staged fin alignment

[arthur dent]

I am building an ESTES CC EXPRESS and im wondering if its absolutely necassery for the fins on the booster stage to be exactly aligned with the fins on the main stage or is it possible to fly with fins set at 45 degrees to each other thanks for any help...

 

[saxophone]

Non-aligning the fins is okay and is actually preferred as long as
the fins are far enough out of alignment that the lower fins
are not in the wake of the upper fins. Being 45-deg out of alignment
is good. Being 1-deg out of alignment is bad.

--Greg

 

[saxophone]

Oh, and by the way... I was given a CC Express kit as a gift.
I modified it so that it uses gap staging instead of taping the
motors together. I also modified the upper stage motor mount to
take an E9 motor. So, I'll be able to fly it using a D12-0 and an
E9-8. I can't wait to take this out to the dry lake bed.

 

[sramberg]

Why is it bad to be 1 degree out of alignment? Too much drag or is it something else?

 

[GoBang]

Originally posted by saxophone
Oh, and by the way... I was given a CC Express kit as a gift.
I modified it so that it uses gap staging instead of taping the
motors together. I also modified the upper stage motor mount to
take an E9 motor. So, I'll be able to fly it using a D12-0 and an
E9-8. I can't wait to take this out to the dry lake bed.

Ya know, I have a Comanche and I have considered doing the same thing. I have a CC and I also have a Quest Navaho and of the two the gap staged Navaho is way easier to prep. Anyone else tried it with a Comanche? Any suggestions?

 

[arthur dent]

yep cheers saxophone...i thought i read that it was ok to have the fins alligned at 45 degrees.I have fitted a coupler to the booster stage so that there is a gap between motors it did'nt seem to fit correctly with just the motors taped together.We can't get E motors over here in the u.k. so i'll be sticking to a double D but it should go mighty high

 

[rstaff3]

What saxophone said. Way off is ok, a bit off isn't. Haven't give this a lot of thought or looked in 'The Handbook', but it must cause extra turbulance and drag, neither is good. When doing aligned fin versions, I generally clamp a light ruler to the top and lower fins to make sure they are aligned.

 

[arthur dent]

I really enjoy building staged rockets and using a ruler to keep upper and lower fins alinged is truly marvelous infact i can't wait to get another staged rocket on the go just o i can try it out...

 

[sandman]

I have an Astron Avenger clone where the lower and upper stage fins actually "nest" together.

https://www.dars.org/jimz/k-38.htm

I went so far as to cut a slight grove in the leading edge of the lower stage fins for the upper fins to "nest" into.

The upper stage fins are 3/32" balsa and the lower are 1/8" so they appear "stepped".

sandman

 

[Justy]

I can tell you from first-hand experience that the CC Express flies just fine when you align the fins. I haven't tried twisting the booster 45deg. It seems to me you'd want to do that if you needed an extra edge in stability at the expense of drag, but the CC Express is nice and stable as is. I made mine pretty heavy in the back (basswood fins TTW mounted), so if you make yours according to the directions, it'll be even MORE stable than mine.

This rocket flies well on C11's, but it really comes to life on D12's.... D12-0/D12-5 ejected a bit short of apogee, so next time I plan to use D12-0/D12-7.

With the stock parachute, be prepared for very long walks after any two stage flight, no matter which engines you choose. I launched mine at a recent MPR launch, and I had a longer walk than most of the F- and G-powered flights! Since I built my fins strong, I might cut out the spillhole. I can't imagine what the recovery will be like for Saxophone's D12-0/E9-8 flight!

The CC Express is the favorite rocket in my fleet... but then, I don't have a Deuce's Wild! yet. (I'd have had one on the way this week if my car hadn't broke down, **** alternator...)

 

[arthur dent]

Im just a bit worried because the fins are not perfectly aligned but they are not out by miles if you see what i mean.We can't get c-11's here so its definatley going on 2 D's

 

[saxophone]

Sorry you can't get E's in the UK. If you've modified the CC Express
for gap staging, you could put an 18mm motor adapter in the
upper stage and fly it on smaller motors.

The nested fins like on the Avenger is cool. The Navaho Zenith II also
has a sort of nested fin arrangement but where the lower stage fins extend
beyond the upper stage fins. I've built a couple of scratch
built rockets using this concept. I'm attaching a picture of
my Triad 3-stage rocket with nested fins. I found I can launch it at
my club launches with a D12-0, B6-0, and A8-5 and still get it back.

I also love multi-staging. I have lost my share of upper stages resulting
in a collection of orphaned boosters. As a result, I modified a Quest Navaho
to fly on 3 stages. The bottom stage takes a 24mm motor (a D12-0).
This is necessary because of the increase in weight. I flew this on a
dry lake bed with a D12-0, C6-0, and C6-7. I had a real long walk just
to get the 2nd stage booster. The sustainer was probably a half mile away.

 

[jflis]

Our (FlisKits) Nomad is a 3 stager (24mm/18mm/18mm) that uses nested or aligned fins for the 2nd and 3rd stages and non-aligned fins for the booster.

Here's a pictures of Thomas Tulanko's Nomad: https://fliskits.com/photo_album/naram-45/images02/nomad03.jpg
If you look closely, you can see the fin line between the 2nd & 3rd stages in green.

This graphic image shows the separation point more clearly: https://fliskits.com/products/rocketkits/product_images/nomad.jpg

I'm shocked that i've never lost mine even after 3 flights! Like many, I have boxes of orphaned lower stages... LOL

 

[jetra2]

Originally posted by jflis
I have boxes of orphaned lower stages... LOL

Nah, Jim, they can't POSSIBLY be orphaned....there has to be SOME rocket that you can pop one or more of them on the end!!!

BTW, I think that keeping the fins in alignment looks cooler. But that has no scientific reasoning. Say you have a somewhat heavy (for LPR) payload. You might want to have the fins out of alignment for.....oh crikey....that doesn't work. Ok...say you have a short 3 stage model, with no payload. it would be D-D-C (yeah...I'm crazy). Having the fins out of alignment would help you improve the CG-CP relationship by providing more fin area.

Jason

 

[powderburner]

I can see no engineering reason why it would make a huge difference one way or the other, whether your upper/lower stage fins are aligned or skewed.

Yes, someone could probably generate some mathematical proof showing a small advantage one way v.s. the other. However, there are so many trade-offs that your question does not really address:

If you 'mix' the fin orientations, the lower stage fins should be flying in 'cleaner' air, should have boundary layer transition delayed to a region further aft on the fins, should therefore generate more stabilizing force per fin area, and could be made with smaller fins, less drag, less weight, etc.

If you align the fins, the lower stage fins should be almost drag-free (if the leading edge of the lower fin is CLOSE behind the trailing edge of the upper fin), at least when talking about profile drag.

Who knows?

I think a far greater difference in performance will result from putting a decent airfoil on the fins, wherever they are, and getting a good smooth surface finish. I do not mean cutting out the balsa sheet and gluing it straight on to the BT with square edges all around, no matter how smooth. Subsonic fins need a round leading edge (not sharp!) and a trailing edge that smoothly tapers to zero thickness. Subsonic fins need a smooth surface finish (I laminate mine with copy-machine paper before painting). Subsonic fins do not need huge root fillets, at least not for aerodynamic purposes.

And don't forget the nose cone surface finish, and the gap between the nose and the forward BT.

 

[arthur dent]

Ive been putting a sharp(ish) edge on the front and a rounded edge at the back...oops...Do you think this will make a big difference to performance????

 

[jflis]

Actually, yea it will. An airfoil for low speed (sub-sonic) is rounded in the front then tapering from front to rear to zero thickness. Like the wing of a jet airliner

jim

 

[arthur dent]

yep,yep...of course..I just kept thinking a pointed edge would offer less resistence,but when you look at a wing either on a plane or a racing car the leading edge is rounded.Looks like i've got a good excuse to buy another CC EXPRESS so i can get the fin shape correct

 

[jflis]

well, basically (without getting into a lot of complications...) the main problem that you have when you reverse an airfoil is that as the rocket moves forward, the air that passes over the fin (with the pointed edge forward) gets compresses slowly as it approaches the rear of the fin (where it is fatter).

When this air stream passes beyond the tail of the fin (where it is fattest), it discovers a vaccume (of sorts) behind the fin and rushes to fill it creating all sorts of turbulance affecting the drag coefficients of the fin.

The other (correct) way around, the air stream is quickly compressed at the leading edge of the fin then allowed to gradually dissapate over the length of the fin as the fin thickness goes to zero, with the stream exiting the area of the fin smoothly with no turbulance.

An over simplification, but you get the idea.

(figures would help... if i have time this weekend i'll see what I can do)

jim

 

[sandman]

Think of the shape a drop of water takes as it falls...

Round in front and tapered to the rear...hmmm...

sandman

 

[arthur dent]

Thanks for that Jim.Its not an over simplification,its a great way of describing it.I had'nt been giving much thought to what was happening at the back of the fin and i'd just presumed that the front sliced through the air.

Do you think there would be a noticable difference in flight performance betwwen two identicle kits with different fin shapes even at low power.

 

[jflis]

well, 3 factors will come into play (well, more than *3*, but there are 3 *major* contributors all else being equal)

1) Speed: The greater she speed (subsonic) the greater the effect

2) Fin Thickness: The greater the fin thickness the greater the effect

3) Overall thrust time: the longer the burn time, the greater the effect

By all of that I mean (by way of comparison) if you have a model with thin fins with a fast burn motor that only hits a top speed of say 300 MPH (understand, I'm not doing the math here, these are just representations), you may see a small difference (say 200 feet vrs 210 feet) - very hard to tell with the naked eye (or any other naked part of your body )

But if you had a model with thick fins with a long burn motor and a top speed upwards of 600MPH you would see a significant difference, probably easily decernable with the naked eye (but still not by other naked parts of your body ) (hey, i'm very tired tonight... this is my release... LOL)

Bottom line is there are many things to take into account. Your question was easy to ask, can be very complicated to answer.

A *competitor* wants those extra 10 feet so he will *always* use the right configuration, regardless.

A sport flier, frankly, does it more for looks than performance. While I've never *reversed* an airfoil, for sport flying I doubt it would have enough of an impact to be concerned with.

Case in point, the following FlisKits kits:

Praetor: sharp angle front and rear of fin
Deuce's Wild!: Flat all edges
A.C.M.E. Spitfire: Flat all edges
Corona: Sharp angle front and rear of fin
Richter Recker: Flat all edges
Cougar 440 & 660 (competition models) airfoil on 1/64" plywood fins (yep!)

So, you can see that *I* don't pay a whole heck of a lot to what is the *best* aerodynamically. Sometimes appearances are more important than performance (ask anyone into saucers about that one... )

Hope all this rambling helps!

jim

 

[arthur dent]

Cheers Jim...the rambling certainly helps and it's very much appreciated I am enjoying grasping the basics of aerodynamics and just learning what shape the edges should be is great.

 

[Stymye]

.also.don't forgrt the outer most edge of the fin
..according to Stine ,,leave this edge flat,,rather than rounded or sharpened for the least turbulence and best performance.

 

[jflis]

Stymye,

good catch! I had not considered the effect of tip turbulance. Flat is best.

jim

 

[powderburner]

I have been digging through old textbooks and reference books, looking for a particular illustration for you guys. I can't find the one I really want, so I have attached something similar. Although it is not a perfect 'fit' to address the fin drag due to airfoil shape, it does provide a few numbers that should give you a better feel for the advantages of streamlining.

These drag coefficients are for the component called 'form drag' and do not represent the total drag effect (i.e., this data does not include drag due to surface area, or drag due to induced vortices at the fin tips, or other parts of fin drag). Nevertheless, you can see a major improvement as you scan down the list from a blocky, squared-off fin shape to a well-airfoiled and smoothly finished fin.

The classic subsonic shape would have a rounded or slightly (half) elliptical leading edge. It is OK to leave the middle of the fin at a constant thickness. From the 60 or 70 percent chord line to the fin trailing edge, the thickness needs to smoothly taper down to a knife edge. (If someone wants a quickee tutorial on what a chord line is, speak up)

The round leading edge is necessary for stability purposes as much as for drag reduction. If the vehicle yaws a few "X" degrees off the flight path, then the fins are essentially deflected to X degrees angle of attack (with respect to the oncoming airstream). Here is where your fins go to work for you: they are supposed to act as little bitty wings, and generate an aerodynamic force to pull the rear of the vehicle back into line. And since they are acting as wings, in subsonic flow, a sharp leading edge means that you will have instant aerodynamic stall, and consequent loss of the needed aerodynamic correcting force.

So keep 'em nice and round on the front. Class dismissed.

 

[arthur dent]

yep...good point(or rather flat edge)stymye and leaving the outer most edge flat also means less sanding