SEN 1845
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- Category: Archive 2014
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- Turbulators
- North American Cup - with minor corrections
- In the Press
Turbulators
Roger,
Back from vacation, I just read the thread on turbulators in SEN 1835-36-37. Some comments from my side may, even after 3 weeks, still be of interest.
The triangles! Jim Parker "So what are my triangle objects doing???". Will try to give an answer. A lateral row of adjacent triangles, called a HAMA-turbulator (Francis R. Hama was the first to investigate such structures in the 1950's at NASA Langleigh), form a 3D-turbulator, i.e. a lateral row of closely spaced vortex-generators (as do the cylindrical pins, or the semispherical bumps turbulators). The purpose is, as with all turbulators, to trigger, at the right place, the boundary layer (BL) to change from laminar to turbulent state, which has a much better chance to survive the adverse (growing) pressure gradients downstream before seperating. And this without the large drag of the laminar bubble, occuring with free BL-transition and with forced transition via a solid 2D-trip. An optimized Hama-turbulator for an F1-wing may look like this:
leading edge _______________
V-notches VVVVVVVVVVVV
trailing edge _______________
There are no triangles glued onto the surface, the V-shaped notches are cut into the surface. This was common in the last decades of the past century, with partially great success, on solid balsa wings in the F1A-B-H classes (but cutting 240 tiny notches accurately into a 2,2 meters wing is somewhat frustrating, I can tell). It should work, in simple terms, as follows:
The laminar flow into the notch is guided by the walls towards the center, thus increasing in velocity and reducing in pressure (like in a jet). On each wall, laminar fluid is sucked from the surface, and combines with the flow in the notch, and a vortex-filament is created along each wall (these are btw NOT Karman vortex streets, as generated e.g. by a pin turbulator!). The 2 filaments merge downstream at the corner of the notch and leave it there, with additional accelleration and high kinetic energy/momentum, and will trigger, at some distance further downstream, the transition of the BL. With a typical glider airfoil at very low Reynolds number (RN), this has been made visible during experiments in the water channel at Stuttgart University, already in the 1960's.
Now, with a standard D-Box from an eastern shop, cutting notches into the profile may give some serious problems :-). So one may glue a row of adjacent triangles onto the surface. In prinziple this can work with good results, but it is less efficient. The downstream lateral edges of the triangles add drag. In addition, this edges reduce the amount of laminar BL on the surface ("cutting-off " healthy downstream laminar wedges), increasing drag too.
In general, the cylindrical pin-turbulator is imho better than the Hama-turbulator. Besides that it is basically more efficient, it's also the behaviour at high speed. The Hama-turbulator is always fully active (= always producing full drag), also at high-speed flight. The pin turbulator behaves differently. If the pins Reynolds number (RN) exceeds 400 (flight speed > 9 m/s), the spreading Karman vortex street after the pin will completely dissapear, being replaced by a narrow, irregular turbulent wake. The Cd of the pins stays about the same. But less of the wings surface will be wetted now with turbulent BL, and the area wetted with laminar BL will increase accordingly, thus the wing's Cd should decrease. The pin turbulator may act as a "flex" turbulator - being fully active during glide, but automatically "hiding", to a certain degree, during high-speed flight.
Dimensioning of pins. Apart from simple calculations (as shown in my paper in the 2004 Sympo), the pin dimensions can be calculated more accurate with the help of XFoil. RN of a pin should be in the range 100 < RN < 250, producing stable Karman vortex streets, in the "dynamic" glide-peed range of the particular model. OPER > BL (or BLC) plots the local BL velocity profile and thickness (Delta). With this, the optimum pin-diameter and -height can easily be calculated.
To find the optimum chordwise position for a "real" turbulator, XFoil is imho not really suited. Yes, there are possibilities to "fool" the e^n-algorithm (which XFoil uses to predict free transition), by setting the Nkrit parameter (which mimicks the incoming flows turbulence-factor) to a very low value, e.g. 1,0. The predicted transition will then happen near the predicted begin of linear instability, which is therefor bypassed, like a real turbulator would do. But the results are speculative, biased by the arbitrary choosen values for Nkrit. Black/red magic ;-). Btw, XFoil cannot cope with the inherent drag af a real turbulator...so polars may turn-out somewhat optimistic!
Lateral spacing of the pins. With conventional airfoils, and a basically smooth(!) surface, efficient(!) turbulation needs a spacing < 10 mm. With LDAs the spacing can probably be increased up to 15 mm (the flows pressure-minimum is normally further downstream, compared to conventional airfoils). This increases the area of the laminar wedges after the pins, hence reduces drag. Experiments with an F3K-wing (Drela airfoil) have confirmed this.
Rudolf Hobinger
The Toronto Free Flight Group
Presents the 2014
North American Cup
A World and America's Cup and Team FF Competition
Dates:Tuesday, February 11th 2014 with Wednesday February 12th as reserve day
Place: Bissonette Mirage Field, Lost Hills, CA USA
Events: Individual F1A, F1B, F1C, & 3 Person Team F1A, B & C (Teams can be mixed nationalities)
Entry Fees: $25.00 for F1A, F1B, F1C – Team entry included in fee
Juniors free all events!
AMA liaison CD:Brian Van Nest
Co-Organizer: Peter Allnutt: 527 Philippine St, Taft CA USA (661) 763-5039
Organizer: Tony Mathews: 2 Tralee Street, Brampton, Ontario, Canada L6Z 2X4
cell: +1(705) 754-5553 E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.This email address is being protected from spambots. You need JavaScript enabled to view it.This email address is being protected from spambots. You need JavaScript enabled to view it..This email address is being protected from spambots. You need JavaScript enabled to view it.
Tuesday February 11th
F1A/B/C Round 1 8:00 am – 9:00 am Rnd 1: 210 sec max F1A, 240 sec max F1B/C
Round 2 9:00 am – 10:00 am
Round 3 10:00 am – 11:00 pm 180 sec all other rounds
Round 4 11:00 am – 12:00 pm
Round 5 12:00 pm – 1:00 pm
Round 6 1:00 pm – 2:00 pm
Round 7 2:00 pm – 3:00 pm
F1A/B/C Fly-offs3:30 pm – asap (weather permitting)
Trophies and medals for 1st, 2nd and 3rd place F1A, B&C as well as medals for 1st, 2nd and 3rd place Teams.
Medals for top Junior in each event also! Champagne and snacks will be available Tuesday evening. Final awards ceremony will be after the final flyoffs.
(Sunrise for Feb 11th – 6:50 am Sunset – 5:36 pm)
Deviations from the FAI sporting code and special instructions:
Competitors MUST provide their own timekeepers. Two timekeepers per competitor are required for the flyoffs. Fly-off times will be decided on the day depending on the weather.
Team scores will be decided by the total time of the 3 contestants including flyoff times (ie: not as per the sporting code).
Note: A Starting “area” will be utilized without pole positions (depending on entry numbers).
Please e-mail advance entries to Tony Mathews This email address is being protected from spambots. You need JavaScript enabled to view it. prior to Feb 09th. Entry fees to be paid on the field by Feb 10th (someone will be available to take entries on the field on Monday Feb 10th from 12:00 pm to 4:00 pm) please do not send money.
In the Press
Feb Ffn has a review of proposed CIAM/FAI rules changes. Most are in the electric space with several to correct the anomolous situation created by the German proposal that imposed un-necessary architetcure requirements on the F1Q rules. For the mini event classes there is a proposal to introduce a new event, F1S, that is based on the popular E-36 event that was created by the NFFS and now adopted by the AMA and in other countries too. The proposed F1S rules probably still need some work with obvious editing errors.
...........
Roger Morrell