Vne of vintage aircraft

[old man emu]

How would you determine the Vne of a vintage plane?

 

I’ve got the originals of the 1932 & 33 “Flying and Glider Manual” in which the plans for the Pietenpol Air Camper and Sky Scout were published. Also the plans for the Gere Biplane, Corben Baby Ace are in these books. These planes were designed to use Ford Model A or Model T engines which produce about 40HP and were expected to have top speeds of about 65 to 70 Kts.

 

Eighty-odd years on and now I can get hold of a lightweight engine which will produce about 100HP for about the same weight penalty. With more power comes more speed, but I wonder how much more speed I can ask of the airframe before there becomes a Vne problem.

 

I’ve been googling all over the place, and have found some formulae with which I can calculate Vne. These seem to suggest that Vne is ultimately determined by the wing loading. Here is the sequence of calculations:

 

Wing Loading (WL) = Gross Weight/ Wing Area

 

Design Cruising Speed (Vc) = 33√WL

 

Design Dive Speed (Vd) = 1.4Vc

 

Never Exceed Speed (Vne) = 0.9 Vd

 

Here’s a table showing some results based on a hypothetical wing area of 150 sq ft:

 

So what do you reckon?

 

Old Man Emu

 

Vne.xls

 

Vne.xls

 

Vne.xls

[djpacro]

Looks like those formulae are from FAR 23 - the design requirements which have been around for many years now but the types you mentioned would've predated FAR 23.

 

I suggest that you have a look at the old CAR 04, the US airworthiness regs from the '40s.

 

VL shall correspond to design power in accordance with CAR 04.11.Vg shall not be less than VL + Kg (Vm.-VL), except that it need not be greater than either VL + 100 miles per hour or 1.5VL, whichever is lower. Kg is specified on Fig. 04-1.Vm is defined in CAR 04.115.

 

.....

 

The flight tests shall include steady flight at the maximum certified airspeed. Such tests shall in no case involve indicated airspeeds in excess of 90 per cent of the design gliding speed (Vg) for which compliance with the structural loading requirements (CAR 04.21) has been proved. When Vgis greater than 1.33 VL (CAR 04.111) the flight tests need not be conducted beyond an indicated airspeed equal to 1.2 VL, provided that the operation limits are correspondingly fixed (see CAR 04.743).

 

....

 

04.743-AIR SPEED LIMITATIONS.-The maximum certified airspeed shall be limited to a value at least 10 per cent less than the maximum indicated airspeed attained in official flight tests. (See CAR 04.722.) The effects of auxiliary devices shall be similarly accounted for when their operation lowers the safe operating speeds. Means shall be provided by which the pilot is suitably informed of the necessary speed limitations.

 

04.7430-When the design power (CAR 04.105) used in determining structural loading conditions is less than the maximum (except take-off) power for which the engine is certified, the cruising speed shall be limited to a value at least 10 per cent less than the VL (CAR 04.111) corresponding to the design power used.

The next problem is that those types were probably not certified and maybe not even considered wrt any airworthiness requirements. i.e. I wouldn't assume anything regarding an appropriate VNE.

 

 

[facthunter]

pietenpol. airspeed.

 

Wouldn't expect any great speed with a Rotax with a draggy plane. While th "T" or "A" Ford were not powerful, they swung a big dia prop. that will get a "big" plane along at say 55 mph. A few "modern" Piet's use a Chev. Corvair 6 cyl, but I don't think they break any speed records either.

 

You are very correct to be concerned with flutter, so don't let me put you off. It's just that I don't bekieve you will get the airspeed..Nev.

 

 

[jerrajerra]

Hi, I gotta question about VNE. Would you need to re-calibrate the VNE of your aircraft after so many years or would the difference between the VNE then and the VNE now be so minimal that it need cause no alarm or requirement to take action? For example, all C172's from the factory floor at 1967 let's say would have the same VNE. But given 40 odd years or so would the wing load factor therefore alter becuase of the envirionment the AC has been subjected to?

 

Thanks, Chris

 

 

[rgmwa]

Hi, I gotta question about VNE. Would you need to re-calibrate the VNE of your aircraft after so many years or would the difference between the VNE then and the VNE now be so minimal that it need cause no alarm or requirement to take action? For example, all C172's from the factory floor at 1967 let's say would have the same VNE. But given 40 odd years or so would the wing load factor therefore alter becuase of the envirionment the AC has been subjected to?Thanks, Chris

My understanding is that Vne is mainly governed by the aircraft's susceptibilty to flutter, which is a function of airspeed and the natural frequency of essential parts like tail components. The natural frequency is a function of geometric and material properties and won't change with age, but age related fatigue (ie stress cycles) may affect the strength of the airframe parts.

 

rgmwa

 

 

[naremman]

My understanding is that Vne is mainly governed by the aircraft's susceptibilty to flutter, which is a function of airspeed and the natural frequency of essential parts like tail components. The natural frequency is a function of geometric and material properties and won't change with age, but age related fatigue (ie stress cycles) may affect the strength of the airframe parts.rgmwa

Airframe is not always the determinate of Vne, sometimes the engine is the principal factor. Vne of the Victa Airtourer 100 is 175 knots, cos at that speed the tacho, even with the throttle closed is red line. Same airframe, the 150 Hp Airtourer with a CSU unit is 204 knots.

 

Tiger Moth Vne 139nots. only achieved in the vertical down with full throttle.

 

The then young instructor who taught my Dad to fly, who is now a Cathay B747 driver, always used to refer to Vne as: "very near the end"!!

 

 

[jerrajerra]

Thanks for comments :)