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        Build Log Jetco Thermic 72 from PVHC

#1 micheldeman Jun 15, 2012 09:04 AM

Jetco Thermic 72 from PVHC
 
6 Attachment(s)
I recently bought a Jetco Thermic Replikit from Penn Valley Hobby Centre. After paying VAT to customs, as I live in the UK, I received the kit together with 2 other models nicely in a solid carton box.

The kit looks good and most of the parts are there and nicely being laser cut. Some parts are not there and probably need to be purchased locally. Not sure whether this is correct or whether they were "just" missing from the contents. Anyway:
I am planning to make some changes to the design and convert the model to 2CH RC.

I am planning to give the main wing an angle of attack and place the stabilizer flat on the boom instead. The pod I am planning to build up from formers and cover with balsa stringers as I have seen somebody else doing in another building log.

I am planning to add 2 spars on the top-side of the mainwing to give some more strength to the wing and let them function as turbulators to give more lift. I might keep the left and right side of the wings seperate for easy transport.

I firstly cutted the solid boom into 2 parts with a basa cutter to lead the RC controlls through.

See pics below of the progress so far, it took me 1.5 hours sanding by hand as I had to plane the resulting 2 parts and make them nice and flat and a bit thinner. I started to cut the boom from 2 sides (top and down-side) where the cuts met each other somewhere in the middle with a slight offset that needed to be corrected.

#2 Beardiedrg Jun 15, 2012 01:42 PM

I am thinking of getting this also keep us up to date Are you making this rc and powered.

#3 sailnut Jun 17, 2012 08:34 PM

This is the Ehling version of Frank Zaics classic design. I built 3 of these one in the late 40's and 2 at Parks College in the late 50's

Frank Ehling had his faults but not when it came to force setup. I would be very reluctant to take out the negative incidence in the stab and shim up the wing. Most likely when flying the frontal drag was minimized by his arrangement.

The solid balsa pod provides badly needed nose weight. Don't forget that the boom (in the original) was basswood and weight was required. Whats the point of a built up pod if you are going to pack it with much needed nose weight.

Although it's a very interesting airframe this is one of Ehlings less successful designs . This is my conclusion from the three I built.

Richard Smith

#4 mlbco Jun 18, 2012 01:44 AM

2 Attachment(s)
I built a PVHC Thermic 72 converted to RC when the kit was first released. I think mine had print wood for the ribs, no laser cutting. I hollowed out the pod and made it a bit wider to squeeze the RC gear inside. The model is covered in tissue paper and dope. I launch it with a low-power hi-start (the wings are weak with the stock spar design) and have enjoyed many thermal flights with it.

Steve

#5 micheldeman Jun 18, 2012 04:49 PM

Quote:

Originally Posted by Beardiedrg (Post 21906343)
I am thinking of getting this also keep us up to date Are you making this rc and powered.

No, just RC, I am flying normally on a hill where I can fly on the wind and wait untill the glider gets thermal. There is no need for me to make this plane powered.

#6 micheldeman Jun 18, 2012 04:58 PM

Quote:

Originally Posted by sailnut (Post 21924260)
This is the Ehling version of Frank Zaics classic design. I built 3 of these one in the late 40's and 2 at Parks College in the late 50's

Frank Ehling had his faults but not when it came to force setup. I would be very reluctant to take out the negative incidence in the stab and shim up the wing. Most likely when flying the frontal drag was minimized by his arrangement.

The solid balsa pod provides badly needed nose weight. Don't forget that the boom (in the original) was basswood and weight was required. Whats the point of a built up pod if you are going to pack it with much needed nose weight.

Although it's a very interesting airframe this is one of Ehlings less successful designs . This is my conclusion from the three I built.

Richard Smith

The other two, frank Zaic versions, do have an angle on the main wing and a flat stabilizor, but what do you mean exactly with frontal drag?

Making the pod built up will definitely reduce the total weight of the glider as you only need to add weight to the most front part of the pod, and not over the full length of the pod which is less efficient. I.e. it might only need half of the total extra nose weight to get the CG right.

I just started with the main spars of the main wing, pictures will follow.

#7 sailnut Jun 20, 2012 03:39 PM

If you check around you will see that I am sort of the historian in regards to Frank Zaic and JASCO. and I worked with Ehling for a couple of years.

OK... your correct about the declage set-up on the Zaic original 72 and 72X. What you missed is that Zaic later confessed that he had no idea at exactly what angle of attack a models wing flew at. Remember we are dealing with low Reynolds numbers and the NACAA plots were useless.

Ehling on the other hand had tremendous success from the late 30's to the mid 50's designing contest free flight models. I would be loath to modify Ehlings force setups. If its drawn this way Ehling had a good reason (although I am inclined intuitively to your point of view)

As to the built up pod. The post about concentrating the nose weight rather then distributing it through a solid balsa pod is correct. However you will be massing several ounces of lead in a fragile built up structure. You can be sure that the in a crash the weight will play havoc with a built up pod. On the other hand the Ehling design can take a beating.

Another point to consider is that all 3 examples I built were several ounces underweight (AMA rules) and required ballast. Another observation particular to this model is that I think it would fly a lot better if ballasted. Many early towline gliders flew to slowly to realize their max potential.

#8 Beardiedrg Jun 20, 2012 11:11 PM

Hi sailnut.
Been getting involved with the jetco line. What do you know about the eastwind and the jasco scout suitcase glider. I have the eastwind, navagator. Thermix 50 Rc and the 72 towline. I have the plans for all the line except the scout. Any ideas where to get a set and your thoughts on these. Thanks Mark

#9 micheldeman Jun 21, 2012 04:07 AM

Quote:

Originally Posted by sailnut (Post 21950542)
If you check around you will see that I am sort of the historian in regards to Frank Zaic and JASCO. and I worked with Ehling for a couple of years.

OK... your correct about the declage set-up on the Zaic original 72 and 72X. What you missed is that Zaic later confessed that he had no idea at exactly what angle of attack a models wing flew at. Remember we are dealing with low Reynolds numbers and the NACAA plots were useless.

Ehling on the other hand had tremendous success from the late 30's to the mid 50's designing contest free flight models. I would be loath to modify Ehlings force setups. If its drawn this way Ehling had a good reason (although I am inclined intuitively to your point of view)

As to the built up pod. The post about concentrating the nose weight rather then distributing it through a solid balsa pod is correct. However you will be massing several ounces of lead in a fragile built up structure. You can be sure that the in a crash the weight will play havoc with a built up pod. On the other hand the Ehling design can take a beating.

Another point to consider is that all 3 examples I built were several ounces underweight (AMA rules) and required ballast. Another observation particular to this model is that I think it would fly a lot better if ballasted. Many early towline gliders flew to slowly to realize their max potential.

Thank you for your comments and explanations. I think there is some room for discussions and based on each and everyone's different insights or reasons, one may chose for different solutions. I started building the wings now, see next post, and leave the fuselage for a moment, that will give me some more time to think about the set-up of it.

#10 micheldeman Jun 21, 2012 04:22 AM

center wing sections
 
4 Attachment(s)
I have started building the wings. As the wings are built-up with under cambered ribs, the main spars which are located at the bottom side of the ribs are not touching the building board when assembling the wing. I choose to use small pieces of balsa to lift and support the main spars and fixed them on the building board so that the leading and trailing ends of the ribs touch the building plan. I glued all the ribs of the center sections first, except number 1 and 2. The plan advises to add the trailing edge first and then the leading edge. I chose to add the leading edge first. The trailing edge I will add later as it requires an angle due to the under cambered wing. If I would glue it now, it will have the wrong angle. I will show that later. Next step is to build the outer panels of the wing. I will show that once finished.
I decided to add 2 small spars to the front top side of the wing to add some strength to the wing and which will act as turbulators. My experience with using scalps to cut small slots is not so good. Better is to use a peeler knife with small cuts and a small file. The result is nice small cuts in the ribs to place the spars in. The spars I made out of a 3mm balsa with a balsa cutter set-up on 1,5 mm (sorry, I don't do inches).

#11 micheldeman Jun 21, 2012 05:48 AM

I am thinking of giving the wing tips some washout, anyone having experiences with this? One other guy who built a thermic was suggesting this in order to avoid stall at low speed?
thanks,
Michael

#12 micheldeman Jun 21, 2012 10:52 AM

outer wing sections
 
6 Attachment(s)
Ok, after some hours I have "nearly" completed the main wings apart from the trailing edges and wing tip gussets.
I have added some washout to the outer wing section and I think that is needed anyway as the wing is undercambered. If you would put the trailing edge of the outer wing panels flat on the plan like the leading edge, you will create a negative washout, that will likely causing the wing to stall. Result now is also that the leading edge of the wing ribs are alligned straight as is the leading edge spar. I have added 2 stringers acting as turbulators. Main thing is to fix the main spars on the plan first and be sure that their level is alligned correctly. Then start fixing the ribs and leading edges, then the 2 turbulators stringers can be glued as well.

With these kind of wings, it's a good idea to build them in parallel and at the same time. Otherwise, the risk is high that they will not be symmetrical.

#13 brokenspar Jun 21, 2012 11:31 AM

Good work, micheldeman. Watching this build with interest.

#14 micheldeman Jun 23, 2012 02:22 AM

Wings nearly finished
 
10 Attachment(s)
Progress is going on. I have nearly finished the two wing halfs. It looks like the outer wing panels have the right angle of attack, a little bit washout is noticable. However, after shaping and sanding the first wing, it is not possible to sand the leading edge into the round shape as required and designed for the wing foil. The top side of the leading edge is fine, but the bottom side has not sufficient wood so to say to polish it into the right shape. I need to add a stringer along the bottom of the leading edge to add wood to the bottom side and then polish it into the required shape. Because, as it is now, the wing foil would not perform very well. I don't know whether others who built the same glider did't have this issue, but I have read all the threads and no one did mention anything about this. Using a rectangular leading edge would be a better choice to use in the first case..

#15 micheldeman Jun 23, 2012 03:56 PM

4 Attachment(s)
Done as said, I added a stringer to the bottom side of the leading edge. After sanding, the leading edge has now the perfect shape as part of the air foil.


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