by Kevin Horton » Mon Jan 03, 2005 10:37 am
J.C.,
Please excuse the slow reply. I wanted to do some thinking before replying, and yesterday was a crazy day.
There are a number of things to consider here:
Is the VW a suitable aviation powerplant? I have read concerns about cylinder head cooling problems if you run at high power for too long. If true, this means that the VW might be OK if you had an aircraft that allowed you to run at low power except for take-off, but you might have an increased risk of engine failure if you had to run at high power most of the time. You said the aircraft was originally designed for a 100 hp engine. If so, you might need to run that VW pretty hard to get acceptable performance. I’m no VW expert, so I won’t dive in too deep here - I’ll leave this issue to those who know VWs better than I do.
Will a lower hp powerplant deliver satisfactory aircraft performance? Take a look at other aircraft that use the VW engine. How does your aircraft compare in terms of weight and wing area?
Will the CG be in the right place? Originally you thought you had a Playboy, which was a successful design. We could have looked at the engine weight of typical Playboy engines, and figured out how far forward of the firewall the VW needs to be to get a similar CG. But now we don’t know what aircraft it is. It might even be a one-of design. All I can say is to be very careful to do a proper weight and balance, and to do whatever it takes to get the CG in a suitable place. If you’ve got an aircraft of conventional design, with no wing sweep, you should probably shoot for a CG of somewhere between 20% and 25% mean aerodynamic chord for the first flight. The aircraft might be perfectly acceptable at CGs further aft than this, but you want to work back there a little bit at a time, not start there on flight one.
Strength of the engine mount mod - You said that you figure the weight of the engine and prop at 150 lb. I’ll call this value W. The engine mount you have is 13.5� high at the firewall. I’ll call this value H. We need to know how far forward of the firewall the CG of the engine and prop will be. We don’t have this value, but we’ll label it D. Lastly, we need a design g loading. You don’t want to do any aerobatics or tight manoeuvring. Normal category FAR 23 type certificated aircraft are designed to take 3.8 g, with a 1.5 factor of safety, for an ultimate g loading of 5.7. We’ll call this one N.
Looking at the in-flight loads case at high speed, with zero angle of attack. If pull some g, we have a load down on the engine/prop combination of W x N. If we consider moments around where the engine mount lower firewall point is, we have a moment of W x N x D. This moment must be reacted by the upper firewall mounting point, which means the upper engine mount attachments are being pulled away from the firewall with a force of (W x N x D)/H. We’ve got two mounting points, so the load on each one is half that, or (W x N x D)/(2 x H). Just to get an idea of the forces involved, lets say that the CG of the engine/prop combination was 24 inches ahead of the firewall (value D). We get a force of (150 x 5.7 x 24)/(2 * 13.5) = 760 lb pulling each upper engine mounting point away from the firewall. The lower engine mounting points would be pushed into the firewall with a similar force.
We also need to consider the straight downwards force created by the g on the engine. If we assume that the four engine mount points share this load (this is probably not a good assumption, mind you), we have a force of (W x N)/4 = 214 lb straight down on each firewall point. The 760 lb load and the 214 lb load are at right angles to each other, so the total force is the square root of (760 squared + 214 squared), or 790 lb. If we assumed that the whole vertical load was taken by the upper mounts, as a worst case assumption, that gives us a vertical load of 428 lb per upper mounting point, and a total load of about 870 lb on each upper mount. I’m not trying to design an engine mount for you - I’m just shooting these numbers at you so you have some idea of how much force the mount should be able to withstand.
In some cases the landing loads in a hard landing might be even worse than the flight loads. We don’t have enough info to know how well your landing gear will absorb hard landings. But I am going to assume that you won’t deliberately fly off rough strips, and that you will do a very good inspection of the whole aircraft, including engine mount, if you ever have a hard landing.
There is one big caveat about all this - although I graduated as a mechanical engineer over 20 years ago, I went straight into pilot training, and never did any mechanical design work. So, it is quite possible that I have screwed up these calculations, or have missed some other important aspect.