Ziroli P38

[Chris Freeman 3]

A while ago Byron went to visit one of our flying buddies and when he returned he asked me to come and have a look at something! On the lawn was a mostly framed up P38 lightning! Jon had run out of enthusiasm for the project and asked Byron if he would finish it! The airframe had been around for a while and was starting to pick up damage from being moved around and the number of components and the size made this a very serous build. A big tub contained all the spinners, plastic parts and the Robart Retracts with an electric conversion kit.

 

I started to do some of the woodwork in the areas that needed fixing or strip planking to be finished. Whilst working on the bits we decided it would be best to glass as soon as possible to make it more ding resistant. As a piece was finished it went down the road for Byron to start final sanding and glassing. Byron also spent a lot of time getting all the parts to fit better as the wing is in 3 parts, removable booms and stab. Work has been on and off as other projects had priority.

 

When the DC3 repair moved back to me, Byron started to do more work on the P38 and On Sunday when I went around to his house it was back on the lawn for me to see and help discuss some ideas. I must say that it is a very impressive aircraft but also one that takes a huge amount of perseverance or long suffering to build and also one that needs to be flown with a huge amount of respect. This is were it is now:       

[RICHARD WILLS]

I do love a P38 ! 

I think Ive made six smaller ones from scratch . Thats bordering obsession 😬

Edited November 4 by RICHARD WILLS

[stu knowles]

Steve Ricketts had a lovely Z P38 which went in due to elevator flutter (I believe)  I once read a list of mods that he considered necessary to the original design but where I saw it is long gone.  It would be worth checking in with him.  His father uses this forum.

 

[Chris Walby]

I was talking to Steve a few weeks ago and IIRC he mentioned that the P38 was a great flyer and very eager which resulted in pushing the envelope at times. You know the sort of thing where its just begging for a fast low pass etc which ultimately resulted in a few bin liners 😞

 

I have a FlightLine P38 and although its foam, over 3.5 Kg and only 63 inch wingspan is a real pleasure to fly (it has the motor upgrade pack so good for 94 mph), but quite a few meet an untimely end due to people flying them non-scale. If you wot drive bank and yank I think it will clap its wings, but IMO that's not how it should be flown!

[Jon H]

2 hours ago, stu knowles said:

Steve Ricketts had a lovely Z P38 which went in due to elevator flutter

 

I think he lost 2 with horizontal stab failure but i cant remember for sure. I think the engines from the 38 went into the A20 he built afterwards. 

[Chris Walby]

4 minutes ago, Jon H said:

 

I think he lost 2 with horizontal stab failure but i cant remember for sure. I think the engines from the 38 went into the A20 he built afterwards. 

That might explain my sticky carb barrel on one engine and one carb as there is an external mark on the carb body on the A20. 🙂

[Chris Freeman 3]

12 hours ago, stu knowles said:

Steve Ricketts had a lovely Z P38 which went in due to elevator flutter (I believe)  I once read a list of mods that he considered necessary to the original design but where I saw it is long gone.  It would be worth checking in with him.  His father uses this forum.

 

Thanks Stu, I will follow up on this.

 

Byron has done a bit of research on this subject and the booms are a problem, they break just behind the wings or just in front of the stab. The booms look bigger than they are but when you see how much is removed for the wheels to retract and also to have a hatch to allow them to be removed from the wings it leaves just the sides of the boom for strength. The back of the booms is the same, not much wood as they are rounded and then have the stab in the middle of them, compounded  by the fact that they are a long way behind the CG so you are trying to save weight. The pushrods are piano wire inside a nyrod which is as per the plan, I will be checking on this as servo installation is next. 

 

The original design called for Zenoah 38's but double the power is often used. Dennis Crooks raced one in the Madera races for many years and this was a composite version he made. He had 4.2 motors and apparently on one race the one motor quit just after take off so he flew the entire race on one motor!

 

  

[Chris Walby]

1 hour ago, Chris Freeman 3 said:

Thanks Stu, I will follow up on this.

 

Byron has done a bit of research on this subject and the booms are a problem, they break just behind the wings or just in front of the stab. The booms look bigger than they are but when you see how much is removed for the wheels to retract and also to have a hatch to allow them to be removed from the wings it leaves just the sides of the boom for strength. The back of the booms is the same, not much wood as they are rounded and then have the stab in the middle of them, compounded  by the fact that they are a long way behind the CG so you are trying to save weight. The pushrods are piano wire inside a nyrod which is as per the plan, I will be checking on this as servo installation is next. 

 

The original design called for Zenoah 38's but double the power is often used. Dennis Crooks raced one in the Madera races for many years and this was a composite version he made. He had 4.2 motors and apparently on one race the one motor quit just after take off so he flew the entire race on one motor!

 

  

Just my 2 pence worth and I can totally see why people put overly large engines in to get the C of G and use less lead, but IMHO there are two important trade offs with serious outcomes.

 

1. If overly large engines are installed then to fly scale they will be at less than 1/2 throttle most of the time. Some time back I was at Buckminster for the scale comp and there was an outstanding P38. It completed almost all of its flight 4 stroking as apart from the loop it was never given enough throttle to clear the engines. If I was the pilot I would been pooping myself that one was going to quite and spoil my day!
2. You could under prop the larger engine, but that's unlikely and it will rev its nuts off when tuning so people will probably fit the right prop for the size of the engine. Then you fly around and really start enjoying things until the red mist comes down and its time for a few fast passes. The problem is that its very likely you will exceed the VNE and design limits of the model. As you say its a balance between weight and strength in the booms and tail and the designer would have adding enough strength and lightness for the recommended 38 to 52 cc petrol engines.

I appreciate you don't want marginal power and a bit extra helps if one quits, but two 69cc WOT in a power dive and I wouldn't expect the airframe to stay outside the bin bag for long!

[Jon H]

Assuming I have the same model I don't see a need for massive power. At 114 inch and around 40 lbs it's in the ballpark of 4 inches and 5lbs bigger than a 1/4 spitfire, and those have flown fine on z62 engines for yonks. 

 

Jan hermkins in the Netherlands has a p61 and a26 of 120 and 140 inches, with weights of 40 and 47 lbs. Both models fly fine on laser 240vs, which are broadly compatible to a z38. 

 

I really can't see a need for engines over 40cc, especially given how sleek a p38 is

[GrumpyGnome]

Prop clearance may force the "rev its nuts off" scenario with larger engines........ I'm not a fan of massively overpowering models.......

[Nick Cripps]

But don't forget that Chris lives in Johannesburg where the air density is significantly lower...

[Chris Freeman 3]

A friend of ours had one with DLE 55 and it flew very well but lost it when one motor quit and he heard it to late. They always say if you have height and speed with a twin it will be ok, if low and slow the good motor will take you to the scene of the accident! Reliability is the key to this project. The airframe was originally set up for DLE 35's but we did not get them.  

[Chris Walby]

1 hour ago, Nick Cripps said:

But don't forget that Chris lives in Johannesburg where the air density is significantly lower...

Then its all more important not to add weigh and rely on it flying fast just to stay in the sky. Looking at the spec the wing loading comes out at 57oz/sq ft so unless it comes out under weight Chris F may be heading into corner that will be hard to get out of!

 

My foam P38 only has a wing loading of 10 oz/sq ft and the trauma of the 80 inch Ta154 (may have benefitted from bigger engines, but no more weight!) came out at 52 oz/sq ft, ok its bigger at 114 inch but it will be under a lot of stress if its pushed hard.

 

Chris F, Reliability may well be key, but not having a flying manhole cover will make it a pleasure to fly

 

PS I don't believe the spec as the numbers don't add up because my Tigercat is 116 inch, 21.75kg wet and has a wing loading of 34 oz/sq ft so something is wrong with the figures quoted. Chris F do you know what the actual wing area is and your target AUW?

 

PPS the full size could be recovered if it went into a spin...about  1000 vertical feet per revolution and a couple of rotations to get it out.. that might be more than 3 mistakes high for this model! 

[Chris Freeman 3]

23 hours ago, RICHARD WILLS said:

I do love a P38 ! 

I think Ive made six smaller ones from scratch . Thats bordering obsession 😬

Richard, I have a Spitfire problem, built 10 with more to come! We have 3 1/3 cubs and 3 DB Dakota's so i understand what you are saying!

 

Chris W

From what i remember from the plans which Byron now has is that it has 1400 wing area and wing loading was 44 to 46oz/sq ft. Greg Hann built his one at 42 pounds with Zenoah 38's and reported it was a joy to fly. We plan to keep it as light as possible and will fly it in primer so we can see how much weight we can still add with a finish but we tend to try and keep things as light as possible. The many electric projects I built have taught me how important weight is.

 

I had a Ziroli Beech 18 that was rebuilt from wreck and that flew on Zenoah 38's and weighed 50 pounds so it really taught me how to fly smoothly and power management. 

[Jon H]

4 hours ago, Chris Freeman 3 said:

A friend of ours had one with DLE 55 and it flew very well but lost it when one motor quit and he heard it to late. They always say if you have height and speed with a twin it will be ok, if low and slow the good motor will take you to the scene of the accident! Reliability is the key to this project. The airframe was originally set up for DLE 35's but we did not get them.  

 

More power will require more rudder authority to counter it so its critical speed may have been higher than a model with less power. All that said, most twin pilots never practice engine out procedures so its usually blind panic and confusion that ultimately causes the crash. 

 

7 hours ago, GrumpyGnome said:

Prop clearance may force the "rev its nuts off" scenario with larger engines........ I'm not a fan of massively overpowering models.......

 

not an overpowering fan either, but if larger engines are fitted then more blades might be the answer. Ground clearance is unlikely to be a problem with a P38 though unless its to the fuselage. 

[GrumpyGnome]

Yes, to the fuselage........

[Steve Rickett 1]

Hi Chris...

 

Yes, I have built 2 Ziroli P38's now...and lost both of them.

I had put Laser 360's (because I had one already, so purchased another!) The model, obviously, had plenty of power, but was not overly fast. I used to regularly fly with other WW2 fighters and it wasn't any faster than them.

 

Both of mine suffered from Elevator flutter. Catastrophic, and without any warning at all. The flutter probably only lasted a fraction of a second before failure of the entire Tailplane on the first version, and I lost 1 servo on the 2nd version, and damaged the remaining servo, which gave up 3/4 round the emergency circuit to land...

 

I had done a lot of work with the second model to try and rectify the issue, and it flew for 85 flights, over 3.5yrs before the end. Perhaps the servo gears has started to wear, or linkage slop developed...I don't know. The main issue is that the Ziroli P38 elevator is prone to flutter without some redesign of the Tailplane itself. Just check on YouTube...there are a few videos of the same issue, unfortunately.

 

On a positive note, the model flew beautifully, with a very benign stall. I never had a total engine failure, just a loss of 1 cylinder on one engine once and the model was quite flyable as long as into engine Rudder was used, and the speed kept up. I keep considering building a 3rd...I liked it that much.

 

I'll try and remember some of the other mods I made.

 

The booms as per the plan will crack and break just behind the wing. I made my wing break either side of the boom, with the wing joiner passing through. This kept the boom structurally, in one piece.

 

I didn't like the Undercarriage units being mounted from top down. So I moved the uc plate up and mounted the units from the bottom.

 

I fitted fowler flaps to the first model and plain split flaps to the second. Fowler flaps looked great, but were more trouble and I didn't notice any difference in stall speeds.

 

The second model had 2 servos in the Tailplane, with small bulged hatches. The Tailplane is so thin, you can't get big enough servos in there. I can't remember what servos they were but were probably Futaba minis. I should have fitted HD full-sized servos in the booms and then had short, solid linkages to the elevator. 

 

I strengthen the Tailplane and the 2nd version, improved the Tailplane joining system, and then mass balanced the elevator.....

 

Next time, I think I would increase the thickness of the tailplane, among other ideas...

 

I hope this helps.

 

Fwiw, my Libellula weighs about 70lbs and flies ok on a pair of Zenoah 38's. They will certainly fly the P38, but I think you will be left wanting a bit more ummphf!

 

Best of luck.

[Jon H]

The full size P38 features an external mass balance. Perhaps there is a call to make it functional on the model.

[J D 8 - Moderator]

            Early on in the life of the P38 it was thought that elevator flutter was the cause of a number of lost aircraft. This came as a surprize to designer Johnson as he was well aware of possible issues and had had the tail booms and elevator made of thicker alloy sheeting.

  Wing fillets to smooth airflow and the mass balance were fitted but aircraft continued to be lost.

Problem was found to be the result of compressibility as the speed of sound was approached resulting in controls locking up and aircraft would pitch over into an uncontrolled dive. It was found the P38's critical Mach number at 0.68 was lower than other main stream fighters of the time. Dive flaps were fitted that controlled the problem.

[Chris Freeman 3]

Hi Steve

Thanks for the reply, a real privilege to get a post like this as it will really help. I have followed your work in the magazines from when you were a kid! Byron and I will be getting together on the weekend to see what the requirements are for servo's. At this stage the servos are in the boom, but we well look at the available options. It is also a little strange as the original builder still owns the aircraft and does have some say in the project so we do have to keep that in mind. I think the airframe will be Byron's once finished

 

Was the flutter on the elevator or the stab as that could also influence what needs to be done. I must say I have also found the fact that the elevator is one piece and has a servo at each end strange. My head is more comfortable with 2 elevator half's which is not practical on this design. Many years ago in the 80's my dad built a large stick that we lost when the stab bowed under load and this locked the elevator. Byron's  Grandfather was a great believer in having the control surface thicker than the flying surface as this prevented airflow separation and turbulence which can start flutter. Jack built a 2 50% WW1 aircraft in the 80's and 90's  so he did have some experience.  

 

I might have to use a very strong mouth wash  for saying this but the test flights will be done as an electric set up as this will make the test flying less stressful! we do have 2 DLE 55's and 2 MVVS 58's which can be used once the test flying is sorted. 

[Chris Freeman 3]

Byron had a friend help with the cad drawings for mold for the drop tanks for the P38. The mold has been 3d printed and fiberglass drop tanks will be made in this. They are large and should look good.