Thorp Air Command - T18.net

Guys,

I was looking at the specs on a number of aircraft and noticed that there is quite a range when comparing the maneuvering speeds versus the cruise speed for varous aircraft powered by the same engines. I was looking in particular planes with the same 160 hp engines. I noticed that in the case of some of the faster homebuilt planes with 160 hp engines, they all cruise about 160 mph at 65% power. The maneuvering speeds are quite different. See below:

RV9 - 160 mph cruise, 118 mph Va
RV6A - 160 mph cruise, 135 mph Va
Mustang II - 160 mph cruise, 140 Va
RV8 - 160 mph cruise, 142 mph Va
Thorp T18 - 160 mph cruise, 165 Va

For certified aircraft:
C172 - 120 mph cruise, 112 Va
Piper 161 - 120 cruise, 130 Va
C152 - 115 mph cruise, 110 mph

Interestly enough the Thorp T18 and Piper had design inputs from the same designer, John Thorp. I was surprised that the RV series of aircraft, being a popular as, they are have such a difference. In rough air you really have to slow down. The certified planes don't have much of a speed difference between cruise and maneuvering speeds.

Does anyone know if there have been issues with the RV series in rough air? I don't fly an RV. Frequently when I am in turbulance where I am using large control inputs the procedure is to just tighten the seat belt and throttle back a bit much like I used to do in the Stinson and C172.

Any one want to speak up as to what the situation is with the RV's?

Jim

Yes the RV 's are low compared to the Thorp. I think most RV owners are not aware or chose to ignore the limitation rather than admit the Thorp is superior in this area. In New Zealand the Special cat airworthiness cert issue to A/B aircraft has an additional page of limitations that acts as a poor man's flight manual. The data is taken from manufacturer information or if that is not available from the test flight report required in NZ. If you do not demonstrate the aircraft is capable of operations to the design limitations then the limitations page will limit operations to what was actually achieved/demonstrated .
Tony Schischka
ZK-VMS
T18C

Wish we could ask the airline pilot of the RV8 that had a wing separate in St George last month.
Cubes

As I understand it, maneuvering speed is set so that the wing stalls prior to structural damage to the WING. The idea being that if you yank hard enough the load factor will build and the wing will stall and therefore unload itself prior to be bending at or below that speed.

It says nothing about damage to another structural part such as the tail.

At least that is what I was taught.

Maneuvering speed is determined and set by the designer. The wing is the main design factor for stall speed, but the loads on an aircraft structure are variable and complex.
Both weight and bank angle, for example, directly affect the safe "maneuvering speed". It is my belief that the designer takes much more than the wing only to determine maneuvering speed. As I said, it is complex, and certainly other parts of an aircraft can be vulnerable to damage and failure at speeds below the chosen maneuvering speed, depending on flight conditions and the application of control input. The failure of the vertical tail components on an airliner due to abrupt rudder movement from stop to stop, while flying below the maneuvering speed is a classic example of this. I think that perhaps some designers choose to be more conservative when specifying maneuvering speed. Maybe especially in today's legal climate. That's just my guess... could be wrong.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Maneuvering speed is the square root of the max g limit times the clean stall speed. So if you have two planes that cruise at 160 and both have a limit load of 6 g's, the one with the lower stall speed will have the lower maneuvering speed. The lower stall speed allows the wing to pull g's down to a lower speed so the maneuvering speed is also lower, even though both planes are rated for 6 g's.

sq root of 6 = 2.449
low stall plane = 58
high stall plane = 68
cruise speed both planes = 160

58 x 2.449 = 142
68 x 2.449 = 166

So if you want your plane to have a really high maneuvering speed, keep clipping the wings until it stalls at 100!!

Bernie Fried
N18XS

Well said Bernie, a great explanation.

I second that - thanks Bernie!

I've seen maneuvering speed defined, incorrectly, as the speed below which it's "impossible" to damage the airplane with abrupt control movements. Just like flyingfool clearly stated, the wing will stall, but catastrophic damage can still potentially be done to the airplane. I knew there was a specific formula, but I had been led to believe that a designer had the leeway to assign a speed below that value to a certain degree. That's just one example of why I'm obviously not an aircraft designer. We all know that there is no "magic speed below which you cannot hurt this airplane".

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE