launching with an angled rod / rail Q

I am still a noob in most ways. My question is simply this, but I am not sure how to word it right.

If you have some wind and angle your launch rod a bit to get it to land near you, should the rocket be placed under the angle or on top or it?

[video=youtube;A34em4HOXLA]https://www.youtube.com/watch?v=A34em4HOXLA[/video]

So in the above vid I would say that the rocket is on top of the angle that is leaning to the left. I am guessing that is why the rocket did not launch, but later found a bit of glue on the lug. Rocket still slid down the rod with ease though. I figured the lug binded from wanting to go up but tweaked against the lug holding it there

The reason I am asking is because I picked up a rail and the Rockwell Jaw Stand showed up today and I am working on making a rail holder that I can clamp down in the stand.



It seems most folk attach the rail on the outside part of the wood or device that is in the clamps. As in the left side of the pic.
I was thinking that if I put the rail on the inside (right side of the pic) and closer to the center of the stand it would make it more sturdy. But my thinking is that all of my rockets would be on the bottom of the angle if I do angle it.

Of course I will still work on a blast shield and cut that piece of wood down to proper length, so I am not worried about gasses harming the stand. Just wondering if any of you have thoughts on launching under or over the angle. I also agree with some other thread I cannot find now that any angle usually results in more trouble than it is worth, but figure I might do a few degrees of tilt on slightly windy days in the future.

Any tips on why my rail should be on the outside welcome as well.

I don't tilt much, easy to overdo and adds velocity at ejection for there to be more horizontal travel, and many rockets weathercock anyway. If anything it will be a tilt away from spectators. BUT, typically on a rod the rocket will want to fall onto the low side. If the rocket is heavy and stands far to the side of the rod, flexure can be an issue, and if it can be held on to the high side, that helps. I attached a little bump to where the rod comes past the fin on one rocket to allow that. Note that ideally there should be little force on the rod once the motor fires.

That is something that always happens naturally, I guess. The rocket usually ends up "under." With a rod, you 'd have to take some kind of unusual measures to keep it "above," and I've never done it. I always seem to have good luck angling rods. Of course, at personal launches I usually send up 2 of my traditional opening rockets, a Wizard on a B and some small BT-50 RTF I've had for 20 years, on a C with small parachute. This gives me a really good idea where to put the rod.

With a rail, I might be inclined to position it so the rocket is "above" when tilted. You can probably buff the rail better on the outboard side, and then the rocket would rest on THAT surface with a little less friction.

At our club we find that tilting rods/rails into the wind actually results in the rocket tilting into the wind during flight. The wind pushes on the fins more than the rest of the body causing the rocket to fly into the wind.

We have actually had informal discussions about just welding our club HPR launch stands so fliers can only launch straight up. I doubt it will ever really happen.

Tilt depends on three factors primarily-- 1) the wind speed, 2) the speed the rocket accelerates at (weight/motor choice), and 3) the stability margin of the rocket (noseweight/fin size-- CG/CP relationship).

The windier it is, the more you need to tilt the rod to get the rocket to drift back down near the pad, naturally. BUT, it's not that simple... the other two things have to be factored in as well.

If the rocket is large/heavy/draggy, it's going to accelerate slower, and thus be more prone to weathercocking. In windy conditions, usually a higher peak-thrust engine is a better choice, as it helps the rocket get up and moving faster off the pad, and thus minimizes weathercocking effects. When a rocket is sitting on the pad, the wind is moving at a 90 degree angle to it... IOW, a 90 degree angle-of-attack. If you recall from stability discussions about the cardboard cutout method, this moves the CP as far forward as it can go. At any rate, as the rocket ignites and starts to move, the "apparent" airflow from the mass of air moving downwind (the wind) will appear, from the rocket's point of view, to decrease its angle of attack and the wind will start to "appear" to be coming straight down from the nosecone, or thereabouts. For instance, when the rocket lifts off and achieves a speed exactly the same as the wind speed, the "apparent" wind direction will be from a 45 degree angle off the nose, upwind. When the speed of the rocket is double the windspeed, the 'apparent' wind direction from the rocket will be 22.5 degrees off the nose, upwind. Is that clear as mud?? If you've ever driven in a snowstorm, you'll know EXACTLY what I mean... if you drive slow, the snow apparently comes almost straight down, at a slight angle from upwind... as your speed increases, the snow will start appear like it's coming STRAIGHT AT YOU, from a more and more horizontal level as your speed increases... because you're driving into it! The rocket is "driving into" the wind, and so the wind appears to "shift directions" with respect to the rocket's direction of flight (where the nosecone is pointed).

What this means is, that you want the rocket going as fast as you can possibly have it moving when it leaves the rod or rail (since the rod or rail arrests any turning moments induced into the rockets direction caused by the wind pushing against the sides of the fins). Once the rocket leaves the launch rail or rod, it's a free body in space and any side forces generated by the wind against the fins will tend to cause the rocket to turn until those forces are neutralized... So, faster acceleration on a windy day equals a straighter flight.

The third factor is the CG/CP relationship and weight distribution of the rocket, combined with the fin size-- IOW, the dynamic stability of the rocket. The wider the separation of the CG from the CP, the more "overstable" the rocket is, the greater the tendency to weathercock. The length of the rocket and size/location of the fins, and the distribution of mass within the rocket, determine the moments of inertia and how fast and strongly the rocket reacts to the aerodynamic forces working on it in flight. A rocket with a heavy payload in the nose will be slower to turn, but harder to stop turning (oscillation) than a lightweight rocket. The longer the moments of inertia, the more "leverage" they exert on the rocket-- IOW, fins further aft have more "leverage" to turn the rocket into the wind in windy conditions than fins further forward... and weight nearer the CG of the rocket (shorter rocket, or no payload or payload installed well below the nosecone area) will have a lower moment of inertia and thus is easier to "leverage" into moving by the fins. Make sense?? The size of the fins determines the AMOUNT of corrective force they can generate, which is also directly tied to speed-- the higher the speed, the more corrective force they generally produce. That is why CP moves aft with increasing speed (to a point, not talking transonic/supersonic here).

This is part of the reason why super-rocs tend to go unstable in windy conditions... it's funny how all these factors come into play, and what works great for one rocket isn't necessarily going to work well for another... I've seen guys launch on a breezy day off the club pads, using the same rod angle, (didn't touch it from the last three guys who launched) and one guy will have his rocket come down 50 yards upwind, the next guy might have it land 30 feet from the pad, and the next guy may STILL have to walk a half-mile downwind to get his rocket... So the rod angle really depends on the rocket and the conditions at the time... that's where experience comes in...

As for "on the downwind side of the rod or the upwind side (back of the tilt) on rods, it's pretty pointless to even try to keep the rocket on the other side. Gravity tends to make the rocket rotate to the "lower" side of the rod, on the "inside" of the tilt... If it's gusty, the wind will try to "turn" the rocket around the rod to the "higher side" of the tilt, but when the wind backs off, gravity usually turns it back. (This of course can play heck with your ignitor clip connections! Sometimes putting something like a clothespin or something like that to keep the rocket from twisting around on the rod as the breeze catches it can be quite handy!)

For rails, it doesn't particularly matter, since the wind will either be pushing the rocket against the rail if it's on the "inside" of the tilt, while gravity is tending to pull it away from the rail, or if you install the rocket on the "outside" of the tilt (on the back of the rail WRT to the wind) the wind will be trying to push the rocket away from the rail, and gravity will be pulling it towards the rail... If the rocket is on the rail at 90 degree angles to the tilt (either on the "front" or "back" of the rail, if it's say tilted to the left) then the wind will be "torquing" the rocket or causing it to twist with respect to the rail... which might be more prone to cause some binding in the buttons...

Just some things to think about! OL JR

DUDE

OMG Luke thanks for such a great response. Such good info and reasoning behind it.

and that is how it went down for those at home.

WOW

After goofing off all evening after posting this, I realized that I can simply turn the rail mount 180 easy.

Thanks for the great info though. I am very grateful.

Still not sure where I will end up in all of this, but it will be fun for sure.

I live in Oklahoma, so windy launches are a given for us. I recently built a model I call "Miss Atomic Bomb of 1957" that has a lot of fin area.



Rocksim shows a stability margin of 2. During the first launch we all noticed this rocket really weathercocked. The rod was straight (launch angle 0) for the first launch, then on the next launch, the rod was angled into the wind--the flight looked like a cruise missile--it went up, then nearly horizontal :y: That was my bad, I didn't check the rod angle :eyeroll:

I went back to Rocksim to analyze what happened. I finally hit upon plotting the 2-D flight profile (x axis = range; y axis = altitude). Hopefully the following plot will illustrate what Ol JR described above...

Wind is 8-14 mph, and comes from the left side of the plot...all flights on D12-3

Launch angle = 0 (straight up)
Maximum predicted altitude = 292 ft, deployment velocity = 38.5 ft/sec


Launch angle = -10 (angled INTO the wind)
Maximum predicted altitude = 250 ft, deployment velocity = 62.2 ft/sec


Launch angle = +10 (angled down wind)
Maximum predicted altitude = 315 ft, deployment velocity = 16.6 ft/sec
Note how the rocket starts down wind, then the weathercock causes it to fly nearly vertical.


It is counter-intuitive unless you keep in mind what the CP/CG relationship is doing for you.

At our recent launch (8 mph w/ 12 gusts), I tried aiming MAB down wind, and by golly she flew straight up!! Physics is phun!!