Robertson STOL = Slow Me Down

[Chris Kuyoth]

Scott,

I have some time in a turbo-normalized 400 with the Robertson kit. While I do not have any time in a regular 400, based upon my Killough operating manual, the 400 with the Robertson is quite a bit slower than a regular 400. Estimating as much as 10-12 knots. That is largly off-set bu its ability to get up high in the thin air with the turbos, but down low, my 260 is not much slower.

Don't know how much speed loss is contributed by the Robertson kit because the turbo mod adds "gills" to the cowl for added drag and some would argue that without the turbos spooled up, they rob the engine of HP by creating exhaust back-pressure. Don't know enough about aftermarket turbo-normalizing to comment. I can tell you that airplane tends to ride nose high, as if it wants to "plane-out" but can't, especially when heavy. Lastly, if you think a regular Comanche floats, look out with the Robertson STOL.

Not trashing the mod or the airplane. There are a lot of great benefits to the STOL but like everything else in airplanes, nothing is free and there are trade-offs. I wonder how VG's compare to the Robertson mod?

I would imagine that removing the mod would require re-skinning the leading edges of both wings.

Chris

[Zach Grant L1011jock]

Yes, the Robertson modified birds are slower. Depending on how straight the install, it could be 5 kts slower. Honestly, the Twins aren't that much faster than the singles if you really look at it, even in the books. As for believing ASI's for speed claims...hell half of them are indicating 30 kts sitting on the ramp! There are only two Comanches that I know are indicating the correct speeds, and one I'm not too sure of...if you get my drift. GPS ground speed 3 way or two way after finding the wind are the only real ways to verify your air data. Going for a flight and drag racing with another aircraft is valid for bragging rights but it doesn't verify anything as far as instrumentation accuracy. It will give you an idea where you stack up speed wise.

-Zach

PS.- Robertsons carry LOTS of ice...so I have heard, and Kris, I'm with you...never had a problem with tail ice issues in anything with a stab, but it sure did get ugly on occasion. I believe the constantly changing surface angle without changing the effective AOA of the stab by use of an elevator probably causes the stabilators to be more forgiving than conventional tails.

[Scott Ducey]

Zach - but I think my airplane is 10-12 knots slower than it should be. I talked to Dave Fitzgerald (who also has the same mod), and his conclusion was the same. This mod was put on in 1972. Thereafter, the owners threw on every speed mod that is out there, i suspect to offset the Robertson STOL drag.

Steve - maybe we can try your test down in the Bahamas this week.

Regards,

Scott Ducey

[Zach Grant L1011jock]

Scott,
I have never had a Robertson normally aspirated twin, but mine like Daves is a Robertson Turbo. The turbos are probably 8-10 mph slower than book down low, but get to book speed about 10-11K and if the turbos are good, beat book above that. Jay has a turbo Robertson also and sees similar numbers. The only other non turbod Robertson I have been around is Al Powers, and he is at most 3-5 mph slower, but has regained the speed lost with speed mods. I guess the question is what is your wind corrected ground speed, and how does that compare with book. Comparing IAS to book is a wasted effort.
-Zach

[James Oates]

I've been doing a lot of speed study lately, and with a 260C with the robertson STOL, I can't see more than between 1-2 knot loss over book across all power settings. Some power settings (around the 24 square space) appear to be right at book.

[MULEFLY]

James... I agree with you.. I think that there is a 2x2 matrix here that applies... you have No Roberston/Roberston on one axis... You have poorly rigged airplane/well rigged airplane on the other axis. I firmly believe that a well rigged, Robertson equipped airplane will be a poorly riggled, non-Roberston anyday of the week.

Overall is the Roberston a negative?... most certainly... but after that... I know I'm faster (not just indicated airspeed indicated -- GPS tests too) than many of my peers that have well maintained aircraft without Roberston... but they may still have brakes hanging in the wind etc.

All the best!
Jim