Transferring prop loading to airframe

[bfanger]

Aircraft engines are somewhat of an oddity to me in that the thrust is carried by the crank and transferred to the engine case on its way to the airframe. In an auto engine, only the torque is transferred to other components that make the thrust to move the vehicle. I could imagine that the original application had no allowance for the case being used to pull the vehicle around. In a direct drive I don't think that it could generate enough thrust to matter much but with a reduction drive I could see it being more significant. Being as the cases are already prone to cracking in the car form, has anyone tried to make a structure or girdle to help carry the additional loads and make it more ridged?

 

[JamesG]

This is not true in all cases. If an engine has a reduction drive or other final gearbox, then there is no lateral load on the crankshaft. Most high power reciprocating and turbine engines also have final thrust bearings that take load off the main shaft. Most aircraft engine cases, being made as light as possible are mounted in frames or mounts which support the weight, torque, and tension loads.

 

[rv6ejguy]

The average static thrust created is on the order of 4-5 pounds per hp so an 80hp engine might see 400 lbs at most- insignificant for almost any automotive case. There are enough direct drive engines flying to know that the prop thrust loads are generally no problem either since they are usually quite a bit lower than the clutch pressure plate force on a per hp basis. Gyroscopic and bending loads on the crank are usually much more of a concern and this is usually where we see front crank/ bearing failures rather than case failures.

 

[bfanger]

I intend to use a reduction drive, and definitively do like the added benefit of not having the crank take those loads. It is still attached to the front of the engine and imparting them on the case. I would love to mount the engine by the pulley end but there is just nothing there to mount to. I haven't seen a flywheel mount reduction drive for VW yet.

My first thought is a tensile rod on top and bottom squeezing the case lengthwise to offset the the thrust but I really don't see that working and if done wrong could be worse by compressing the case with easily 2-3 times the force that the thrust imparts.

There is also the possibility I'm overthinking this.

The average static thrust created is on the order of 4-5 pounds per hp so an 80hp engine might see 400 lbs at most- insignificant for almost any automotive case. There are enough direct drive engines flying to know that the prop thrust loads are generally no problem either since they are usually quite a bit lower than the clutch pressure plate force on a per hp basis. Gyroscopic and bending loads on the crank are usually much more of a concern and this is usually where we see front crank/ bearing failures rather than case failures.

The difference I see in a clutch is that the thrust load is applied close to the bellhousing and should not be going across the thinner sections where the cylinder bores are, depending on which is the thrust bearing. I could be wrong though, its been a while since I've had a VW engine apart.

 

[JamesG]

Dont get the old guys started on that...

Any car engine is going to have cases stronger than anything a prop can dish out. They are already to heavy, don't make it worse.

 

[rv6ejguy]

I intend to use a reduction drive, and definitively do like the added benefit of not having the crank take those loads. It is still attached to the front of the engine and imparting them on the case. I would love to mount the engine by the pulley end but there is just nothing there to mount to. I haven't seen a flywheel mount reduction drive for VW yet.

My first thought is a tensile rod on top and bottom squeezing the case lengthwise to offset the the thrust but I really don't see that working and if done wrong could be worse by compressing the case with easily 2-3 times the force that the thrust imparts.

There is also the possibility I'm overthinking this.


The difference I see in a clutch is that the thrust load is applied close to the bellhousing and should not be going across the thinner sections where the cylinder bores are, depending on which is the thrust bearing. I could be wrong though, its been a while since I've had a VW engine apart.

You're overthinking this. 350-400 lbs force on any part of the case is miniscule compared to the strength of the case. You could lift the entire car up from a couple of bolts anywhere on the case. You could do a quick calc on the case strength. Take the cross sectional area of the thinnest part of the case and multiply by 20,000 if you're working in inches and pounds. You'll see it exceeds the weight of your aircraft many times over.

 

[BobbyZ]

While I agree with Ross I dont know why people are afraid to extend the mount some for added support.Especially if using a redrive or a added bearing like some conversions use.A few well engineered bends would add very little weight and allow the loads to be transferred away from the crankshaft.Although these ideas of mine are directed more towards torsional loading than thrust loads.

I know this might be a belt and suspenders approach but nonetheless I feel that in some cases it'd be worthwhile to integrate this into a auto conversions mount.There's no shortage of places to attach to and the actual weight I feel would be very little.Granted this would only work as a way to mount another bearing or a way to tie into a redrives case etc,I still think its something worth thinking about.

 

[mcrae0104]

Being as the cases are already prone to cracking in the car form, has anyone tried to make a structure or girdle to help carry the additional loads and make it more ridged?

I've never heard of an engine (automotive or aircraft) having a cracked case due to carrying thrust loads. No need to solve a problem if it doesn't exist.

Most aircraft engine cases, being made as light as possible are mounted in frames or mounts which support the weight, torque, and tension loads.

Um, not really. The thrust load (which presumably is what you meant by "tension") goes through the crank to the thrust bearing and through the case before being supported by the mount. The vast majority of reciprocating aircraft engines are in fact cantilevered. The statement above would lead one to believe cases are weak and must be strengthened by external means, which is not the case (no pun intended) at all.

While I agree with Ross I dont know why people are afraid to extend the mount some for added support.

Generally, it's not done because it's not needed and there would be no practical benefit (only additional weight and reduced performance). However, here is an example where it was needed.

 

[Riggerrob]

Turboprop engines tend to have long engine mounts that transfer prop loads directly from the reduction gearbox to the firewall.
But like another poster said, automobile crankcase are plenty strong enough.