Posted Thursday, February 24, 2011 @ 01:00 PM
Hi Ray,
I am very interested in the subject of achieving a TE compensated vario without resorting to running another tube back through the fuselage and up the fin myself. I am in the process of getting a Vivat back in the air and it does not have a TE probe. Neither does the Milan.
There is a difference between a vario requiring a volume bottle and a vario requiring a TE compensated signal. If you look at the installation manual for the Tasmin vario is says to hook it up to a TE probe but if you don’t have one hook it up to a static port. I am very curious as to how the Tasmin compensates for airspeed if it is only hooked to a static port (i.e. it only sees changes in potential energy) It has no way of “knowing” what the airspeed is at any given time. A TE probe does an analog calculation to add the potential energy and the kinetic energy together and give a corrected static signal to the vario. I also can’t imagine how the Tasmin decides whether the signal it is seeing is compensated or not (some electronically compensated varios require something in the set up menu to tell the vario how it is configured but the Tasmin manual does not provide this option that I can find). Internally compensated vario's have a connection to both the static port and the pitot in order to do the compensation internally.
I have spent many hours trawling the internet on this very issue. I think that on a Milan / RF4D fuselage installing a Dick Johnson / Oran Nicks mid fuselage TE probe is quite easy. It would even be easier if you move the radio antenna to an internal dipole type and install the TE probe in the old antenna hole aft of the baggage compartment. From what I can read the TE probe should be at least 1 meter from the trailing edge to get the probe far enough away from the distortions to the flow lines caused by the wing. I would think that the depth of the fuselage plus the distance back behind the wing would be good on a Fournier. This is my plan for solving the problem on the Milan.
On the Vivat it is a much bigger problem. The aft fuselage is a monocoque structure and so putting holes in it is not a good idea. The wing is also mid fuselage mounted and it has flaps which will distort the flow lines more than an unflapped wing. I came to the conclusion that a TE probe in the fiberglass non-structural cover over the wing center section was just too close to the wing. I looked at what would be involved in trying to get another tube into the aft fuselage but even if I bribed a small child to slither down the tail cone I could not be certain that the tube could be secured so that there was no way it could come adrift in turbulence and get tangled in the control cables.
Another location I have seen for a TE probe (on another Vivat) is behind the firewall ahead of the windshield. It doesn’t do much for you when the fan is on but it will work well when it is off and arranging the plumbing is really easy. I could see a similar approach working well on a Fournier too.
My research through old Soaring magazines (now available in an online archive to members) revealed a lot of work on this issue back in the 1960’s and 70’s. I guess that the top contest pilots of the day were figuring out how important a TE compensated vario is to improving contest standings but many of them were stuck with gliders that were not plumbed for TE probes at the time they were built – like all modern gliders are. Wil Schuemann developed what is probably the best mechanical TE compensation system using bellows and diaphragms to take a Pitot signal and a Static signal and combine them to provide a TE compensated signal that could be fed to a mechanical vario. The August 1961 issue even had DIY plans on how to roll your own using, what were freely available at the time, Link trainer bellows. Also jumping in to filling this gap in the market were many of the early electronic varios. Most newer ones require a TE compensated input because it costs a lot of money to incorporate all the extra diaphragms and so on to do the compensation calculation. In the end I managed to buy a used Westerboer VW910 vario with audio and a VW921 glide computer for $500 from Wings and Wheels to solve the problem in the Vivat. I figured that it was cheaper than trying to install a TE Probe and using a Tasmin vario or similar. In addition, I have another box with knobs and a screen to fiddle with with more capability at no additional cost! It is also heaps easier to install because right behind the vario in the panel I tee into the pitot and static lines and provide both signals to the vario.
Best regards,
Dave
