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Oct 06, 2021, 07:21 PM
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Quote:
Originally Posted by burkefj View Post
You are making assumptions, the model is not pitchy at all, .
What I mean is that if you moved the CG aft from where you have it now, without a gyro, it would be “pitchy” in boost, but would be trimmed at a lower AOA in glide (giving you lower drag and less wing wobble). Adding a gyro (with the right gain setting) should damp the “pitchiness ” but still give you a lower AOA in glide.

I should clarify that the gyro mentioned above is only acting in pitch, or elevator channel. Some damping in roll (aileron channel) may or may not be of benefit.
Last edited by sr205347d; Oct 06, 2021 at 07:36 PM.
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Oct 06, 2021, 11:09 PM
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X30PILOT's Avatar
Hi Frank,

Those flights on the SR-71 look great!
My experience with my SR-71 and all 4 of my of X-30 models has taught me at high AOAs the wing is blocking the airflow to the vertical stabilizers causing the rocking.
Like you said, it only happens when you get slow. Iíve come to embrace this characteristic as itís a great visual indicator of an impending stall, and no a gyro wouldnít be much good with this.

Dave
Oct 07, 2021, 11:17 AM
Registered User
Quote:
Originally Posted by X30PILOT
and no a gyro wouldn’t be much good with this.

Dave
Dave,

I am not successfully communicating what the benefit of a gyro might be to an RCRG, so let me back up.

In order to boost with an adequate stability margin, the CG must be sufficiently far forward of the neutral point (NP). If the CG isn't quite far enough forward, the rocket will become "pitchy" as Frank says (less stability margin). If the motor is in the tail, then after the propellant is consumed the CG will be further forward. (This is all model rocket 101.)

In order to have a trimmed glide, with the CG forward of the center of lift, the tail must balance with a downward force. The further forward the CG, the greater tail down force required for balance. The lift required of the wing is the sum of the weight AND the downward force of the tail. Moving the CG forward will increase the lift required (either more AOA or speed) (much like adding weight). Moving the CG aft will decrease the lift required (allowing less AOA or speed). (However, there is an aft limit to CG due to inadequate stability margin.)

With the propellant burned off, the CG is way further forward than necessary for a stable trimmed glide.

But if you move the CG aft in order to reduce the AOA/speed required in glide, the stability margin in boost will be inadequate("pitchy").

A gyro in the elevator channel can damp the pitch sensitivity in boost ("pitchiness"), thereby allowing you to put the CG not so far forward, lessening the down force required of the tail and reducing the lift required of the wing (AOA/speed) (much like making the glider lighter). With lower AOA at low speed, there will be less tendency for wing wobble and stall.

Merely adding a gyro, without adjusting the CG aft, will do nothing as you say.

BTW, this kind of gyro stability augmentation in the early F-16 fly-by-wire flight control systems was called "relaxed static stability" in the marketing from General Dynamics. It helped make the F-16 more maneuverable to some degree.

Andy
Last edited by sr205347d; Oct 07, 2021 at 11:52 AM.
Oct 07, 2021, 12:26 PM
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burkefj's Avatar
Andy, I think in this case your proposal is correct in theory, but in practice with these sport flyers that have significant non optimal shapes, thicknesses, flat airfoils, exposed servos/controls etc the gains in moving the CG back to reduce glide trim and relying on the electronics for boost will be low. In this model I'm only using 3/16" of up trim, and on my others similar so not a lot of trim but some for sure. But, cost for the electronics built into the receiver now is not very significant to add this so why not try and measure the difference in actual glide times and see. In my hands I think they fly great and feel no need to optimize another 5 seconds of glide time out of them, but for you why the heck not. I find a lot of times I'll get flight time differences of 10-30 seconds in glide time just from wind changes and approach requirements and boost profile. Boosting at non vertical profiles will also be more efficient and can have a big impact as well, so it's going to be very hard to measure apples to apples conditions and impact. Report back on what you find.

I enjoy personally hand flying these and not having something inbetween me and the model but I do use cruise control in my car

Frank

Quote:
Originally Posted by sr205347d
Dave,

I am not successfully communicating what the benefit of a gyro might be to an RCRG, so let me back up.

In order to boost with an adequate stability margin, the CG must be sufficiently far forward of the neutral point (NP). If the CG isn't quite far enough forward, the rocket will become "pitchy" as Frank says (less stability margin). If the motor is in the tail, then after the propellant is consumed the CG will be further forward. (This is all model rocket 101.)

In order to have a trimmed glide, with the CG forward of the center of lift, the tail must balance with a downward force. The further forward the CG, the greater tail down force required for balance. The lift required of the wing is the sum of the weight AND the downward force of the tail. Moving the CG forward will increase the lift required (either more AOA or speed) (much like adding weight). Moving the CG aft will decrease the lift required (allowing less AOA or speed). (However, there is an aft limit to CG due to inadequate stability margin.)

With the propellant burned off, the CG is way further forward than necessary for a stable trimmed glide.

But if you move the CG aft in order to reduce the AOA/speed required in glide, the stability margin in boost will be inadequate("pitchy").

A gyro in the elevator channel can damp the pitch sensitivity in boost ("pitchiness"), thereby allowing you to put the CG not so far forward, lessening the down force required of the tail and reducing the lift required of the wing (AOA/speed) (much like making the glider lighter). With lower AOA at low speed, there will be less tendency for wing wobble and stall.

Merely adding a gyro, without adjusting the CG aft, will do nothing as you say.

BTW, this kind of gyro stability augmentation in the early F-16 fly-by-wire flight control systems was called "relaxed static stability" in the marketing from General Dynamics. It helped make the F-16 more maneuverable to some degree.

Andy
Oct 07, 2021, 12:54 PM
Registered User
Quote:
Originally Posted by burkefj
Andy, I think in this case your proposal is correct in theory, but . . .

Frank
I am happy that I was finally able to communicate my point for anyone who might be interested. But, as you say, it doesnít really matter unless you are flying in a glide duration contest (but then you wouldnít be flying an SR-71), or want to reduce wing wobble tendencies.

You take care to build light, so why not fly light?

Andy
Oct 08, 2021, 12:28 PM
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burkefj's Avatar
Another beautiful blue sky cold morning with lots of smoke! This is the YF-12 version today, flies so nicely....
1/32 scale R/C YF-12 Interceptor(A12/SR-71 based) rocket glider (5 min 42 sec)
Oct 09, 2021, 01:49 PM
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burkefj's Avatar
I asked Bob Parks who was the original designer of the Aerotech Phoenix and an MIT educated aerodynamicist and all around good guy about the SR-71 style Wing plan form and the rocking here was his reply.

"All highly swept wings tend to do that. And the tendency increases at higher angle of attack.

Having a lot of vertical tail area can reduce it. the various blackbirds really do not have that much vertical tail area.

Its not really a stall kind of thing, but you do get a vortex shed by the leading edge that could be contributing to it.

Also, on the blackbirds, the fuselage strakes will shed a vortex and that could be interacting with the vertical tails too.

Vertical tail blanking is not really an issue. The high sweep keeps the flow pretty attached. "

Bob
Oct 09, 2021, 01:58 PM
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X30PILOT's Avatar
Quote:
Vertical tail blanking is not really an issue. The high sweep keeps the flow pretty attached. "
Build one with 30% taller vertical stabs an see what happens.

Dave
Oct 10, 2021, 12:52 PM
Electric Coolhunter
Thomas B's Avatar
I had a couple of engineers at Lockheed Martin offer the opinion that the Hobbylab SR-71 rocked in the glide due to tip and other vortices shedding asymmetrically from the model while at a slight angle of attack.

Ties in with what Bob Parks said above. Also, the strakes were pretty thick on the Hobbylab SR-71.

My twin 64mm ducted fan LX SR-71 has a good bit less rocking on approach (still has a little), likely due to thinner strakes.
Last edited by Thomas B; Oct 10, 2021 at 12:57 PM.
Oct 10, 2021, 02:08 PM
Registered User
Quote:
Originally Posted by Thomas B
I had a couple of engineers at Lockheed Martin offer the opinion that the Hobbylab SR-71 rocked in the glide due to tip and other vortices shedding asymmetrically from the model while at a slight angle of attack.

Ties in with what Bob Parks said above. Also, the strakes were pretty thick on the Hobbylab SR-71.

My twin 64mm ducted fan LX SR-71 has a good bit less rocking on approach (still has a little), likely due to thinner strakes.
Nice sharp leading edges on highly swept wings could help prevent asymmetric vortex shedding. See my discussion of vortices here: https://rcairplaneinnovations.com/st...-your-plane-2/
Last edited by sr205347d; Oct 10, 2021 at 02:14 PM.
Oct 10, 2021, 10:13 PM
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burkefj's Avatar
It's interesting to understand the concept but in the end in these particular models it doesn't matter, the plane doesn't depart or stall it just rocks the wing
and settles when almost at zero air speed, any other regime it doesn't do it and there's absolutely no problem with the flying characteristics and it doesn't lose any control or pitch authority throughout the entire range of airspeed. Ie it's an interesting phenomenon but not a problem that needs to be fixed imho
Oct 12, 2021, 12:09 PM
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X30PILOT's Avatar
Quote:
it's an interesting phenomenon but not a problem that needs to be fixed imho
I certainly agree.
Back 22 some odd years ago when we were building small test models before building the big X-30 this issue was so bad that the model wouldnít fly.
We increased the size of the verticals 15% from scale and all of a sudden we had something that performed very nicely.
Just to be safe, we settled on 20% for the final versions. We didnít want to go so big that it didnít look scale.

Dave
Oct 20, 2021, 04:43 PM
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burkefj's Avatar
More rocket glider fun, this time a mini AQM-37 Jayhawk test flights before applying the markings.


Mini AQM-37 Jayhawk Target Drone R/C rocket glider (4 min 48 sec)
Last edited by burkefj; Oct 23, 2021 at 03:07 PM.
Oct 23, 2021, 03:05 PM
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burkefj's Avatar
Another mini model, having a lot of fun with these, allowing me to bring back a few kits that I can't do in a larger scale any more due to lack of 9mm depron availability, but 6mm is great for these small models. Test flights before applying markings
R/C X-15 Delta Configuration rocket glider (5 min 0 sec)
Oct 29, 2021, 02:31 PM
Registered User

Legalistic Question


Maybe I shouldn’t ask a question if I can’t stand the answer. But, here it is:

Has it been determined if radio controlled boost gliders are legally considered model rockets and therefore governed by Title 14 (federal aviation regulations) Part 101, which has no altitude restrictions, or, are RCRGs considered (like model airplanes and drones) as small unmanned aircraft governed by 49 U.S.C. section 44809 which are limited to 400 feet AGL in uncontrolled airspace?

I am not interested in opinions, I want to know if there has been an authoritative determination.

Andy
Last edited by sr205347d; Oct 31, 2021 at 05:00 PM.


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