7.74X Cherokee N (Is there a mechanical engineer in the house?)

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dixontj93060

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Been doing some design work on a planned build during the last 24 hours... It is the rocket which will utilize the superb nosecone that Dan Schwartz of Python Rocketry completed from this purchase: https://www.rocketryforum.com/showthread.php?66980-SOLD-Discount-Python-Rocketry-nose-cone. I sure wish I had an independent picture of the nosecone as it is soooooooo nice (maybe I can post it later). In any case, I decided to do a upscale of the Estes Cherokee K-47 (original kit) as shown below.

Cherokee Upscale_10inMOD_v3.jpg

Obviously this type of upscale has been done before. Here is an example of an 8" upscale of the Estes #1247 kit version. Personally I wanted to make the shorter K-47 version work because it was the first rocket kit I built at a young age. The problem is this kit is a bit tough with respect to stability and for sure I cannot use solid plywood fins as shown in the example above because of the weight and stability penalties. As most know, the Cherokee fins are honking big, so something needs to be done to build these correctly.

This particular build will utilize the mid-size range of 10" Sonotube which is 10.25" O.D. actual. Fins at this 7.74X upscale end up with a root cord of 27", tip cord of ~16" and fin span of ~17". So, to save weight, the planned fins are 1/2" ply frame with "paper" honeycomb inserts with a couple of 5.7oz. biased carbon fiber layers as skin. The "frame" of the fins will be ~1.5" around the perimeter with the TTW tab kept intact.

With the background above, now comes the need for a mechanical engineer (which I am not)... Doing flight simulations along with laminate and flutter modeling, it looks like the structure is good to ~1.5 Mach; which is great. Still, for some reason I have a compelling drive to stabilize the leading edge/tip of these large fins. This may, or may not be needed(?). I would like to add a single "stringer" or "spar" (I don't know the right term) from root to tip. Again the question(s)... Is this needed? Does this help with flutter damping (I think it would)? If the first two questions are "yes" or partially "yes" then the question is, what shape/size/location is best? My current thought is to cut out a rounded triangle and rounded parallelogram leaving one "spar" shown as Position 1 below extending from the approximate center of the root chord to the fore of tip chord (cut outs again filled with paper honeycomb). Other options considered were Position 2 and Position 3 below which may be better or worse(?). So, are there any mechanical engineers here on TRF that can help provide feedback on the internal fin structure?

CherokeeFinReinforce1.jpg CherokeeFinReinforce2.jpg CherokeeFinReinforce3.jpg
 
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CarVac

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Been doing some design work on a planned build during the last 24 hours... It is the rocket which will utilize the superb nosecone that Dan Schwartz of Python Rocketry completed from this purchase: https://www.rocketryforum.com/showthread.php?66980-SOLD-Discount-Python-Rocketry-nose-cone. I sure wish I had an independent picture of the nosecone as it is soooooooo nice (maybe I can post it later). In any case, I decided to do a upscale of the Estes Cherokee K-47 (original kit) as shown below.

View attachment 181527

Obviously this type of upscale has been done before. Here is an example of an 8" upscale of the Estes #1247 kit version. Personally I wanted to make the shorter K-47 version work because it was the first rocket kit I built at a young age. The problem is this kit is a bit tough with respect to stability and for sure I cannot use solid plywood fins as shown in the example above because of the weight and stability penalties. As most know, the Cherokee fins are honking big, so something needs to be done to build these correctly.

This particular build will utilize the mid-size range of 10" Sonotube which is 10.25" O.D. actual. Fins at this 7.74X upscale end up with a root cord of 27", tip cord of ~16" and fin span of ~17". So, to save weight, the planned fins are 1/2" ply frame with "paper" honeycomb inserts with a couple of 5.7oz. biased carbon fiber layers as skin. The "frame" of the fins will be ~1.5" around the perimeter with the TTW tab kept intact.

With the background above, now comes the need for a mechanical engineer (which I am not)... Doing flight simulations along with laminate and flutter modeling, it looks like the structure is good to ~1.5 Mach; which is great. Still, for some reason I have a compelling drive to stabilize the leading edge/tip of these large fins. This may, or may not be needed(?). I would like to add a single "stringer" or "spar" (I don't know the right term) from root to tip. Again the question(s)... Is this needed? Does this help with flutter damping (I think it would)? If the first two questions are "yes" or partially "yes" then the question is, what shape/size/location is best? My current thought is to cut out a rounded triangle and rounded parallelogram leaving one "spar" shown as Position 1 below extending from the approximate center of the root chord to the fore of tip chord (cut outs again filled with paper honeycomb). Other options considered were Position 2 and Position 3 below which may be better or worse(?). So, are there any mechanical engineers here on TRF that can help provide feedback on the internal fin structure?

View attachment 181528 View attachment 181529 View attachment 181530
The skin is what will provide the stiffness to stabilize the fin against flexing, in any direction, even on the leading edge. The material near the centerline of the fin is not going to be contributing much, if anything, compared to the much stiffer and farther-away-from-the-center carbon fiber. If you're worried about the leading edge, then you should add unidirectional plies in the direction indicated in Position 1: they will help prevent divergent flexing.

However, if the paper honeycomb you are using doesn't have much shear strength, then Position 1 would be a good idea, adding a nice long higher-shear-strength section.
 

dixontj93060

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If you're worried about the leading edge, then you should add unidirectional plies in the direction indicated in Position 1: they will help prevent divergent flexing.
Yeah, I can do that.
 

Binder Design

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So, to save weight, the planned fins are 1/2" ply frame with "paper" honeycomb inserts with a couple of 5.7oz. biased carbon fiber layers as skin. The "frame" of the fins will be ~1.5" around the perimeter with the TTW tab kept intact.
Rather then one spar, you might consider "skeletonizing" the whole fin. Triangular shaped holes, preferably with 1/4" radius in all corners to minimize stress risers. As an example of what I'm talking about, reference the decals for our Excel fins and imagine those as cut outs with radiused corners. Fill those with your honeycomb, then skin both sides.
 
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dixontj93060

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Rather then one spar, you might consider "skeletonizing" the whole fin. Triangular shaped holes, preferably with 1/4" radius in all corners to minimize stress risers. As an example of what I'm talking about, reference the decals for our Excel fins and imagine those as cut outs with radiused corners. Fill those with your honeycomb, then skin both sides.
Mike, I did consider something like that (I think), but opposite of the decals on the Excel. There really is no value of reinforcing the aft half of the fins. At least when it comes to flutter, although hard landings may be a different story. Although considering overall weight and fin construction here, I don't think I will have problems with "normal" landings. (BTW, sorry for the crude sketches.)

Excel4.jpg versus CherokeeFinReinforce4.jpg
 
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dixontj93060

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This is closer to what I'm trying to convey.

View attachment 181537
Well, I can certainly see how that would aid impact strength, but, from what I understand from flutter/divergence, I don't see how it helps much more than the single strut to the leading tip shown in the Position 1 drawing. Flutter is impacted by a stiffer, more stable leading edge, thus the addition of the extra strut to the mid point of the leading edge in the Position 4 drawing.
 

Binder Design

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I've seen the aft edge of a fin flutter more then the leading edge in a few instances. Rigidity of the whole fin is what I would go for, but I like overkill. :)
 

dixontj93060

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I've seen the aft edge of a fin flutter more then the leading edge in a few instances. Rigidity of the whole fin is what I would go for, but I like overkill. :)
OK, sure. I do agree that once a fin starts fluttering all bets are off, i.e., aft could be really flapping; but to negate the undamped state to begin with, it is the leading edge that matters. Saying that, I may just go ahead and add a strut to the rear tip also, again, for strength/durability during impact.
 

PhlAsh

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As Binder says (though I prefer twice the thickness for the radius), lightening holes are your friends:

CherokeeD.jpg

Carbon fiber over those should provide a good stiff light finset.
 

Rex R

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some time back when I built a k47 I 'papered' the fins...:) I don't know how they would hold up to high speed though, I've only launched it on Ds.
Rex

c fin 001.jpg
 

dixontj93060

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As Binder says (though I prefer twice the thickness for the radius), lightening holes are your friends:

View attachment 181556

Carbon fiber over those should provide a good stiff light finset.
I like that drawing. I do believe the 1" radius is "about right" given the size of the fins and cutouts, but I don't really have any mechanical/structural backing for that SWAG. I don't like giving up the material in the TTW/root fillet area though as that attach area is critical. How were you able to net the remaining material and get a weight estimate?
 

dixontj93060

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OK, I don't want to overthink this. As I said, I believe I'm OK on flutter with just the CF skin and a very close secondary goal is weight reduction with the tertiary goal of impact strength. Given that, I'm just going to go with the template below (shown next to a card deck for some reference--although it isn't a straight on picture so the card deck looks bigger than it really is). The lead, tip and aft edges are 1.5" with 1.0" margin above the TTW tab and three 1.0" spokes radiating from the center. Radius on cut-outs is 1.0". I will run uni-carbon plies along spokes in addition to the two 5.7 oz. weave layers.

IMG_1157.jpg
 
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Binder Design

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That looks real nice to me! You hit all the fin tips, plus a mid support on the leading edge.
 

PhlAsh

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I like that drawing. I do believe the 1" radius is "about right" given the size of the fins and cutouts, but I don't really have any mechanical/structural backing for that SWAG. I don't like giving up the material in the TTW/root fillet area though as that attach area is critical. How were you able to net the remaining material and get a weight estimate?
AutoCAD's MASSPROP command reports volume and "mass" (mass always equals volume despite numerous attempt to get them to include a density modifier). I found a generic plywood mass of 34.08 pounds per cubic foot then did the math.
 

dixontj93060

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Per the first post in this thread, I did want to post pictures of the nosecone I got from Dan at Python Rocketry.

The Cherokee nosecone is tricky and Dan helped me work out the math and shape to fit my 10.25" OD (I ended up going with a 0.69 Power Series). Here it is shown with two of the three Sonotubes (coupler tubes edges can also be seen just to left in picture).

IMG_1161.jpg

A better close up of the nosecone to body tube edge and it is perfectly square and fits beautifully. Just enough lip on the nosecone to accommodate the fiberglass layers to be put on the body tube.

IMG_1162.jpg

Cone slid out, you can see the perfectly round shoulder and sharp edges on the shoulder to cone transition. Also you get a glimpse of the nosecone compartment at the bottom of the shoulder.

IMG_1163.jpg

Final picture is the tip where you can see the 1" aluminum rod that extends deep into the nosecone structure and ties into the internal compartment/centering rings. The tip may need a bit of rounding although I am trying to use the original Astron design as reference which seemed to be shown with a more pointed nosecone that the later Estes Cherokees; we'll see.

IMG_1164.jpg

Needless to say, I am very happy with the results provided by Python Rocketry and would recommend them whole-heartedly. Quality work, great price and a real help during the design stages. (www.pythonrocketry.com)
 

pythonrock

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Tim,
Thank you for the extremely favorable endorsement!!
Do you want to be my advertising manager?

I have now also done separate coupler tubes and tailcones. I have tested cutting fin slots into the tailcone as well but haven't sold any with slots yet.

What kind of foam are you using to fill the cutouts in your fins?
Dan
 

dixontj93060

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What kind of foam are you using to fill the cutouts in your fins?
Dan, although I have used foam in the past, in this case I am not. I believe I put a link in the first post to the cardboard honeycomb I'm using as inserts--extremely light.
 

dixontj93060

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Dan, although I have used foam in the past, in this case I am not. I believe I put a link in the first post to the cardboard honeycomb I'm using as inserts--extremely light.
I did a foam board cut template just to keep me on track when I work with the Baltic birch and cardboard honeycomb. The cuts take right at 50% of the material out. I also went back and checked density on Baltic birch and the honeycomb. Baltic birch is 41.3 lb/cu-ft and the honeycomb is 3.4 lb/cu-ft (or less than half of typical Home Depot foam board). So all in, with the CF laminate I'm at 11.5 lbs for the three fins versus 18.7 lbs for solid fins, ~40% savings.
 

pythonrock

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Tim,
With the wt you also save at the nose, it will make a big difference in flight. The cardboard honeycomb is not expensive either.
Looking forward to seeing that one fly.
Dan
 

chris m

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It would be Great to find the honeycomb in smaller sizes , don't need a pallet . I may have to look for some.
 

dixontj93060

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Chris I have a sheet or two (30"x42"x1/2") I'll sell you for $10 a piece.
 

chris m

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Thanks , but it will be awhile . For a 10-12" goblin I should have my mind made up in 6months
 

dixontj93060

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Well, cut some big honking fins this morning. Total weight for three is only 7.25 lbs--that's good! It means each fin is going to come in at less than 3 lbs when finished--20% less than I was estimating :).

ImageUploadedByRocketry Forum1408808991.322776.jpg


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dixontj93060

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Boy, it's working out better than I thought. After cutting the frame I could get a bit of flex, well actually twist in the structure. Now with the honeycomb in place with Gorilla Glue it is solid as a rock. BTW, the inserts and adhesive added 4 oz per fin.

ImageUploadedByRocketry Forum1408846171.057550.jpg


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DRAGON64

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Boy, it's working out better than I thought. After cutting the frame I could get a bit of flex, well actually twist in the structure. Now with the honeycomb in place with Gorilla Glue it is solid as a rock. BTW, the inserts and adhesive added 4 oz per fin.


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I have always wanted to work with some honeycomb reinforced structuring. Your process is working very nicely, looks great!
 

dixontj93060

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Any progress on this mega Cherokee?
Thanks for asking and yes, unfortunately some delays here. Hasn't been the best year for our family health-wise. Saying that, this build is still part of the five or six in-process builds I have going. Had some time this morning and decided to take a big step forward and laminate the fins. I have had the fabric cut for some time so I just had to pull it out of the bag, unfold and take stock of what was there. Note the carbon fiber uni-tape. I decided to use this to reinforce the spokes of the fins versus adding the weight/cost of another full layer of carbon fiber.

IMG_2152.jpg

Used some 5-gal buckets my wife had sitting in the garage to set things up.

IMG_2151.jpg

After applying uni-CF, 5.7 oz CF, 6 oz FG and 3 oz veil FG. Clearly going to need to use some SuperFil given spoke ridges. Actually, this is the worst one, the others look less pronounced.

IMG_2154.jpg
 
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rharshberger

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Looking good, giving thoughts to my L3 project which I am planning as a Cherokee upscale.
 
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