# Thundergull

### Help Support HomeBuiltAirplanes.com:

#### Victor Bravo

##### Well-Known Member
HBA Supporter
Propeller referenced above (too late to edit it in to the previous post)

#### Speedboat100

##### Banned
I agree, a 20 ft spanning motorglider would probably perform very well for an R/C model. You might be able to win numerous R/C model contests with it. Or you could have it piloted by a small Chihuahua or Rhesus Monkey, and it would achieve very good soaring performance.

I have actually once flown a 20 ft spanning motorglider myself, it was designed by the same aircraft engineer who designed the aircraft in the photos above. It had an unusual feature for motorgliding... after climb to gliding altitude, the propeller spinner departs from its mounting and strikes the propeller. This feature causes a very effective warning alarm, which reminds the pilot to shut down the self-launch engine. This results in approximately 45 seconds of memorable gliding flight.

It also provides a delightful souvenir of the gliding experience, the remaining portion of the (post-alarm) propeller, which is suitable for hanging on the wall of your living room.

Cool...what a ride !

I recall mr Jim Bede had an idea to use the BD-5 fuselage also for a glider. I cannot think of much more than 20-25 ft as a span for it.

It is about aspect ratio, total drag and power to weight ratio and wing loading...if those number are still good you can fly it at 250 lbs pilot very well...but if the plane is 800+ kg like some carbon gliders are...it certainly no longer can be small in any measurement.

#### Victor Bravo

##### Well-Known Member
HBA Supporter
Cool...what a ride !
Thank goodness it was an RV-3 aircraft. One of a very few that fly just as well power off as power on.

#### Topaz

##### Super Moderator
Staff member
Log Member
Well...I like the idea of a 20 ft spanning motorglider anyway.
No you don't. Because the moment you turn off the motor, it's not much of a glider.

L/D performance is dependent directly on span and equivalent drag area. Minimum sink rate is dependent upon aircraft weight, equivalent drag area, and span loading.

Note that "span" figures into both of those critical glider-related performance parameters. A 20' span "glider" isn't going to be much of a glider unless it's exceptionally light and clean - in fact, it'd have to be a model, just like VB is saying.

Physics doesn't care what we want. If you want to design airplanes, Topspe... er, Speedboat100, you have to go beyond pretty drawings and start doing the math.

#### b7gwap

##### Well-Known Member
To bring this thread back on topic, has anyone heard any news about Earthstar or the Thunder Gull lately? It really is an attractive little ultralight.

#### pictsidhe

##### Well-Known Member
No you don't. Because the moment you turn off the motor, it's not much of a glider.

L/D performance is dependent directly on span and equivalent drag area. Minimum sink rate is dependent upon aircraft weight, equivalent drag area, and span loading.

Note that "span" figures into both of those critical glider-related performance parameters. A 20' span "glider" isn't going to be much of a glider unless it's exceptionally light and clean - in fact, it'd have to be a model, just like VB is saying.

Physics doesn't care what we want. If you want to design airplanes, Topspe... er, Speedboat100, you have to go beyond pretty drawings and start doing the math.
Pffft, maths is only for people who need stuff to actually work.

#### Topaz

##### Super Moderator
Staff member
Log Member
Pffft, maths is only for people who need stuff to actually work.
Oh, THAT.

#### RogFlyer

##### Member
HBA Supporter
To bring this thread back on topic, has anyone heard any news about Earthstar or the Thunder Gull lately? It really is an attractive little ultralight.
Hi, no response to e-mail or phone from here. Peter has bought a plane and is cheerfully commuting Cowra/Canberrra/Orange (J170, I think). So curses, I don't get to build one with/for him. Back to dreaming.

#### Victor Bravo

##### Well-Known Member
HBA Supporter
There is a complete Tornado kit (knock-off of the Gull) for sale... very reasonably priced... on the San Diego Craigslist if anyone is interested.

If any HBA member buys and builds this Tornado kit, please understand that a heavily armed HBA Delta Force team is standing by with direct orders to shoot on sight if the builder uses the "Wicked Witch Hat" Titan version of the nose cone.

It's just an unforgivable crime against aesthetics.

#### Seawarner

##### New Member
A couple of questions regarding Earthstar in California. Airplane is Thunder Gull 2000:

1. Is Earthstar still a going concern? So far, no response to e-mail or phone, but my mate has only been trying for a couple of weeks. The web-site feels pretty current.

2. Peter is 1.87m and 95kg and 71. Roughly 6'2" and 200lbs. Any ideas on cockpit size, pilot weight and electric start?

3. Are the Gull 2000's only kits, or can one get a fully built one?

4. Does anyone know if there are any flying in Australia (or, at a pinch, NZ - though international flight restrictions could be a problem for a while yet).

Cheers,
Michael.
I corresponded With Mark briefly a few years back. He said that 250 Lb pilot would be no problem with soaring wings. He did a staggered tandem version too.

#### Seawarner

##### New Member
Hi Craig
The Load limits are 6+ 4- and we have tested the wing to failure. It yields as a bow in the wing and fails at the aileron flap juncture as it was designed to do. Every structure has a failure mode and I wanted it to fail in a manner that would present the best stability and clearance for the ballistic parachute, if you install one.

All the models are designed to the same limits and characteristics.

The power system complete with battery's and all is currently $13000 The Soaring Gull 2000 kit is$14,590.

Yes the electric power system should be purchased last.

The wing structure is a Dcell with the wash out built in, in our jig. The Dcell ribs are foam bonded in to the Dcell. The main spar is built up of aluminum angles riveted and bonded to a shear web. The aft ribs are formed from aluminum sheet. and the wing skins are of varying thickness sheets in relation of the local load. The skins are riveted and bonded.

The airfoil is my own design, the product of 20 years of study and 10 tests by taking a wing on a flying airplane and changing the airfoil by cutting the new shape out of foam and attaching it to the wing and measuring its characteristics.
It generates a lift coefficient of 2.0 as compares to the Cessna at 1.6 and the average ultralight at 1.1 this means that my wing only needs to be 1/2 as big as, say a rans for instance.

I am still studying ducted fans but I don't think that they are ready yet for prime time.

I really like Joby motors and will be spending several days at his place this week end doing testing and design work with him, JoeBen.

290# is not to much but I would encourage you to loose some weight since it takes power to lift every pound.

I hope I answered enough of your questions for now, feel free to ask whenever you come up with more questions.

Happy Flying
Mark

#### Speedboat100

##### Banned
No you don't. Because the moment you turn off the motor, it's not much of a glider.

L/D performance is dependent directly on span and equivalent drag area. Minimum sink rate is dependent upon aircraft weight, equivalent drag area, and span loading.

Note that "span" figures into both of those critical glider-related performance parameters. A 20' span "glider" isn't going to be much of a glider unless it's exceptionally light and clean - in fact, it'd have to be a model, just like VB is saying.

Physics doesn't care what we want. If you want to design airplanes, Topspe... er, Speedboat100, you have to go beyond pretty drawings and start doing the math.
This was a good advice. I got it now at 36 ft span.

#### Topaz

##### Super Moderator
Staff member
Log Member
And you know that is the right amount for how much performance, exactly?

My point wasn't "make the wings bigger." My point was that you need to (learn and then) do the math so that you can calculate how much span you need to obtain a particular performance goal. Eyeballs and guessing isn't going to get you there.

Do yourself a favor and buy at least this book. It will be a very good beginning, isn't expensive, and the math is not difficult:

#### Speedboat100

##### Banned
But I really hadn"t read it too much. Now I did and got it right. Raymer and Torenbeek with Roscam and Howe are pretty much aligned now in my design. What I am tuning is the LG mechanism and poor man area ruling.

Last edited: