Open Class

A few observations from one who flies a Nimbus 3 now and previously raced a 604.

Climb is dictated more by span squared loading. 604 and Nimbus 3 both at 750 KG, both at about 9.2 psf, Nimbus 3 has slightly lower sink rate (from the polars) but noticeably better climb. Based on my experiences with 22 meter 604 and 22.9 plus winglets Nimbus 3 and 25.5 meter Nimbus 3. Why? More span and aspect ratio.

Run is dictated most by percent of total area getting laminar flow (or maybe amount of surface NOT getting laminar flow) and close second by wing loading. Again with my 604 and Nimbus 3 comparison. Same weight, roughly same wing loading, but at 80 knots, the 604 just falls away. Why? Less of the wing has laminar flow (more of it does not), and the wing is a LOT thicker (so the wake is bigger, too). So, it presents a much larger frontal area to the air.

Now, we will step to Nimbus 3/4 and ASW22 to JS1. With the Nimbus 4 or ASW22 at 850KG, and the span at 26.5 or greater, the span loading is much lower on the bigger wing ships, so they should have an advantage in climb. They are also at a lower wing loading, so in theory, they can fly slower, turn tighter, etc.

Roll out of he thermal and go running. Not only is the JS1 at a MUCH higher wing loading, it has 40-50 square feet less wing area. So, MUCH less wetted area that is not laminar even if they have the same percentage of laminar flow. And, the wing is a little thinner, so the run is likely much better at speed on the shorter wing ship.

I can only guess that the EB 29 goes well because it has low span loading for climb, and high wing loading for cruise. Sort of like Concordia. I watched it go by my Nimbus 3 about as fast and with as much better glide angle as the Nimbus 3 had over my 604.

The days of just more span for more overall performance are gone. Span leads to area which leads to reduced high speed performance, even if you get the wing loading up. Low speed, best L/D, you can get almost as much with less span and more aspect ratio. Each generation of airfoil sections gets more laminar flow. And can get it at lower Reynolds Numbers. So, aspect ratio can be boosted again. 604 was high aspect ratio at 28.6. Nimbus 1 was amazing at 31. JS1 and Quintus seem almost "low" at 36. My thoughts from several years back were 22 meter span and 40 aspect ratio.

So, what would I like as my stepup up from my Nimbus 3? JS1C, Quintus, and Antares 23E sure look good to me... JS1C probably at the top of my list because for the added cost of the engine in the others, I can buy a whole lot of tows! But, I think there is another thread somewhere that talks about the merits of self launch...

Steve Leonard

On Monday, February 16, 2015 at 8:10:08 PM UTC-7, Steve Leonard wrote:
A few observations from one who flies a Nimbus 3 now and previously raced a 604.

Climb is dictated more by span squared loading. 604 and Nimbus 3 both at 750 KG, both at about 9.2 psf, Nimbus 3 has slightly lower sink rate (from the polars) but noticeably better climb. Based on my experiences with 22 meter 604 and 22.9 plus winglets Nimbus 3 and 25.5 meter Nimbus 3. Why? More span and aspect ratio.

Run is dictated most by percent of total area getting laminar flow (or maybe amount of surface NOT getting laminar flow) and close second by wing loading. Again with my 604 and Nimbus 3 comparison. Same weight, roughly same wing loading, but at 80 knots, the 604 just falls away. Why? Less of the wing has laminar flow (more of it does not), and the wing is a LOT thicker (so the wake is bigger, too). So, it presents a much larger frontal area to the air.

Now, we will step to Nimbus 3/4 and ASW22 to JS1. With the Nimbus 4 or ASW22 at 850KG, and the span at 26.5 or greater, the span loading is much lower on the bigger wing ships, so they should have an advantage in climb. They are also at a lower wing loading, so in theory, they can fly slower, turn tighter, etc.

Roll out of he thermal and go running. Not only is the JS1 at a MUCH higher wing loading, it has 40-50 square feet less wing area. So, MUCH less wetted area that is not laminar even if they have the same percentage of laminar flow. And, the wing is a little thinner, so the run is likely much better at speed on the shorter wing ship.

I can only guess that the EB 29 goes well because it has low span loading for climb, and high wing loading for cruise. Sort of like Concordia. I watched it go by my Nimbus 3 about as fast and with as much better glide angle as the Nimbus 3 had over my 604.

The days of just more span for more overall performance are gone. Span leads to area which leads to reduced high speed performance, even if you get the wing loading up. Low speed, best L/D, you can get almost as much with less span and more aspect ratio. Each generation of airfoil sections gets more laminar flow. And can get it at lower Reynolds Numbers. So, aspect ratio can be boosted again. 604 was high aspect ratio at 28.6. Nimbus 1 was amazing at 31. JS1 and Quintus seem almost "low" at 36. My thoughts from several years back were 22 meter span and 40 aspect ratio.

So, what would I like as my stepup up from my Nimbus 3? JS1C, Quintus, and Antares 23E sure look good to me... JS1C probably at the top of my list because for the added cost of the engine in the others, I can buy a whole lot of tows! But, I think there is another thread somewhere that talks about the merits of self launch...

Steve Leonard

Paul MacCready once told me the essence of a racing sailplane was a glider with the lowest span loading and the highest wetted area loading shaped to attain the greatest % of laminar flow.