Easy Built Hellcat

[Piecost]

Easy Built Hellcat

This is a continuation from the thread on the previous incarnation of the HPA forum; which I hope one day can be recovered.

I had completed the kit with the following changes

Laminated fuselage formers replacing those heavy full sheet formers with a small hole in the middle

Removable wings for storage and repair – not knock-off

Band on (one piece) tailplane - for storage and trimming

Maxfliart type propeller of same 10" diameter as kit prop

Model stored in a foamboard box with the flying surfaces rubber banded onto boards to prevent warping

The model was not as neat as I would like: the covering had several wrinkles, but was reasonably well built. The placement of wing/fuselage ribs and stringers were not ideal for neat covering. I avoided going down the
rabbit hole of making too many changes and ending up redesigning the whole thing.

I used this kit as a means to gain experience in outdoor freeflight scale. I wanted to try the Maxfiart propeller
construction and rubber recommendations. I also built a winding stooge and employed a blast tube and used
a K&P outdoor winder for the first time. I was really pleased with how well it all worked.

At the end of last year's flying season; I achieved my objective of getting the model to circle overhead in
reasonable trim. The motor was:

8 Strands of 3/16 Tan II, loop length of 20" and 20g mass

Wound to 400 turns gave a climb to 8' and about 20 seconds on a tight left turn. Not a great duration; but I
was happy

[OZPAF]

That sounds pretty successful - now you could redesign the structure - lighten it as well perhaps and do even better.

happy flying .
John

[Piecost]

Thanks OZPAF (John). I am pretty happy with the model as-is and don't intend to build a better one; but to get the most out of the exsting model before progressing to other outdoor scale projects.


As part of recommissioning the model for the 2026 season;I am looking back with a little time distance from last year to see what improvements I can make and came up with the following observations:

I used rather more strands/thicker rubber than typical for Maxfliart and that Calgoddard used (8 Strands of
1/8 as merntioned in the thread on the previous HPA forum). I noted that my motor of 8 strands of 3/16" was not even on Maxfliarts graph of maximumn turns!

I don't think that my propeller pitch was too high. I noted from Maxfliarts video "propeller from a cheese container" that during the sanding of the spar he uses a 30 degree pitch gauge and when attaching the blades; seemed to use 35 degrees at 50% radius (Prop P/D = 1.1).

I noted that the Greek Yogurt pot plastic used to construct the blades were very flexible; when combined with the stub-spar at 50% chord; it is likely
that the blades are flexing and increasing pitch in flight.

I did brake a 5mm diameter "bamboo" skua spar on the short grass, so the replacement propeller incorporated a 5/32" aluminium tube sleeve, slightly overlapping the blade root. I think that Maxfliart might have used thicker spars. Indeed, he seemed to not have suffered from the blades peeling off; which I have had issues with. Perhaps my slender spars don't have enough glueing area. My stooge featured a telescopic tube to engage with the propeller to stop a wound motor unwinding; but this tended to cause the problem. So, I stopped using it and also bound the spar to the propeller with cotton.

The CG was very forward, relative to the plan instructions and on similar models. This may be related to excessive destabilisation due to propeller forces/moments, slipstream etc from the effective high prop pitch and thick motor. But I also have a
tendancy to trim with very forward CG on my scale models.

I have started to benchmark the CG position it relative to those models in Flying Scale Models of WW2 book
and may go into more detail in a later post. Indeed, the proportions of my model look less true to scale than
the Hellcat in the book. But from eyeballing it; the CG looks very forward on my model.

I was enamoured by the discovery and employement of metal Figure 8 Jigging fishing rings to attach the
motor to the propeier in the manner of a Crocket Hook. As others warned me in the old HPA forum; this often caused severe vibration as they rotated sideways on the prop shaft with high turns. I had already ordered thick neoprene O-Rings as a replacement; as recommended.

My new stooge worked well. Small details; like a hook to hang the blast tube-spacer-winder from made the
whole winding process smooth and stress free. I have subsequently added a shelf to rest my mobile phone so it is quickly accessable to film the flight immediately after launch.

I was never confident about the wing twist achieved and the model strangley needed into-turn Gurney flap,
So perhaps it has Warp causing a right hand roll built-in. A photo of the model flying towards the camera shows the wing leading edges looking crooked.

[Piecost]

I can't figure out how to attach multiple photos, so heres one of the model on its stooge

[Piecost]

The crooked wing

[TheLurker]

That ^ is a cracking picture!

[OZPAF]

Thanks for that detailed report Piecost - it was interesting to read pf your detailed approach.

Although I haven't trimmed a scale FF rubber model - I have found the CG advice offered by Don Deloach  helpful. It is based on an empirical approach by Bill McComb but it has worked well on my KK Elf. Following the CG calculation I trimmed the model decalage by gliding the model without the propeller and the power trimming was very easy.

The article also includes his views on prop P/D and rubber sizes which may also help.

I agree with lurk - the head on photo is remarkable. however it seems to indicate why you need into turn rudder - that is a fair bit of wash in on the left wing. Did you try flying it to the right?

Are you using braided motors? This appears to be the secret to using long motors. I have been using a braided motor around 2.5 prop - rear hanger distance on the Elf and it works well, reducing to the prop - hanger length after run down in the model.

I used Maxfliart's simple braiding approach and it worked well.

Re the blades breaking off the shaft's - were the blades pegged through the shaft as well as glued?

Happy flying.

John

[RolandD6]

... Indeed, he seemed to not have suffered from the blades peeling off; which I have had issues with. Perhaps my slender spars don't have enough glueing area. My stooge featured a telescopic tube to engage with the propeller to stop a wound motor unwinding; but this tended to cause the problem. So, I stopped using it and also bound the spar to the propeller with cotton. ...

Check your yogurt container for the plastic symbol. It is likely to be 5 or PP which is polypropylene. Polypropylene has very low surface energy which means it resists adhesives. Also bamboo can contain silica which also makes it resist adhesives. If I remember correctly Maxify also rivets his blades to the spar.

Paul

[Piecost]

Hi John,

Thanks for the link to the Don Deloach article. I am aware of it, but it was extremely useful to reacquaint
myself with its contents; especially after gaining more trimming experience. It certainly looks like I have a
perfect example of too forward a CG needing lots of downthrust. Infact, I was running out of thrust line adjustment. I was particularly taken with the observation about this set-up not working at high winds and torque. It was, perhaps, lucky that I did not get that far.

I have entered the geometry of my model and that of the Flying Scale Models of WW2 Hellcat and Wildcat into an old Mega-spreadsheet where I was comparing different CG calculators. It will be interesting to use the Deloach criteria. I will report back when I get round to the analysis. I can see an epithany coming; since my indoor scale models seem to share such an extreme set-up.

I didn't like the wing construction in the kit with its very wide rib spacing and ribs with convex lower surfaces making it difficult to eliminate twist. But decided to keep the heavy kit wood ribs and faithtully cut along the printed lines. I recall setting the leading and trailing edges on Lego bricks to avoid warps, but perhaps the uneven leading edge slots ribs forced them out of alignment. I spent some time in trying to ensure that the wings were attached to the fuselage at the same angie. I won't elaborate about the terrible trouble I had in achieving anywhere near similar dihedral on both wings!

I did try the model with a right turn as was its natural tendancy, but, it tended to spiral dive and i needed excessive, LH roll Gurney Flaps on both wings. The left turn was much happier.

I do braid my motors according to Maxfliarts advice and was supprised to find that the shortening effect only works after winding the motor. I had not realised this for 30 years!

Paul,

I checked and the pot was indeed polypropylene, so you were right. I did use cocktail stick rivets; again following Maxfliarts advice. I was a bit supprised
that he successfully used quick setting epoxy which I consider inferior to the longer drying stuff.

Tim

[OZPAF]

Thanks again for your detailed response Tim.

My limited rubber FF experience leads me to prefer less stability - thus a more aft CG, as this is far more capable of handling the speed changes from powered to gliding flight - following my experiences in HLG and CLG and RC gliders.


Using DeLoach's recommendations- my Elf is balanced at 55% of the wing MAC which still provides a Static margin of 9% by my NP calculator, due largely to the very large tail volume of almost 1.0 arising from the 34% tail area! It has a stable flight pattern with a flat floating glide. Here is an early clip of it flying on a loop of 3/32.

https://www.youtube.com/watch?v=X1TyichvEHA

I'm not a fan of 5min epoxy as well - except for quick jigs - 30 min is fast enough for me.

John

[lincoln]

How much does it weigh? From the bones picture, I imagine it's not very heavy. Keep trimming. Unless it weighs a ton, you ought to be able to get significantly more time out of it.

You might you might try making a sacrificial motor and winding it up, outside the model, to see how many turns per inch it can take before breaking. 3/4 of that is probably safe. I suspect that you can get more than 400 turns safely from a lubed motor. Even better, wind to a certain torque. I'm sure you can find instructions for making one at freeflight.org, in old newsletters or the PFFT archives at flyingacesclub.com Note that there's a partial index of the old newsletters. If you can be as sloppy as me, maybe you can make your first torque meter in 15 minutes.

[Piecost]

Hi Lincoln, thanks for your comments.

The weights are as follows
Airframe         58g
Nose weight  11g
Rubber            25g
 
It is not a heavy model and certainly has much more duration potential than I have achieved so far.

With that in mind; my changes for this year include:

This tissue was damaged on the lower nose so I sheeted the forward bay of the nose between the lower stringers. I removed about 1g of lead to compensate and to retain the same CG.

l am making new propellers with 30 degree (P/D=.9) and 32.5 degree (P/D=1.0) pitch at 50% radius. I am still employing the same flexible Greek Yogurt pot blades. I will employ the same construction as the original prop, but may extend the aluminium tube sleeves slightly further outwards for added strength.

If one of the new props works well; then it may prove illuminating to install stiffer blades with the original 35 degree propeller pitch to see if it makes a difference. i.e. the same effect as reducing the pitch allowing a thinner motor to be used.

I built a wire torque meter to replace the spacer in my blast tube winding setup. I test wound and unwound the motor used successfully last year and was supprised that I was going into the sharply increasing torque part of the curve with my launch turns. The old TAN II took less turns than expected, this may have been to the slightly lower temperature in my garden during testing than encountered during the sunset flights last summer.

l attach a plot of the winding/unwinding torque versus Handle turns (10:1) for the motor. The data should be taken as very approximate. It indicated that I could not get many more turns on the motor without significantly increasing the torque with the expectation of trim problems.

I removed the reminants of a wing Gurney, which I had nealry completely wittled away during trimming and replaced the hastily improvised Gurney flaps on the fin with neater sheet examples.

I will initially fly the model with the same motor and propeller set-up to check that it is still in trim and that the warps haven't changed. It will be interesting to note down the launch torque in addition to turns.

I will then switch to the 32.5 degree pitch propeller, starting on low turns. I hope that it proves over-powered so that I can switch back to 8 strands of 1/8" rubber to gain more duration.

I will then retrim with a further aft CG following the Deloach advice. I allready have a propless nose block that Incannuse forntest glides.

[calgoddard]

Piecost -

Consider using a 9 1/2-inch Gizmo Geezer (GG) prop assembly with a spinner on your model. Cut the diameter of the plastic prop down to 9-inches with scissors.

The 10-inch diameter Hallman prop you built may be a little too large for the 28-inch wingspan.

The P/D of the 9-inch GG prop assembly is about right for this model. It was for OK for my Easy Built Hellcat (picture attached). My model was heavy at 76.86 grams as it was only the second or third stick and tissue Rubber Scale model that I had built at the time. My notes indicate that I was consistently getting one minute flights with my Easy Built Hellcat. The plastic prop of the Gizmo Geezer prop assembly will be much more durable than the Hallman prop you built.

Adding ballast to stick and tissue model airplanes is a last resort when needed to adjust the CG. It is better to have any needed weight of the model attributable to functional components. The GG prop assembly will be much heavier than the Hallman prop you are using. Therefore you can probably remove some ballast if you swap in a 9-inch GG prop assembly.

My records indicate that I flew my Easy Built Hellcat with an 8 x 1/8-inch TSS rubber motor, 25 grams, and 33-inches in length. I wound in 1450 turns and launched my model at 9 inch-ounces of torque.

In addition, the GG prop assembly provides three very useful advantages, namely:

1) reliable free wheeling;

2) easy and precise thrust line adjustments; and

3) no need to braid the rubber motor. 

I can tell from your posts that you have the skill, experience, and dedication to significantly increase the flight times of your Easy Built Hellcat.