Vortex Generators on a Rocket

I know what everyone is thinking... Why?

Rather than argue the question, I am publishing the flight test results of installing StolSpeed VGs on my HR-II.

Summary: StolSpeed Vortex Generators were installed on a Harmon Rocket II (C-GVRL). Flight test results showed improved stall behavior and a 3 KIAS reduction in full-flap (VS0) stall speed; with a 1.7 KTAS reduction at fast cruise speed. Little reduction in clean stall speed or partial flaps (15 degree extension) stall speed was indicated.

Full report: http://www.vx-aviation.com/sprocket/documents/StolSpeed Vortex Generators.pdf

Cheers.

I loved the VG's on my old Cessna 182. But on an RV, getting 4 knots to play with at stall.... but being 1.7 nm behind where you used to be each hour...not so sure. And I hated washing the wings. YMMV

Guaranteed to keep people with normal vision and logic from sitting on your rocket wings. On a Lake Renegade 250 with a38 ft wingspan they lower the stall speed 12 knots which is a worthwhile investment and saves a lot of hull pounding on rough water take off and landings. Thanks for posting the numbers for the Rocket.

Vern; you are a gadget kind of a guy, how about making them retractable!

Yeah, what Tom said.... or maybe just make a full set of retractable leading edge slats.... that would be cool! I've always wondered how a Pegazair type wing would do on one of these beasts.

Now, to go design some even longer titanium gear legs.......

Up here in Canada I thought we had natural vortex generators... Raindrops!

NAH!!!!!............

.....those VG's up in NORTHERN Canada would be....

BUGS!

( easy to get the height, and profile...but the correct spacing takes a pilot with the 'right stuff'!)

and thanks Vern for being our 'test pilot'.....I think StolSpeed owes you a full credit for all the good press they are getting here!

Vern:

Interesting results. Do you intend to leave them on? If so, what benefit do you see for the Rocket?

Thanks

John

For Tom: After I put in all the speed mods (with your great advice) to increase my top speed, I went and put VGs on my wings and lost a couple of knots. Sorry!

The intent is to improve the landing behavior of the Rocket so that my average day landings don't require superior skill or excessive braking. The slower stall speed, coupled with the better stall behavior is encouraging. I will keep lowering my approach speed until I find the sweet spot.

It's all a relatively low cost, easy to reverse modification. I have a feeling that I am now running out of elevator authority as indicated by lack of a definite stall break with VGs. I have ballast in the baggage compartment, but I think the Rockets tend to be nose heavy. I don't really want VGs on the elevator, though.

I have been spoiled by the docile landing behavior of my RV-9A!

BTW, the 'catch' in the ailerons during full deflection rolls has been muted into more of a burble, to use the engineering term.

flyboy1963 said:

.....those VG's up in NORTHERN Canada would be....

BUGS!

( easy to get the height, and profile...but the correct spacing takes a pilot with the 'right stuff'!)

and thanks Vern for being our 'test pilot'.....I think StolSpeed owes you a full credit for all the good press they are getting here!

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I've seen the June bugs, the size of a hummingbird! Usually come out in July in the Okanagan... an oxymoron.

Tom Martin said:

Vern; you are a gadget kind of a guy, how about making them retractable!

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...because I would forget to put them out. You know the story... thems that have and thems that will...

Thanks Vern. We saw similar results on an RV-8, and the owner came to the conclusion the VGs were of little value other than slowing you down in cruise. I'll see if I can get him to post his flight test results. I assume 92" was in the middle of your CG range?

swaneymj said:

Thanks Vern. We saw similar results on an RV-8, and the owner came to the conclusion the VGs were of little value other than slowing you down in cruise. I'll see if I can get him to post his flight test results. I assume 92" was in the middle of your CG range?

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Range 88.7-97.4 inches aft of datum. I had a 19 L water jug for ballast.

vlittle said:

I have a feeling that I am now running out of elevator authority as indicated by lack of a definite stall break with VGs. I have ballast in the baggage compartment, but I think the Rockets tend to be nose heavy. I don't really want VGs on the elevator, though.

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How is your nose-down authority? Is it possible you need an incidence angle adjustment on your horizontal stab?

Oh, and if you were to try VG's at the tail, you'd want them on the Horizontal Stab, not the elevator... But you'd just lose another knot in cruise...

Bottom of the horizontal stab.

As far as the stall break, it could be related to CG as well. Many aircraft will not have a nose down pitch that cannot be arrested with a forward CG. The stall is recognized when the stick is moved full aft and no further increase in pitch occurs over a short period of time (2 seconds). You might try testing with a further aft CGs, but work up to that slowly. Probably good to test it closer to the forward CG limit too.
Mark Swaney
F1 Rocket N76TR

Snowflake said:

How is your nose-down authority? Is it possible you need an incidence angle adjustment on your horizontal stab?

Oh, and if you were to try VG's at the tail, you'd want them on the Horizontal Stab, not the elevator... But you'd just lose another knot in cruise...

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wouldn't it be nice to have the space to tuck some VG's on the elevator leading edge, like on the Zenith......!
No drag unless significant deflection of the surface......

Snowflake said:

How is your nose-down authority? Is it possible you need an incidence angle adjustment on your horizontal stab?

Oh, and if you were to try VG's at the tail, you'd want them on the Horizontal Stab, not the elevator... But you'd just lose another knot in cruise...

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What I need is about a 200 lb pax volunteer in the back seat ;-). While I'm at it, I want to test it flying at the lower speeds that we sometimes fly in formation, particularly with the Swifts and CJs. Dropping 15 degrees flaps is annoying and requires turning off my flaps overspeed alarm.

Now I see why Bryan flys his Rocket at the back of the formation... the low speed dynamics are quite different than the other RVs and no one can see him slew around back there.

you run out of elevator because you have to go to higher AOA to stall with VGs. So it could be that you don't have the elevator authority to fully stall the wing at fwd CG. Airplanes have to have all the parts designed to work together. IF you change one thing, it impacts others. Either the forward CG and the tail size were matched (presumably) to allow you to stall the wing at the fwd CG limit. Now if you increase the stall AOA by 2 or 3 deg, at fwd CG you might run out of elevator before you get to the stall, depending on if there was margin built in.

Another consideration is that if the stall AOA is now way above the 3 point attitude, you might not be able to use the extra lift that the VGs buy you in a landing unless you want to drop it in on the tailwheel. But you will have more margin and perhaps better roll control in the stall, so that might still be desirable.

When people started putting slats on super cub type airplanes, they soon learned that you can't just bolt them on and go. You need a bigger tail and longer gear and/or a wing incidence change if you wanted to land at 5 or 6 deg higher AOA. It's the famous domino effect. VGs are sort of like poor man's slats. They do the same thing to a lesser degree.

How slow can you go...

Vern, I debated installing a set on my HR2 given my home strip was 1500' long with obstacles. My buddy "DS" proved their worth in the Idaho back country on his HR2 "The Screamer" by shortening his landing distances by 20%.
Being an F16 driver, AOA is my forte". I preferred to install a Lift Reserve indicator (AOA) and fly my HR2 at it's max AOA limit. I found I could consistently fly my approach at 60-63KIAS and land in the first 500' of my strip. Also found I could really pull to the limit in acro...

I just thought VG's were aesthetically unpleasing on my wing...
V/R
Smokey

vlittle said:

I know what everyone is thinking... Why?

Rather than argue the question, I am publishing the flight test results of installing StolSpeed VGs on my HR-II.

Summary: StolSpeed Vortex Generators were installed on a Harmon Rocket II (C-GVRL). Flight test results showed improved stall behavior and a 3 KIAS reduction in full-flap (VS0) stall speed; with a 1.7 KTAS reduction at fast cruise speed. Little reduction in clean stall speed or partial flaps (15 degree extension) stall speed was indicated.

Full report: http://www.vx-aviation.com/sprocket/documents/StolSpeed Vortex Generators.pdf

Cheers.

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