How fast can a PML Ariel safely go?

[maxpower]

I realize this is a very subjective question but I am asking people with experience with PML QT kits.

Again this is very subjective but when I built my Ariel I sanded all surfaces to be glued with the scratches perpendicular to the direction of the motor thrust. I used Aeropoxy without any fillers. The fins are glued to the motor tube with glue fillets added to the motor tube joint. The inside fin to body and of course on the outside fin to body joint.

I heard that with QT you should stay below 0.7 mach

According to Rocksim here is the max velocity and accelerations for various motors:

I161 - 0.37 mach - 26 gs - I have flown it on this
I300 - 0.52 mach - 21 gs
I600 - 0.74 mach - 36 gs
J350 - 0.73 mach - 30 gs
J570 - 0.91 mach - 45 gs

The datasheet does not list the I600 or the J570 and says motors not listed may require some modifications to the kit. What mods? The J350 is shaded in grey saying that building has to be absolutely top notch! This was my first HPR kit (and so far only) so I doubt I built it top notch but as you can see I used, what I understand, to be the strongest epoxy out there. I did not used milled fiber which I will use on my next HPR.

Before I did an I600, I would go with something in between the 161 and the 600 first

How far would you push this rocket?

Thanks

 

[hartlch]

You need to go to their website and look at the "motor recommendations" link. It shows a color coded chart for all the motors (though I didn't see the I600). It also explains what modifications or procedures need to be followed for building. Also refers to their airframes FAQ about strengthening techniques.

From that chart for the J570, the coding says you need the strength mods and other mods to fit the extra long motor - i.e. it is probably too long to fit the kwik-switch motor mount.

They say mach 0.85 is the limit for QT and the strengthening mods don't apply to it. That is, you need to have asked for a phenolic airframe so you can glass it.

Check out their FAQ(s) and they will explain it all...

You can probably assume the guidelines for the J350 apply to the I600 too.

 

[BHP]

That QT is neat stuff.

I flew my QT Pterodactyl Jr. modded only with a 54mm MMT and a 6" extension for an altimeter bay on a K570 to 8,100' and 777 mph. It came back ready to fly again.

 

[kevin_mcgrath]

Those are some pretty high accelerations -- maybe bad sim data?

An I161 in my Ariel sims at 9Gs and the recorded data matches that. Now, its a bit heavy since I have 8 ozs of electronics and have stretched the payload section 4 inches.

But that is still quite a difference between your sims and my sims/data. Did you use your actual built weight in the sims?

To your original question -- I have flown my Ariel on a J350 with no problems.
The fastest flight was a J275 -- 5990' and Mach .74 (measured).

Kevin.

 

[daveyfire]

I'd mostly be worried about the thin 1/16 G10 fins fluttering off. Check that and you should be fine.

 

[maxpower]

Originally posted by kevin_mcgrath
Those are some pretty high accelerations -- maybe bad sim data?

An I161 in my Ariel sims at 9Gs and the recorded data matches that. Now, its a bit heavy since I have 8 ozs of electronics and have stretched the payload section 4 inches.

But that is still quite a difference between your sims and my sims/data. Did you use your actual built weight in the sims?

To your original question -- I have flown my Ariel on a J350 with no problems.
The fastest flight was a J275 -- 5990' and Mach .74 (measured).

Kevin.

Well... I noticed that Rocksim is always high. I got the Rocksim file off the net and it said that my Ariel would go 2800 ft on an I161. That is even after I added the weight of my electronics. It actually went 2144ft, according to my Perfectflite data. To get the sim to agree with the Perfectflite altitude, I tweaked the Cd. That is where I got the numbers from for my post above. Your post reminded me that I had the Perfectflite data so I calculated velocity and g-force. It is possible I did it wrong but I get (for the I161):

11.9 gs with a max velocity of 0.51 mach (using 1140 ft/s for sos)

Odd, which says it accelerated slower than Rocksim but the rocket ended up with a higher velocity. That is not impossible, just interesting.

With my new Cd, my sim numbers do not agree with PMLs. In fact the J350 max altitude is around 4000ft and PML says 6063ft. This number is important as the ceiling at the field I launch at is 4500ft.

Even before I tweaked the Cd, and on the I161, PML = 3057ft, Rocksim = 2800ft, Perfectflite measured 2144ft. There is such a difference. That is why I am asking you guys for thoughts and not trusting what PML says. Not that PML is wrong but there are certainly variances in build techniques. I know it is very subjective but I appreciate getting the real world information from you guys. Thanks!

 

[hartlch]

But besides the weight of your electronics, did you measure the final built weight of your rocket and used that? The finished weight that PML has in their simulations is usually less than what most people seem to end up with. PML even says they tend to "BUILD light" - don't remember where I read that - either in their FAQs or an interview with the owner.

 

[maxpower]

I had weighed my Ariel on a scale accurate to 0.01 oz. That is the weight I used, not PML's

 

[cjl]

What delay did you use?

When I get unreasonably high accelerations, it is often because the delay is wrong, and that is the real peak acceleration - but the peak is where the main chute opens (not during burn). If you choose the right delay, I bet that the number of gs predicted will vastly decrease.

 

[maxpower]

In Rocksim I always use All delays. For the real flight, I used my Perfectflight to trigger at apogee. I got a peak right when the charge went off. This could be because I didn't seal the bulkhead well enough. I did not use that data anyway for my calculation of the max acceleration.

I will try the sims again with different delays and see if the max acceleration goes down.

Thanks

 

[maxpower]

Originally posted by cjl
What delay did you use?

When I get unreasonably high accelerations, it is often because the delay is wrong, and that is the real peak acceleration - but the peak is where the main chute opens (not during burn). If you choose the right delay, I bet that the number of gs predicted will vastly decrease.

I went back an picked a delay (instead of "All") and that gave me an acceleration close to what my Perfectflite measured. It says, 10.4 gs and I measured 11 gs.

Thanks for everyone's input!

 

[smurf]

If you are using electronics to do deployment then you can just select deploy at apogee and you don't have to worry about which delay to choose. Rocsim is usually pretty optimistic in any case when it comes to altitude and airspeed.

 

[maxpower]

Right. I understand that it was just in Rocsim when I chose all it gave me a high max acceleration. Selecting a delay, like 10, gave me a more reasonable acceleration. In fact it nearly matched my electronics. When I use my electronics I don't even load the charge in the motor.

 

[bobkrech]

Max

Several words of advice.

When you ask a sim question, always include the weight of the rocket without the motor, and it's diameter. That's all you need to do a simple sim on any rocket. It take less than a minute to put the data into wRASP, webalt, or any other sim program.

All sims will give the same peak acccelerations. This usually occurs at low speed there Cd is irrelevant.

Altitude depends on Cd. Real Cds are between 0.6 and 0.75 for virtually all smaller rockets.

Make sure you have a good motor file in your sim. Every motor file listing I have seen has at least one bad motor data set in it.

Anytime you do a sim your really have to go a sanity check.

For example look the the thrust curve for a I161. https://tripoli.org/tmt/new_motors/AT I161W dec 08.pdf

The peak thrust is ~58 pounds. If your rocket is close to 59 oz with the e-bay but without a motor, with an I-161 it would weigh 72 oz, or 4.5 pounds.

The peak acceleration of your rocket is approximaitely the (peak thrust divided by the rocket weigh)-1 so

A peak = (58/4.5)-1 ~ 11.9 G. This would be reported peak acceleration if the simulation time step is ~ 0.001 s like Rocksim. wRASP use a time step of 0.1 seconds, so it reports the peak accleration as 9.55 G.

To match your altitude to the estimated weight, I need a Cd=0.75.
Loading a J-350 into the sim, I get an altitude of 4033 ft, with is almost 500 ft below your waiver altitude. (Peak velocity is M=0.74)

My guess is that the PML predictions are optimistic, and that you are not properly using RockSim.

The real issue for your flights is not the QT, since even with the J570 you would only reach M=0.94, but fin flutter. I don't think the stock 1/16" fins are stiff enough to go over M=0.8 or 0.85 without fluttering and ripping off. I would have used 3/32" or more likely 1/8" FG fins.

Bob

 

[DJDeadParrot]

Originally posted by daveyfire
I'd mostly be worried about the thin 1/16 G10 fins fluttering off. Check that and you should be fine.

I saw that exact thing happen on an Ariel powered by a J415.

 

[cjl]

Originally posted by bobkrech


The real issue for your flights is not the QT, since even with the J570 you would only reach M=0.94, but fin flutter. I don't think the stock 1/16" fins are stiff enough to go over M=0.8 or 0.85 without fluttering and ripping off. I would have used 3/32" or more likely 1/8" FG fins.

Bob

I have pushed 1/16" fins to M.9 before without issue on my PML amraam-3. I wouldn't push it supersonic, but it handles mach .8-.9 just fine.