RV-8 VN Diagram

[pvalovich]

Anyone have an electronic versin of the RV-8 VN diagram? If so, can you email it to me at:

pvalovich@dcscorp.com?

Or point me to an online source
Thanks.

[Andy Hill]

Never seen one but pretty easy to devise:

1. +6/-3g @ 1600lb
2. VS ~55K @ 1600lb (someone will give a more accurate figure, but it can vary esp due IAS error)
3. VA 123K
4. VNE 200K => VD 220K

Not sure if the gust lines can be mathematically added, or more info is required?

Andy

[penguin]

Gust lines can be calculated from the info in FAR 23 (but may not extend beyond basic envelope)

Use Lift = 1/2 * rho *V^2 * S * Cl to calculate the positive and negative curves, where

rho = density of air = 1.225 kg/m^3 (sorry, don't know what that is in lps)
V = speed - but be careful of the units (m/s in SI)
S = wing area (span * chord - don't worry about part in fuselage)
Cl = lift coefficient - can be calculated from the stall speed
Lift is in Newtons in SI, and is weight * g

The biggest gotcha here is the units - Cl should be somewhere around 1.5, so if it is much different for the stall case then the units are wrong.

Pete

[hendrik]

I have made a vn-diagram, but I'm still unsure about the gust lines. I read the d(CL)/d(alpha) from the diagrams in the NACA note NACA-TN-1299, and while the +50fps line agrees with v_no quoted in the flight testing section of the plans, the -50fps line crosses -3g much earlier. Okay, that's not exactly a surprise, since the gust lines are almost symmetrical, but still I'm not sure how to interprete this. Maybe some aeronautical engineer can comment?

I also used a stall speed of 55 kts, so v_a is higher than the number you find in the manual for the ASI markings (that assumes 50.4 kts stall speed).

Here is the file, still preliminary, and comments are very welcome: http://rv8.linta.de/files/vaf/vn-diagram-rv8.pdf

[plehrke]

Originally Posted by Carl J. Niedermeyer

The yellow areas of your V-n diagram should be red. Yellow implies one can fly there, but with caution. Limit Load (+6g, -3g) should never be intentionally exceded in flight. The 1.5 factor of safety to Ultimate Load (+9g, -4.5g) provides a margin of safety to cover various items.

I agree with it should be red above the limit (design) load factor. Above limit load the airframe can deform permanentely but can not fail until ultimate load factor. If you pull more gs then limit load then you wil need to inspect the aircraft for any permenantly strained structure. I have not heard of Van publishing or anyone posting a list of the places to look; most likely places to find the strained structure (wing spars, tail, engine mount, fuselage longeron, etc)

[Alan Carroll]

The diagram looks very good; thanks for sharing it.

I wonder if the use of color should be limited to showing specific airspeed ranges within the loading envelope? (ie yellow from Vno to Vne, red from Vne onward). This would keep it consistent with colors used on the airspeed indicator and avoid any confusion about limit vs. ultimate loads.

I'm curious about the signficance of Vno at negative Gs as well.

[hendrik]

Thanks for the comments. I've changed the colors, though that's probably the part of this diagram I'm least interested in. The gust lines are what I'd like to learn more about. Any insights on this question?