but a friend has suggested possibly the brake callipers can rattle. I will try taxiing with the brakes gently on.

Finally, I would just like to say what excellent and warm service I have had from Aerosport Power, E-mag, Robbins Wings, Larry Vetterman, AFS, Becki Orndorf who made the seats, and John Stahr, who painted it.

The first 100 days! - Part I

The first 100 days with an RV4! What a blast its been. This is the second VANS aircraft I have built. Last time I was disappointed, I built one of the designs for geriatrics, this time I am ecstatic. I thought it might be useful to other -4 builders, to gather my impressions and experiences while its still all new and fresh in my mind.

As I was building, I had to make many decisions that would have a bearing on its future utility, and as I moved towards the first flight I had many queries as to how she would handle.

Details of my -4 are covered in much more detail elsewhere on my blog, but G-IKON is powered by an Aerosport Power O-320 160 hp engine, driving a two blade MT c/s prop. She is the first aircraft in the UK approved to run on twin P-mags.

I’ll start with the flying, because that is what this is really all about, but then cover my experiences with some other items in Part II. Before I start I should say I have over 400 hours on Supercubs, and more on other taildraggers, so tail draggers are not strange to me.

To date I have about 29 hrs on G-IKON so am far from experienced. I have just finished a much longer than normal flight test because of the P-mags. Because of the flight test I have only once flown 2 up. I have only flown on/off grass so far, so all-in-all, plenty yet to learn.


Takeoff. I have a flap position indicator on the wing. Early takeoffs I set this to around 10 degrees, though I now set around 15, and find this may be marginally better. This gets me off the ground in about 300’ which is VANS number. I had seen tail dragger RV pilots have quite a bit of a problem with torque steer as they initially put the power in, but I have not experienced this at all. I do find a distant tree or cloud to focus on, so perhaps this is the reason. I am not aware of putting any rudder in, but some say I must.

By the time I am off the ground I start putting the flaps away, and start the transition into the climb as soon as I see 100 mph. Below 100 she is very draggy. This speed comes up about 1000’ after the start of the takeoff roll. My immediate reaction on the first flight was one of a solid predictable and responsive aircraft.

I feel I still do not make quite the clean transition from rolling to flying that I would like. There always appears to be an instant when we don’t quite know which we are trying to do. Perhaps I think I get a message saying its time for flight half a second early. What I have found is that its best to get the tail wheel off the ground ASAP, but then leave the tail low, so once we are flying, during the acceleration I am actually pushing ever so slightly forward as the speed builds, until its time to climb.

Landing. My home runway is short. Fence to fence is 1020’ and it has many other complications. Confidence to return to it, came after 27 landings at a local grass airfield where we got to know each other, after the initial take off from my home strip. I still don’t quite believe it, but a –4 is a remarkably easy plane to land compared with a Supercub, and needs about the same landing distance. I typically use 600 to 700’. At first I struggled a little doing 3 point landings, which didn’t always work so well. The problem was the tail wheel would hit a small bump which would throw it back in the air, sometimes worryingly high. Now I flare until the tail wheel is, at a guess, just 3” off the ground. That so far has produced a winner every time. Not to do quite fully held off landings has been quite hard to learn, but she still lands nice and slow. (I should qualify this. Landing two up is completely different, though I have only done 3; very ragged. I havn't cracked that yet. Its a completely different aeroplane! As the speed bleeds away in the final second, possibly it needs a push on the stick to hold the attitude. Perhaps its just very light. It has caught me out each time somehow.)

I said easier to land than a Supercub. This is true, but I suspect much less forgiving. Partly for that reason I have an AFS AoA installed, and I expect 'Jennifer' to give me a good talking to, if I fly abusively slow. So far she has only done it as the wheels touch. "Angle, angle, push!"

Somewhere in the circuit, with full flap, I trim for, and settle at 60 mph, solo, and let that start to bleed away just before the fence. She is still very ‘solid’ at that speed. I approach on the flat side which requires quite a bit of power, but I find that helpful, since as you start feeling for the ground, as soon as you are confident there are only inches between the wheels and the grass you can shut the throttle and she gently settles with no intention to bounce. I usually lift the tail once the mains are on. I can see better, my strip is narrow, and the rudder is more effective, though I still find myself using differential braking.

I have little cross wind landing experience with G-IKON so far, but the one I had to do went well. I crabbed the approach and converted to wing low quite late. This worked. Once on the ground, rudder, with the tail high, plus differential braking, enabled me to track as I wanted. As the autumn winds set in across my strip I will soon have much more experience. The equinox was 2 hours ago.


Cruise. Not a lot to say here. She is extremely exact to fly which is very pleasing. VANS say I should see 207 mph with 160 hp, and that, if you wait long enough, is exactly what she does. I have been breaking in the engine, so have far too much experience at high power in the local area. (I am limited during flight test to 35Nm from base.) In the last few days I have been hunting for a normal operating regime. Its beginning to look like perhaps 2300rpm and 23.5” which gives me a 180 mph cruise. 2200 rpm at 22.7” which leans out to about 24lph - about 6 little US galls. – and results in 165mph is also a regime I return to. I will be more exact about these numbers once I have been on some trips and have an established regime.


Handling in general. Fantastic, harmonised, delightful. I don’t know what else to say. If you are still banging rivets to build a –4, keep at it. If you are disappointed, there is something wrong with you. If you cant get in a -4, stop going to McDonalds. Do build it light though. (See 9/5/06 and 6/10/08)

I will come back with some more observations in Part II about the technical problems I had to fix, and how some of the systems have worked out, but that’s it for now.

Flight testing - 2

Flight testing continues. I have done about 8 hrs now and am beginning to settle in. The rudder is now trimmed to put the ball in the centre and the ailerons are also now in balance. Its a much more comfortable way to fly.

As you can see from the attached graph, double click for a larger version, the oil pressure is now under control.

These 6 flights all occurred on the 4th July. I took off, and soon landed back to adjust the aileron trim. I then took off and flew at quite high power for a little over an hour, to work on breaking in the engine, and at the same time flew calibration triangles. The yellow line is calculating VANS approximation for HP. So 45 is 65% power and 48 is 75%. It is multiplied by 10 for clarity. There is a break in the middle while I worked on calibration of the AoA.

At the end you will see after a break, I did 4 circuits to work on landing technique.

There are some interesting points that show up:

1. At about 11:49 there is a tiny leaning of the mixture (magenta). There is a clear, if slight, increase in power (yellow line), and a very significant change in the EGTs. In particular EGT1 (green) goes from second coolest, to hottest.
2. At the same time CHT4 does the same thing.
3. By 12:18, with 75% power set, and fuel flow increased the EGT and CHT relationships have switched around again.
4. CHT1, the green line always sits lower than the others. I wonder if there is a case to reduce the airflow to this cylinder.

At the end of the flight I did 4 circuits in about 16 minutes. It is interesting to see that with the low air speed involved, the oil temperature climbs quite a bit higher than it had previously reached. It ended at 190F, still a well controlled temperature, but worth remembering. The ambient temperature recorded at this point was around 70F.

I am sure I should be observing far more information in this data, than I am. at least I am learning.

My main conclusions are that the CHTs, oil temp and pressure are well under control. I am spending a significant amount of time with the power above 65% which is good for break in.