I play outside with my group several times per year, and busk as well. If your embouchure is good, your flute carries much better than you, the player, thinks. There have been several times at our renfest where I couldn’t hear myself at all but 20 rows back all the audience can hear is me on whichever flute I have and the whistle.
Just sayin’…
Ah, the good old Paddy Keenan method! 
" I need to play several times outdoor with a loud uilleann pipe and sometimes with a bagpipe too. That’s why loud is necessary for me, and I think for all buskers in the world that don’t use amplification." Timberflute22
Well try and get the uilleann piper to close his reed up a little so you are more evenly balanced volume wise. Better still improve your embouchure and breathing technique. If the “other bagpipe” is a Scottish Highland pipe, or gaida, then there’s no hope for ye, unless you try a small battery driven guitar amp and a contact microphone. I’ve busked with an acoustic guitar so you don’t need to be loud, if you’re any good people will listen and some will give you money
Another distinction to consider is whether the flute is loud to the player or the audience. Some flutes give a big sound under the ear, so the player thinks she’s shaking the rafters, but the flute isn’t projecting very well. Projection doesn’t get talked about a lot in these circles, but it’s a big consideration for classical flute players: can the back row in a big hall hear you? Like many things, projection is partly the flute’s job, with the burden of proof resting on the player. And it is challenging outside! (Wear a hat, put a wall behind you–anything for the sound to reflect against.)
Thanks for the answers.
I play with a Hammy and I feel well with bagpipes. Specially with a sound pushed at his maximum limit.
However I’m curious about Grinter references, because I’ve heard that it has a tone similar to a Pratten (robust, warm and full sound) despite it’s a kind of Rudall.
I’ll try with an hat 
Ps: why you can say that Hammy’s one “cuts” between other instruments not due for its volume?
Thank you
I’ve heard a lot of Grinters being played here in sessions and
they’re not terribly loud — except when Peter Woodley is behind the wheel!
I guess that the particular balance of fundamental and harmonics can stand out.
I’ve heard a lot of Grinters being played here in sessions and
they’re not terribly loud — except when Peter Woodley is behind the wheel!
I had a lesson with Kevin Crawford a few weeks ago. When he played his Grinter he produced a very powerful (and beautiful) sound. I’m convinced that the player was the more significant factor than the specific make or model of flute being played though.
Agree. I don’t find Grinters/Kev particularily loud.. Nice full sound, oh yes, but not (overly) loud.
Again, the loudest i’ve heard was Hernon, Murray and Hammy (flutes, not players ) but that may well be the players as much as the flutes.
Agree with Larry that the loudest flute player I’ve ever heard is Marcus Hernon-easily heard at an Irish music week session with 30 people. So much air at such high pressure through such a small lip opening…
Hugh
It is exhausting to play that way and it means you have
to practice playing that way all the time to stay in shape for it.
It’s just not fun; it’s why I’ve taken a flute-cation the last
few months and sold my Olwell.
The loudest flute I’ve played is a Casey Burns Large-Hole model. I could blow super hard in that flute and create a very loud tone.
That said, I play small-holed flutes now and have no problem being heard or projecting a loud enough tone. It really is the player.
It really is.
I have my stunt double play all the loud bits for me.
I might be able to fit the order? I make the Elaphone.. a flared fipple flute, that I believe to have the advantage when it comes to volume. It is actually the characteristic that I believe to be worth the fact that this design does sacrifice some of the second octave. The flared tube works to direct the low note frequencies, in the same way that a didgeridoo does.
Yesterday I had my display at a flute fair where I was the only wooden flute maker, in a room full of silver flutes and piccolos, and I was able to draw eyes from the other side of the room with a couple of my flutes. This being in a room full of discordant scale runs and overtone sampling, almost like a white noise of fluty cartoon sound effects..
http://www.etsy.com/listing/172186552/new-a-flat-minor-elaphone-woodwind?ref=shop_home_active_2
I have a G minor from this same batch that I think you would like more, because it’s even a little louder.
probably my loudest flute?
What key are you looking for?
I’ll break you a good deal if your willing to come back here and give me an honest review?
I’m still trying to get my name out there…
corylittle.com
phone works best.
There are many loud things you can try, now that door to random suggestions has been opened:
This is a thread about transverse Irish flutes, if you couldn’t tell 
.
Sean Gavin has a real foghorn of a flute, a Grinter I think, but man, he can blow all the windows out of a room when he wants to. We didn’t see much of that though; he’s much quieter playing with others.
Truth is, if you play too loud at sessions or with other folks in general, it’s obnoxious and usually unmusical.