Could someone pleae tell me how much the Robertson STOL conversion affects a PA30’s cruise speed?

Thanks.

Marcus,

Several folks on this Forum know that answer but visit infrequently so it may be awhile.

Hello Pat,

Thank you, I'll just have to be patient!

Marcus.

I do not have a "before and after"... so I may not be able to give you an adequate answer. N8982Y is a PA39 (SN147) and has the Robertson STOL... the Miller nose, and almost every other mod that can be hung on a twinco... I do not have the Arapaho windshield, and I have the flap track covers but I have not installed them yet. Matt Kurke has my gear tuned so tight, that I almost grimice when the gear is retracted

Yesterday, my wife and I heavily loaded flew 6:23 minutes from Beloit, WI to Santa Fe, NM... we used 96 gals on the flight. Stopped for fuel in Garden City, KS and then had a routing of KGCK, DHT, FTI, KSAF... there were a couple of other minor turns. I file every flight at 165 kts and quite honestly, almost always get it. Yesterday on the second leg, we were at 10,000 ft ... and I was only seeing 190 MPH... the first leg was at 6,000 ft and it showed 195 mph the entire flight.

If I am alone and full fuel but nothing else (ok maybe 70# of sand ballast in the back)... and if I keep the altitude at 6,000 to 7,000 feet. On occassion I will see speeds as high as 203 MPH on about 15.5 gph. That ain't bad.

I know that I will never out run Hans or Jeff... but I do outrun a signficant number of the non-STOL equipped twicos that are in the fleet. Supposedly the PA39s (counter rotating) are slower. Could be, but I'm faster than many of the airplanes that are not counter rotating and do not have the STOL. BTW... airspeeds have been verified on a 3 leg GPS test flight.

I do not have tip tanks... and I love the 3,800 MGTOW that the Robertson allows... I have close to 1,300# useful load with a heavily modified airplane.
All the best!

Jim

BTW... Flight aware gives GROUND SPEED... head winds and tail winds will have obvious effects. But you can look at the two flights mentioned on flightaware.com N8982Y We had a headwind on the second leg... it is obvious in the ground tracks. And we had a bit of a tail wind on the first leg (at times too!)

Marcus, based on Jim's post and since I have a PA39 with lots of mods and 10 tanks(so its heavier than some) but not the STOL, I'd guess 5-10 mph delta.

Stock 39s are 1-2kts slower than stock 30s as flight tested when they came out of the factory by Marion Jayne, famed race pilot and my Mom. Piper promo lit said different but we were based at a Piper dealer distributor and she tested a lot of them.

Fast forward 40-50 years and any Twin can vary widely on speeds based on how well they've been maintained, rigged, mod-ed, age of engines etc.

Hope this helps a bit.

My Robertson conversion was either the second or third one converted by Robertson, and I got a special price of 3500 dollars for the job. My sense at the time was that I lost 2 to 3 mph at cruise.
I didn't worry about this loss of cruise speed because at the time there were lots of PA30s crashing due to training accidents when instructors were pulling single engine out practice at low altitudes.

I had experienced a 1/2 turn spin and recovery when I had a demonstration ride with a Piper dealer and asked for a VMC demonstration. I knew nothing about twins at the time and thought it was a normal situation. Later on, after I had bought the airplane, the training accidents and Piper raising the VMC from 80 to 90 mph, etc. raised my concerns about single engine controlability. Robertson marketed their kit emphasizing safety, and I took a demonstration ride. Their pilot let me fly and I went to altitude and with an engine out slowed to the point the airplane went over the top. I shoved the nose down and stood on opposite rudder. The airplane recovered nicely. He wanted to show me short field landings, etc., but I told him the demonstration was over, and I placed my order that day.

Later on, when my engines were due for overhaul, Lycoming gave me a counter rotated engine for no extra cost. This, in combination with the Robertson mod, results in 75 mph VMC and 63 mph dirty stall speed. I am told that counter rotating costs some speed, but I did not notice that when I changed engines. Subsequent to all this, I have installed the Lopresti nose bowls, spats, splitters, and Knots 2U stainless engine pans. These mods were all done at one time, so I can't speak to individual results, but I now feel I get handbook or better speeds and fuel burns at my usual cruising altitudes of 6 to 8000 feet. If I fly higher I get better speeds for the same fuel burn, but my usual routes don't justify the extra climb time.

As Mulefly notes, the Robertson mod adds 180# usefull load. I, too, have lots of mods, but still have over 1300 # usefull load available. I know I will never be racing with Hans and the Southern California guys, but I like the Robertson and C/R characteristics of my airiplane when I am using the 2500 foot grass strip that I have been landing on for 4 decades.

Al Powers ICS 2978 N88AP

Thank you, I really appreciate you all giving me an education in Robertson STOL.
Regards,

Marcus.

I think that with my Robertson I'm about 3 - 5 kts slower at low altitudes, about even at 12K and maybe a hair faster in the mid teens.

Jay