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Jun 30, 2005, 09:24 AM
VTPR & Slope Aerobatics
surfimp's Avatar
I'm so glad you guys are taking the time, thank you! We will all benefit as a result. Very, very cool!

Steve
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Jun 30, 2005, 11:02 AM
Long to be flyin'
Antonsoarer's Avatar
Quote:
Originally Posted by Peter Wick
Hej Tony

I agree totally. I just wanted to state, that it is easy to do some airfoils that are looking a little bit better than those I posted, but there is to things to remember
- you have to built these airfoils, so fx 100% laminar flow on the underside of the airfoil at low angels of attack looks certainly good on the comp. in fx Eppler, but can you build it on a foamie....and for how long?
well, you might say...ok, so the transition is earlier, but what the heck?
I mean actually if you are going with an airfoil and consider only 80% laminar flow on the underside, you actually get some design possibilities on the upper side and in the end can come up with a better airfoil i reality.....but the computer doesn`t know.
- the other thing is to use the airfoil as their designer thougt they should be used. That is especially important when you trying to make a flying wing flying, because every thing is connected together: handling, performance, structure and so one.

yours Peter
Peter, whoa! I certainly don’t think the GS20 is in any way better than any PW foil (just different in some areas, context really matters when things get subjective) and I agree such microscopic differences evaporate during a build, that’s why I made the comment: ‘But once again only a small difference during a build could change everything.’

I agree with your observations and understand your design considerations.

I noted your bottom hinge comment in the earlier post, good info for maintaining laminar or attached turbulent where it may already exist at low positive alpha.

And now for something completely opposite….
A few builders have asked why I put such large control surfaces on many of my models; I think I get to ‘trip the flow’ (hey! that suddenly sounds like a rip-the-bringism to me) on top with the hinge-line at about 60-65% (supposed to be the zone for maximum benefits on faster flying designs), seems to help at high alpha when floating about. The theory is that it should give lower drag and improve control surface effectiveness at low speeds, field tests with small control surfaces and turbulaors at that position seem to confirm this with some foils. However I can’t get xfoil to see turbulators to any discernable level but they appear to work in reality, yet another discrepancy between the computer and reality.

That’s two approaches, keeping it smooth on the bottom and tripping the flow at 60-65% on top, I did both on the model pictured below (bottom-hinged and effectively top-turbulated). The foil looks bad on Cl-max and most wouldn’t choose to use it but it is designed to be very low drag, as a result it won’t bang a hard turn but for such a light model it has a very wide range of flying speeds and will float well also. When I cover the hinge gap on top with tape the float performance and low speed control degrades but it doesn't seem any quicker (need a radar-gun to be sure).

We have a lot to thank Stefano and Drela for; anyone can now relate subjective performance back to a consistent reference. Even if real world experiences don’t always manifest themselves in xfoil, that’s still good data.

I think we are going to see a whole new generation of high performance planks on the slopes as the PW51 has enough pedigree for home builders to be confident about trying it out.

Tony.
Last edited by Antonsoarer; Jun 30, 2005 at 11:38 AM.
Jun 30, 2005, 06:33 PM
Registered User
Thread OP
well

if I would do a mouldie plank, there will be some improvments in the design of plank airfoils....thickness, laminar flow on the underside and so one. These will not be big steps, but....
The underside of plank airfoils is a critical task, because:
at low angels of attack, especially at low re numbers, the air can not follow the shape of the airfoil at the rear, if you are using a very pronounced reflex. Look at the newer HQS foils, fx the HQS 1.5/9. The difference in drag compared with a PW 51 is huge and so is also the difference in pitching moment, the HQS is very unstable in this configuration. Well you might say, you don`t fly low cl`s and low re......I do not agree, because
You fly straight and slow and wont to dive to ge´t some speed...you push the flaps down, the re is still low and the cl too! In this moment the pitching moment gets negativ and is contributing to lower the cl even further, actually trying to go inverted, so the pilot is forced to put ind some elevator to keep the dive straight....now the re raises and the pitching moment gets somewhat more positiv or less negativ and again imatches the flap setting not the situation the pilot wants the plank to fly ind...so he had to correct again. Given the sensitivnes og planks....you are not going to have a proper handling and you are going to use a higher static margin......and thereby loosing all the bedst of planks.
So look at the underside of plank airfoils or use trips.
And there is some more connected to building and airfoil design:
What may work as an airfoil for a foamie might not be that good for a mouldie. The rougher surface of a foamie might work like a trip causing the transition to happen on the underside early enough to make the reflex work, but the same airfoil on a foamie might see very long laminar flow on the underside and when transition sets ind, it might be so late, that there is no reattachment....causing all the troubles.
I just wanted ti illustrate, how much things are connected and how important it is to consider, building possibilities, and the generel design.
Jun 30, 2005, 09:04 PM
EPP Rules!
aussie's Avatar
Quote:
Originally Posted by Peter Wick
first, Xfoil isn`t the truth. Just compare Selig`s windtunnel results with Xfoil.
That's something I've been planning on doing for a while now and hadn't got around to it... It's always interesting to see how closely computer predictions match "reality". Although they may be closer to reality, wind tunnel tests still differ between wind tunnels and/or tests as well, so I guess there's no substitute for actually flying different designs and trying to accurately build and measure/experience the differences (hard to do though).

Although not always perfect, I think Profili/XFOIL is an incredibly valuable piece of software and helps narrow the field of likely candidates for a particular application enormously.

Thanks very much Peter for the insight into your design approach and philosophy. It's great to hear your airfoils have been working very well in real-life applications too!

I find the top hinge versus bottom hinge issue interesting as well. Do you (and others) feel the bottom surface flow is more important for all airfoils, just reflexed ones or just specifically these PW ones? The top/bottom hinge experiement (and compression hinges) is another thing I've been meaning to do for some time and trying to "scientifically" see if there are noticeable differences in flight performance.

PS. Thanks for sharing your airfoil too Tony!

Kye.
Jun 30, 2005, 09:34 PM
Sand it round!
DennisRB's Avatar
Peter, nice foils you have there

If I wanted to make a fast single fin combat wing with a fairly wide sweep (less sweep than usual, similar to a halfpipe) What one of your foils should I use? Could I use the 51? The aim is to have a fast wing that is still strong for combat, with decent recovery. (with speed being more imortant that recovery).

Thanks.
Last edited by DennisRB; Jun 30, 2005 at 09:45 PM.
Jul 01, 2005, 01:34 AM
Registered User
Thread OP
Hej Kye and Dennis

@Kye: I agree totally. About top hinge...I think almost every airfoil will have a profit form bottom hinge, but it is certainly important for flying wing airfoils, because if your are flying with higher angels of attack to produce som lift, the acceleration of the airflow on the underside of the airfoil is raising and thereby extend the laminar flow to 100%, so why trip it with a top hinge before?
I am not sure wether you will be able to recognize it on an EPP plane, maybe only when it is very clean and new. I did`nt made ant comparisons, so I don´t know.
@Dennis: The plane and flying situation will fit ver good together with the PW51 or the GS20. So, yes use that one and you won`t be dissapointed.

Yours Peter
Jul 01, 2005, 02:48 AM
Registered User
Florrain's Avatar
Hi,

Thank you Peter and Tony for sharing your work with us.

@ Peter, I'm not sure I understand very well what the hinge theory is so correct me if I'm wrong.

You mean that the flow on the underside of the foil if laminar for to long could not be able to follow the reflex of the trailing edge and thus seperate from the foil causing stability problems and drag?
Do you suggest to turbulate the flow to keep it attached with an adequate hinge position.
Do you mean that the kink caused by the hinge causes the tripping or the gap on the other side causes it? Or both?
Or do you seal the Gap and consider the kink of the hinge acts as a turbulator?

Or do you mean tripping the upperside with an unsealed gap rather then the underside that usually stays laminar 100%.

I'm a bit confused...
Jul 01, 2005, 02:58 AM
Registered User
Florrain's Avatar
I'm thinking about building a hollow molded 60inch plank.
I have not decided yet if it's going to be Bluto style or with a fuselage but I'm trying to decide on a section at the moment.

On post 33 Peter discusses the suitability of foils for foamy or glassjobs.

Before this post I intended to use the French AZ (Alain Zutter) 9%-1.4% cmo=0.015 airfoil but now I consider the PW51 since it has a very similar polar but with a more stable CM curve (according to X-foil).

@PW: do you have airfoils that you consider more suited to moldies? Is the PW51 suitable? Do you suggest to trip it fi used on a moldie?
Jul 01, 2005, 10:52 AM
Registered User
Thread OP
Ok I am trying to answer your questions:

"You mean that the flow on the underside of the foil if laminar for to long could not be able to follow the reflex of the trailing edge and thus seperate from the foil causing stability problems and drag?"

Yes, at very low angels of attack and for airfoils with "big" reflex on the underside to compensate a "difficult" upper side. fx HOS1,5/9 or the EH 2.0/9 and so on.....this will be and it is, I have seen it, especially a problem on mouldies, because they will allow the air to be laminar that long....here a trip could be a solution.....or better say correction of a failure.

"Do you suggest to turbulate the flow to keep it attached with an adequate hinge position."

Here I don`t really understand your question....


"Do you mean that the kink caused by the hinge causes the tripping or the gap on the other side causes it? Or both? "

Yes I think so, if fx the hinge is on the upper side and the gap on the underside....so laminar flow at low angels of attack will probably not reach 100%

"Or do you seal the Gap and consider the kink of the hinge acts as a turbulator?"

Best will be to make a very good hinge on the underside and hoping for that the laminar flow will get over it....and it will I guess, at least at high angels of attack because of the higher acceleration of the air on the underside. Upper side is anyway turbulent in this region....so make the gap there.
Hope this will help.

The PW51 is certainly ok and proven on mouldies as well. But I yes, I have some airfoils which look a little bit better for a mouldie, but they are thinner and more difficult to construct.....and most of all, I havn`t tried them. The gain in performance compared to a PW51 is anyway surprisingly low.

Yours Peter
Jul 01, 2005, 11:29 AM
Free as a bird now.
I'd like to see some pictures of planes built with the PW foils.
Jul 03, 2005, 08:29 PM
Sand it round!
DennisRB's Avatar
Me too Bring* on the pics.
Jul 04, 2005, 12:14 AM
VTPR & Slope Aerobatics
surfimp's Avatar
Quote:
Originally Posted by Brian Courtice
I'd like to see some pictures of planes built with the PW foils.
Dig it with a shovel, yo:

http://www.epp-fun.de/frame.htm?/brett-nf.htm

The Frettchen is an AWESOME plane. Floats like a Weasel, rips as well or better than a Moth.

Steve
Jul 04, 2005, 09:37 AM
Sand it round!
DennisRB's Avatar
Some cool planes on that site.
Aug 05, 2005, 02:44 AM
Registered User
h@bib's Avatar

planes built with the PW foils...............


i've built a couple of planks with the PW75

Procoon Pics (German)

Procoon plan (German)

small brother of the Rocoon => the Racoon (german)

best regards from austria

[email protected]
Feb 28, 2006, 02:47 PM
Flags blow'n, I'm go'n
My Precioussss!'s Avatar
How will the PW51 foil work as a DS'er. I was thinking of doing a composite 60" JW type of plane. Any comments!
Todd


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